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Senior Form Physics – Nature of Light
Nature of Light
Teacher’s Version (with video exemplars)
NOS/STSE Features
1. Scientific laws are descriptions of relationships among phenomena or patterns in
nature. Scientific theories are the explanations of those relationships or patterns.
2. A good scientific theory should have strong explanatory power.
3. People tend to submit to authority when there is not enough evidence to make
judgment.
4. Science is evidence-based. Evidence can establish or refute a theory.
5. Building or developing scientific knowledge is hard work, often building on hard
work of other scientists in the past or present.
6. Scientific knowledge is changing, sometimes even revolutionary.
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activities in this package.
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Table of Contents
Introduction
(i)
Activity 1: Nature of light
P.1
Activity 2: Revision: The history of the photoelectric effect
Activity 3: Experiment 1: Generating a current using UV radiation
Activity 4: Experiment 2: Generating a current from visible light,
and determination of Planck’s constant
Appendix 1: Proofs of law of reflection & law of refraction by
geometry
Appendix 2: Notes for Technician: Electroplating sodium onto the
inner surface of a neon bulb
Appendix 3: References
Appendix 4: Lesson plan
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© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Introduction
Rationale
People have thought about the nature of light for many centuries. The ancient Greeks thought
of light as streams of tiny particles. Christian Huygens (1629-1695) was the first scientist that
proposed the wave theory of light. Isaac Newton (1642-1727) took the idea of the ancient
Greeks and developed the particle theory of light. In the 17th century, Thomas Young (17731829) showed evidence for his hypothesis that light consisted of waves by the famous doubleslits experiment, illustrating wave-like interference of light. He confirmed the wave theory of
light proposed by Huygens. However, Albert Einstein (1879-1955) explained the
photoelectric effect by thinking of light as a stream of particles called photons. de Broglie
(1892 – 1987) showed that the particles making up all matter have wave-like properties. This
package aims to help students to build up the following views of the nature of science. 1.
Difference between laws and theories in science. 2. Evidence is important, not the status of
any scientist. 3. Developing scientific knowledge requires hard work often building upon the
hard work of others. 4. Scientific knowledge may be durable but is subject to change.
Changes may sometimes be revolutionary.
The significance of this package is that it uses both traditional and innovative investigations
to help students to understand the photoelectric effect. Students will learn that it is possible to
electroplate sodium onto the inner glass surface of a neon bulb. A neon bulb with a sodium
coating is used to study the photoelectric effect induced by visible light and UV radiation.
Though an accurate measurement of Planck’s constant is difficult, the demonstration provides
a good qualitative illustration of the particle nature of light.
Level of study
Number of lessons
Form 6 – Form 7
8 lessons (each lesson 35-40 minutes)
Contents
1. Investigating some important aspects of the nature of science through exploring the
nature of light.
2. History of the photoelectric effect.
3. Generating a current with UV radiation.
4. Generating a current by visible light and determination of Planck’s constant.
Teaching package includes
1. Worksheets (Teacher’s Version and Student’s Version)
2. A Teaching PowerPoint
(i)
© 2006 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Activity 1
Nature of light
Christian Huygens
1629 - 1695
Isaac Newton
1643 - 1727
Answer the following questions while listening to your teacher’s presentation about
the nature of light.
Q1 What is a scientific law? What is a scientific theory? What are the differences
between them?
[ PowerPoint P.18 ]
Scientific laws are descriptions of relationships among phenomena or patterns in
nature.
(e.g. Ideal Gas Law, Laws of Reflection, Laws of Refraction)
Scientific theories are the explanations of those relationships or patterns.
(e.g. Kinetic theory, particle theory of light, wave theory of light)
Scientific theories and laws are different kinds of knowledge, a theory will not
turn into a law or vice versa.
Remarks: It is a common misconception that students think there is a
hierarchical relationship between laws and theories.
Laws are descriptions of relationships among phenomena or patterns in
nature while theories are the explanations of those relationships or patterns.
Difference between a law and a theory
Relationship of law, theory and hypothesis
Q2 If you were one of the scientists in the 17th century, would you believe the
“particle theory of light” or the “wave theory of light”? Why?
Hint: Which theory has a greater ability to explain the characteristics of light?
[ PowerPoint P.23 ]
Most scientists believed Newton’s particle theory of light because it has greater
explanatory power:
A good theory should have strong explanatory power, i.e. can explain more laws.
1. Waves do not travel only in straight lines but light only travels in straight line, so
light cannot be “waves”.
P. 1
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
2. Only particles can travel through a vacuum, therefore light could not travel
through a vacuum if it consists of waves as there is no “medium” for waves to
spread.
3. Only the particle theory of light can explain why there are different colours of
light.
Another reason for the success of the “particle theory of light” is due to the high
reputation of Newton, people tend to accept “authority” when there is not enough
evidence to make judgment.
People tend to submit to authority when there is not enough evidence to
make judgment.
Properties of light
Believe which theory?
Q3 Can the wave theory of light explain the result of the photoelectric experiment?
[ PowerPoint P.35 ]
No.
The current should increase with the intensity of light and there should be no
“cut off” frequency if using the wave theory of light to predict the results.
Q4 What can we learn from the long story about the nature of light?
[ PowerPoint P.40 ]
Evidence (e.g. Young’s double slit experiments, photoelectric experimental
results…etc) can establish or refute a theory.
Building or developing scientific knowledge (the nature of light) is hard work,
building upon the hard work of others. (collaboration across time)
Scientific knowledge is ever changing, sometimes even revolutionary
(Einstein re-discovered the particle nature of light, de Broglie discovered the
wave-particle duality of all matter)
Science is evidence-based, evidence can establish or refute a theory.
Building or developing scientific knowledge is hard work, often
building on hard work of other scientists in the past or present.
Scientific knowledge is ever changing, sometimes even revolutionary.
Scientific theory and laws are tentative
Summary (NOS)
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© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Activity 2
The History of the Photoelectric Effect
Thomas Young
1773 - 1829
Albert Einstein
1879 - 1955
The History of the Photoelectric effect
In late 19th century it was discovered that electrons were emitted from a zinc plate
when it was exposed to ultraviolet (UV) radiation. As visible light and UV are
electromagnetic (EM) waves of similar nature, the result suggests that EM radiation
carries energy and is capable of knocking electrons (displacing
them) from metal atoms.
From Young’s double slit and diffraction grating experiments, we
learn that visible light behaves like a wave with wavelengths
ranging from 400 nm to 700 nm. However, in the early 20th century,
physicists discovered that results of the photoelectric effect cannot
be explained simply by the wave nature of light. For instance, the
kinetic energy of photoelectrons emitted from zinc is independent
of the intensity (energy) of the incident UV radiation, but no
photoelectrons are emitted if visible light, no matter how intense, is used.
In 1900, Max Planck studied the radiation emitted by hot bodies. He found that the
energy of vibrating molecules is an integral multiple of h, where  is the frequency
of vibration and h is a constant now called Planck’s constant.
h = 6.626  10-34 J s
Based on Planck’s work, Einstein established the quantum theory
of light by assuming that light consists of particles called photons.
Each photon carries an amount of energy h. In this case,
increasing the frequency of the light (its colour) will increase the
energy of each photon, but increasing the intensity of the light
(brightness) will increase the number of photons.
According to Einstein, the maximum kinetic energy of a
photoelectron is given by
hc
KE max  h  W 
W
(1)

where W is the energy required to take an electron out from a metal surface known as
the work function for the metal being bombarded. So from (1), a photon can knock
out an electron from the metal only if its frequency satisfies the inequality
KEmax  h  W  0 . The energy of photons and photoelectrons is conveniently
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© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
expressed as units of electronvolts (eV). 1 eV is the energy gained by an electron
when it is accelerated by a potential difference of 1.0 V (Figure A).
+ 1.0 V
e0.0 V
Figure A:
An electron moving across a region
with a potential difference of 1.0 V
Revision:
(i)
Find the energy supplied by a dry cell of voltage 1.5 V when a
charge of 1.0 C is driven across the cell in an electric circuit.
From
Energy supplied (J) = Charge (C)  Voltage (V)
E = qV
= 1.0 C  1.5 V = 1.0 C  1.5 J C-1 = 1.5 J
(ii)
Find the kinetic energy gained (J) by an electron (1.6  10-19 C) in
moving from the bottom plate to the top plate in a vacuum (Fig. 1).
From
E = qV
The gain in KE is E = 1.6  10-19 C  1.0 J C-1 = 1.6  10-19 J
(iii)
How many electronvolts are there in 1 J?
By definition, the answer in 2b is equal to 1 eV.
∴ 1 J = (1.6  10-19)-1 eV = 6.25 x 1018 eV
(iv)
Express Planck’s constant, h, in eV s-1.
h = 6.626  10-34 J s  6.25 x 1018 eV J -1 = 4.14  10-15 eV s
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© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
The following table shows the work functions of some common metals in units of
electronvolts (eV).
Metal
Sodium
Calcium
Magnesium
Iron
Zinc
Copper
Chemical symbol
Na
Ca
Mg
Fe
Zn
Cu
Work function (eV)
1.82 – 2.36*
2.52
3.66
4.67 – 4.81
3.63
4.53 – 5.10
Sodium can appear in the form of crystalline metal or polycrystalline metal. The
crystal lattice of the monocrystalline sodium is continuous and unbroken to the edges
of the sample. The opposite of a single crystal sample is a polycrystalline sample,
which is made up of a number of smaller crystals known as crystallites. Almost all
common metals are polycrystalline.
For monocrystalline sodium, the work function is 1.82 eV. For polycrystalline
sodium, the work function is 2.36 eV.
Revision:
(i)
The work function of sodium (Na) is 1.82 eV. What is the wavelength
(in nm) of light required to generate photoelectrons on the metal
surface? Find also the wavelength of radiation required for emission of
photoelectrons from zinc with a work function of 3.63 V.
From (1), the condition for emission of photoelectrons is
or
For sodium,
= 0.683 m = 683 nm (Red light)
For zinc,
=0.342 m = 342 nm (UV)
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© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Activity 3
Generating a current by UV radiation
Generating a current by UV radiation
A bulb contains neon gas in low pressure with two iron electrodes sealed inside
(Figure A). The bulb glows with a distinct reddish-orange colour when the voltage
across its electrodes is increased above a threshold starting level. Since the operating
current is no more than 100 A, small neon bulbs are commonly used as indicators.
The typical starting voltage for a neon indicator is 60 V to 80 V d.c., or 43 V rms to 57
Vrms a.c. Once ignited, a lower voltage is needed to sustain the glow until the applied
voltage drops below the “extinction level”, a few volts lower than the starting level.
Figure A: Three samples of neon bulbs: the two bulbs on the left are indicators; the
bulb on the right is a flicker lamp used to give an effect like a candle in the home
The glowing of a gas discharge tube, including the neon bulb, can be explained in
atomic terms. Electrons, produced by cosmic rays or other means, are accelerated
to a high speed by the voltage applied across the electrodes. Gas atoms become
excited or “ionized” when they are bombarded by fast moving electrons. Light is
emitted when atoms return from an excited state to a lower energy state.
P. 6
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Investigation - Generating a current by UV radiation
Object: To generate a current by shining UV radiation onto a neon indicator
low voltage a.c.
supply
Apparatus:
1
1
1
1
1
1
2
1
low voltage a.c. power supply
digital a.c. voltmeter
step-up transformer with turns ratio 1:20
neon indicator bulb
mini torch with UV emitting diode (UVED)
power torch emitting visible light
leads with clips at both ends
250 k potentiometer
Precaution:
Prolonged viewing of the UV beam directly from
the UVED may have a harmful effect on the eyes.
Wear safety goggles to reduce any chance
exposure (spectacle wearers are protected to some
degree)
Transformer with turnsratio 1:20
Neon
bulb
250 k
potentiometer
Circuit to initiate glow discharge in a neon bulb
Procedure:
1.
Connect the circuit as shown in the figure.
2.
Slowly increase the voltage of the power supply until the neon bulb starts
glowing. Note the ignition voltage [in a.c. or (root mean square)] shown on the
a.c. voltmeter.
3. Reduce the voltage across the iron electrodes of the neon bulb until extinction
occurs and then increase the voltage again to a level just below the ignition
threshold. (about 0.5 V)
4. Hold the UV torch about 5 cm above the neon bulb. (Precaution: If the
diode is too close to the neon bulb, the change in capacitance between the
UV torch and the neon bulb may initiate glow discharge in the bulb. Try
this with a finger.)
5. Direct a beam of UV radiation onto the surface of the neon bulb. What
happens to the bulb? (The UVED gives out not only invisible UV radiation,
but also a small amount of visible light.)
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© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Result and conclusion:
UV radiation__ (visible light/UV radiation) can cause a current to flow in the
neon bulb when the voltage applied across its electrodes is slightly below the
normal ignition voltage. This suggested that photoelectrons (photoelectrons/
thermal-electrons) are emitted from impurities such as zinc in the iron electrodes,
when the surface of the electrodes is illuminated by UV radiation. As these
electrons are given an initial kinetic energy by the potential difference across the
transformer, a lower ignition (ignition/extinction) threshold voltage is required to
start a glow discharge in the bulb.
Our result is consistent with the prediction that the minimum wavelength of EM
radiation required to generate photoelectrons in zinc is 342 nm, which is longer
than the minimum wavelength of 330 nm (from handbook) emitted by the UV
diode (Figure B)
Relative Intensity
Threshold wavelength
of zinc = 342 nm
Visible region
 400 nm to 700 nm
.
300
350
400
450
500
550
600 / nm
Figure B: Spectrum of UV diode
P. 8
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Activity 4
Generating a current by visible
determination of Planck’s constant
light
and
The neon bulb with sodium coating (Na-Ne bulb) will be used to study the
photoelectric effect induced by visible light. These bulbs can be purchased from the
Faculty of Education, CUHK. Since normal indoor illumination can cause sodium to
emit photoelectrons, a local blackout is required to find the blackout ignition voltage in
the following investigations.
Investigation - Generating a current by means of the photoelectric effect using
visible light
Object: To find a relation between KEmax of photoelectrons and the intensity of light
Apparatus:
1 low voltage a.c. power supply
1 digital a.c. voltmeter 0-200 Vrms
1 step-up transformer with turns’ ratio 1:20
1 Na-Ne bulb
1 mini torch with 970 nm IRED
1 mini torch with white LED
1 piece of black paper with a small hole at the center
1 piece of plastic red filter
Several leads fitted with plugs at both ends
1 250 k potentiometer as coarse adjustment
2 50 k rheostat as fine adjustment
Procedure:
1. Connect the circuit as shown in Figure A, or use the built-in circuit provided
in the photoelectric kit (Figure B). Cover the neon bulb with the black paper.
low voltage a.c.
supply
Transformer with turnsratio 1:20
50 k
rheostat
250 k
potentiometer
Na-Ne
bulb
Figure B: A photoelectric kit with coarse &
fine adjustment
V
Figure A:
Circuit to initiate glow discharge in Na-Ne bulb
P. 9
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
2.
Slowly increase the voltage of the a.c. power supply until the Na-Ne bulb
starts glowing, detectable through a small hole in the black paper. Note the
ignition voltage shown on the a.c. voltmeter in the blackout condition.
3.
Reduce the voltage across the Na-Ne bulb until extinction and increase the
voltage again to a level just below ignition.
4.
Direct a beam of infrared light onto the surface of the Na-Ne bulb. What
happens to the bulb? Compare the ignition voltage of the Na-Ne bulb in
blackout and under infrared light.
5.
Repeat step 3 using a more intense infrared beam. Can we reduce the
ignition voltage by increasing the intensity of infrared light?
6.
Repeat step 4 using a mini-torch that emits white light. No blackout is
required if the illumination by the intended source is brighter than the
background light. Starting with an a.c. voltage about 1.5 V (by adjusting
the 250 k potential divider) below the ignition level, increase the voltage
in steps of 0.1 V (by adjusting the 50 k rheostat) until a glow discharge is
observed. The glowing of the Na-Ne bulb can be detected more easily
under white light by viewing through a red filter. Note the new ignition
threshold of the Na-Ne bulb under white light.
7.
Repeat step 6 using a more intense white light beam. Is there any
significant change in the ignition threshold?
Result and conclusion:
Complete the following table from what you have observed.
Condition of illumination
Blackout
Approximate ignition
voltage (a.c.)
56.0 + 0.2 V
Voltage reduction
under illumination
0.0 V
Infrared
56.0 + 0.2 V
0.0 + 0.4V
More intense infrared
55.9 + 0.2 V
0.1 + 0.4 V
White light
55.2 + 0.2 V
0.8 + 0.4 V
More intense white light
55.2 + 0.2 V
0.8 + 0.4 V
Relation between ignition voltage and intensity of incident light
The ignition threshold of the Na-Ne bulb under infrared light (white
light/infrared light), no matter how intense, is the same as that in the blackout
condition. This suggested that no photoelectron is emitted from sodium when
illuminated by infrared_ (white light/infrared). On the other hand, the ignition
threshold of the Na-Ne bulb is lowered (lowered /raised) when illuminated by
white light (white light /infrared light). However, no significant change in the
ignition level is observed by increasing the intensity of white light (white light
/infrared light). The result is consistent with Einstein’s theory that the
maximum (maximum/minimum) kinetic energy of photoelectrons emitted from
a metal surface is independent of the intensity of the incident light.
P. 10
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Investigation - Determination of Planck’s constant
Object: To determine Planck’s constant using photoelectric effect
Apparatus:
1 low voltage a.c. power supply
1 digital a.c. voltmeter 0-200 Vrms
1 step-up transformer with turns’ ratio 1:20
1 Neon bulb
1 laser pointer
1 power torch with at least 0.5 W UVED
1 power torch with at least 0.5 W green LED
1 power torch with at least 0.5 W blue LED
1 piece of black paper with a small hole at the center
1 piece of plastic red filter, several leads with plugs at both ends
3 250 k potentiometer as coarse adjustment
4 50 k rheostat as fine adjustment
Procedure:
1. Connect the circuit in exactly the same way as in Figure A.
2.
Repeat the experiment with the Na-Ne bulb illuminated in turn by a laser
pointer, power LED’s of different colours, and UV light. The distance
between the light source and the Na-Ne bulb should be about 5 cm.
3.
Check the ignition voltage before and after illumination by each light
source. Record the reduction in ignition level, V, in Table V if the
blackout ignition level is the same (within + 0.1 V) before and after
illumination. In this way, the change in ignition voltage due to changes in
temperature of the Na-Ne bulb can be taken into account.
4.
Plot a best-fit straight line graph with KEmax (KEmax in eV is numerically
the same as V in V) against frequency  (in s-1) of light. Determine
Planck’s constant and the work function of sodium from the slope and the
y intercept of the line.
Result and conclusion:
Source of
illumination
Minimum
Maximum
wavelength of
frequency /
light or UV /nm 1014 s-1
Voltage
reduction
V/V
Red laser light
660 + 10
4.55 + 0.06
0.1 + 0.2
Approximate
KEmax of
photoelectrons
in eV
0.1 + 0.1
Green LED
530 + 20
5.36 + 0.12
0.4 + 0.3
0.4 + 0.3
Blue LED
430 + 20
6.98 + 0.12
1.0 + 0.3
1.0 + 0.3
UVED
330 + 10
9.09 + 0.06
1.9 + 0.2.
1.9 + 0.2
P. 11
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Relationship between KEmax of photoelectrons and frequency of light
2
KEmax/eV
1
0
1
-1
2
3
4
5
6
7
8
9
10
Frequency (104s-1
-2
From the Einstein’s equation and the graph,
∴
h (Planck’s constant) = slope of graph = (4.3 + 0.5) x 10-15 eV s
and WNa (Work function of sodium) = negative of y-intercept = (1.8 + 0.3) eV
P. 12
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Appendix 1: Proof of the Law of Reflection and the Law of
Refraction by Geometry
Reflection
In Figure A, the wavefront AB is incident on the reflecting surface and A has just
reached it. To find the new position of the wavefront when B is about to be reflected
at B’, a secondary wavelet is drawn with center A and radius BB’. The tangent B’A’
from B’ to this wavelet is the required reflected wavefront.
In ∆ AA’B’ and
∆ ABB’
AA’B’ = ABB’ = 90º
AB’ is common
AA’ =BB’ (by construction)
Therefore ∆ AA’B’ = ∆ ABB (R.H.S.), hence
BAB’ =
A’B’A
These are the angles made by the incident and reflected wavefronts AB and A’B’
respectively with the reflecting surface. The incident and reflected rays, e.g. CA and
B’D, are at right angles to the wavefronts as are the normals to the surface (the dotted
lines at A and B’) and so the angle of incidence i equals the angle of reflection r. This
is the law of reflection.
A
Figure A
B’
Huygens’ construction for reflection of a straight wavefront
Refraction
Figure B shows the end A of a plane wavefront AB about to cross the boundary
between media 1 and 2 in which its speeds are v1 and v2 respectively. The new
position of the wavefront at time t later when B has traveled a distance BB’ = v1t and
reached B’ is found by drawing a secondary wavelet with center A and radius AA’ =
v2 t. the tangent B’A’ from B’ to this wavelet is the required wavefront and the ray
AA’, which is normal to the wavefront, is a refracted ray. If v2 is less than v1 the
P. 13
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
refracted ray is bent towards the normal in medium 2, as shown. When v2 is greater
than v1 refraction away from the normal occurs.
In ∆ BAB’ and A’AB’
sin i1
sin CAN
sin BAB'


sin i2 sin A' AN ' sin A' B' A

BB' /AB' BB' v1t v1



AA' /AB' AA' v2 t v2
But v1 and v2 are constants for given media and a particular wavelength,

sin i1
 a constant
sin i2
This is Snell’s Law of Refraction and it holds for electromagnetic, sound and water
waves. The constant is called the refractive index 1n2 for waves passing from medium
1 to medium 2. Hence we can write
1
n2 
v1 sin i1 n2


v2 sin i2 n1
where n1 and n2 are the absolute refractive indices of media 1 and 2.
Huygens’ construction thus explains the refraction of, for example, light, on the
assumption that the speed of light decreases when it passes into a medium in which
the refracted ray is bent towards the normal, i.e. when it enters an optically denser
medium. This assumption is verified by experiment.
Figure B Huygens’ construction for refraction of a straight wavefront
P. 14
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Appendix 2: Notes for Technician: Electroplating sodium
onto the inner surface of a neon bulb
Electroplating sodium on the inner glass surface of a neon bulb
To observe the photoelectric effect using visible light, highly reactive metals with
work functions below 3.1 eV (threshold wavelength  400 nm) like sodium,
potassium and cesium can be used. However, these metals oxidise rapidly in air.
They are generally coated on the cathode of a photomultiplier tube for special
applications such as solar cells.
The work function of sodium (1.82 eV) is lower than that of zinc (3.63 eV). What
is the advantage and disadvantage of using sodium to study the photoelectric effect?
Advantage: sodium emits photoelectrons when illuminated by light nearly within
the whole visible spectrum, say any wavelength shorter than 683 nm (red). Zinc can
respond only to UV with wavelengths shorter than 342 nm.
Disadvantage: Sodium metal is dangerously reactive especially in damp air. It
reacts with oxygen in seconds. A high vacuum is often required if sodium is used to
study the photoelectric effect.
It was discovered accidentally that when red light (650 – 680 nm) from a laser
pointer falls on one of the electrodes, glow discharge is initiated in a new neon bulb
when the voltage across the electrodes is only 0.1 V below the blackout ignition
level. As photoelectrons are ejected by red light, the mysterious sodium is of
monocrystalline structure and has a lower work function of only 1.82 V than
normal sodium. The same effect is observed using bulbs from different providers.
These results suggest that at least one of the electrodes is contaminated by sodium
during the manufacturing process. The following paragraph is a summary of what
we learn from the Wikipedia website and related links.
In making neon bulbs, a pair of electrodes is first melted to one end of the bulb
with the other end attached to a manifold connected either to a neon gas source or a
vacuum pump. Before sealing, the bulb is filled with neon gas, heated to cause a
vacuum (evacuation) again and refilled with neon. During evacuation, a high
voltage is applied to one electrode while the other is earthed, allowing an electric
arc to be struck through the interior of the bulb. The arc heats the glass to a
temperature near melting and impurities are driven off in vapour form and removed
from the bulb by suction. We believe that the arc discharge not only gets rid of
most impurities, but also introduces a new impurity in one of the electrodes, i.e.
metal sodium, reduced from sodium silicate (Na2SiO3), a common constituent of
glass.
Our discovery suggested that we may repeat a similar heating-and-electrifying
process to obtain more metal sodium inside the neon bulb. In this section, we
introduce two simple methods which can be used to coat a thin layer of sodium over
the inner surface of the glass envelope of a neon bulb. The working principle of
electroplating sodium using sodium silicate as electrolyte is similar to that of
P. 15
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
electroplating copper using copper sulphate solution as electrolyte (refer to the
figure below). Copper (sodium) contained in the positive electrode changes into
copper (sodium) ions and these diffuse into the solution (glass). Copper (sodium)
ions in copper sulphate (sodium silicate) picks up electrons from the negative
electrode and is reduced back to copper (sodium).
low voltage a.c.
supply
Transformer with turnsratio 1:20
Positive
copper
electrode
To EHT 2500 V
Negative
electrode to be
electroplated
Copper
sulphate
solution
Electroplating Copper
Neon bulb
Heating coil
A
1.0 A
Fibre-glass
mesh tubing
Low voltage
d.c. supply
Electroplating Sodium
The figure ‘Electroplating Sodium’ shows one of the circuit-systems that can be used.
The neon bulb and heater are actually immersed in a salt bath (fused NaCl) not shown
in the circuit diagram. When the neon bulb is glowing, the neon gas becomes
conductive due to the presence of gas ions and electrons. Glass is a supercooled liquid
containing sodium ions from sodium silicate [Na2SiO3  2 Na+ + (SiO3)-]. When the
heater is on, glass is heated to several hundred degrees Celsius and becomes slightly
conducting. Also, a small amount of sodium chloride is vaporized by the heater. A
strong electric field is set up when a high voltage of several kilovolts is applied across
the heater and the electrodes of the bulb. Due to electrostatic repulsion, the inner
surface of the neon bulb is bombarded by negative ions and electrons and becomes
negatively charged. Sodium ions from the salt vapour then diffuse into glass and
migrate towards the inner surface, whereupon they pick up an electron and are
reduced to metal sodium.
P. 16
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Method One:
Electrolysis of sodium silicate by EHT
Object: To obtain metal sodium by electrolysis of sodium silicate in glass using an
EHT
Apparatus:
1 neon indicator bulb
1 heat resistant fibre-glass mesh tubing (1.5 cm long and 5 mm bore)
1 0-6 V low voltage d.c. power supply
1 0-6 V low voltage a.c. power supply
1 step-up transformer with turns ratio 1:20
1 coil of swg 22 nichrome wire with 6 turns and resistance about 1 
1 0 – 5000 V Extra high tension (EHT) supply
1 0 -5.0 A d.c. ammeter
2 salt bath (a ceramic crucible containing 10 ml of common salt)
Procedures:
1. Insert the neon bulb into the fibre-glass mesh tubing, which is wrapped
round by a heating coil.
2. Place the neon bulb, fibre-glass tubing and heater combination into a
crucible.
3. Add common salt to the crucible until the heating coil is immersed in salt.
4. Connect the circuits as shown in Figure 7. All electrical power should be
off at the start.
5. Increase the voltage from the a.c. power supply from 0 V until the neon
bulb glows at its normal rating.
6. Adjust the low voltage d.c. supply so that a current about 1.0 A passes
through the heating coil. Both a.c. and d.c. supply can be derived from the
same power supply. Independent adjustment can be done with the help of a
rheostat.
7. Switch on the EHT and adjust the voltage slowly from 0 V to -2500 V.
Precaution: High voltage! Do not touch any exposed part of the circuit. To
avoid sparking, the earthed heater and the electrodes at a negative high
potential should be separated by the fibre-glass tubing and be kept at least 5
mm apart.
8. When all power sources are switched on, electrolysis of sodium silicate is
taking place in the glass envelope. The process should be closely
monitored for the first 30 minutes to ensure that all meter readings are
stable and no sparking occurs between the electrodes and the heater.
Reduce the voltage of the EHT if sparking happens or the EHT voltage
P. 17
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
falls gradually. The later is an indication of overloading which happens if
the salt melts or there is a flaw in the circuit.
9. Disconnect the circuit and remove the neon bulb after 24 hours. If the
experiment is successful, a layer of metal sodium can be seen as a
translucent brown deposit on the inner glass wall.
Note: Using a large salt bath (Figure A), several neon bulbs can be electroplated
at the same time by connecting the bulbs in parallel (Figure B) and the
heating coils in series (Figure C) with the same high voltage applied across
the heater and the electrodes.
Figure A
A crucible made of ceramic tiles glued
together by epoxy putty
Figure B
Bulbs connected in parallel
Figure C
Heating coils in series wound round fibre-glass mesh tubings
P. 18
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Method Two:
Electrolysis of sodium silicate by an induction coil
Object: To obtain metal sodium by electrolysis of sodium silicate in glass using
an induction coil
Reed
relay
Apparatus:
1
1
1
1
neon indicator bulb
power signal generator
0-12 V d.c. power supply
1100 turns induction coil
with soft iron core
1 reed relay
Several leads with clips
~
Signal
generator
50 Hz, 6 V
0-12 V d.c.
Neon bulb
L  5.0 H
1100 turns
Procedure:
1. Connect the circuit as shown in Figure D.
Figure D:
Electrolysis of Na2SiO3 by the emf
induced in an induction coil
2. Set the low voltage d.c. supply at about 2.0 V.
3. Switch on the signal generator. Set the output at 50 Hz and 6.0 V,. At these
settings, the neon bulb should glow with a normal reddish-orange tint.
4. Slowly increase the voltage of the d.c. supply and/or decrease the
frequency of the signal generator until the negative electrode of the neon
bulb flashes brightly with a light yellow colour.
Danger: Do not touch the terminals of the induction coil. The coil
generates pulses with a voltage over 1,000 V at moments when the
relay is switched off. In the absence of the neon bulb, the voltage of the
induction coil may even build up to 10,000 V.
5. Under the conditions described in step 4, the glass envelope is heated and
electrified by the discharging current and electrolysis of sodium silicate
takes place inside the glass. Sodium ions in the glass are pulled out by the
strong electric field generated by the coil to the inner glass surface and
reduced back to metal sodium. The process is allowed to proceed for 2
hours.
6. Disconnect the circuit and examine the neon bulb. If the experiment is
successful, a layer of metal sodium, appearing as a shiny grayish-brown
layer, has deposited on the inner glass surface near the negative (glowing)
electrode.
P. 19
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
You may also try this circuit for obtaining the sodium metal in the neon bulb:
Points to note:
The primary coil of the transformer is connected to an a.c. power supply. To obtain
sodium metal from electrolysis of glass, we increase the a.c. voltage of the power
supply (primary) until the half-wave rectified output current reads about 8-10 mA and
let the neon bulb overrun for 12 hours, or until a partially reflecting deposit is formed
on the inner glass wall.
P. 20
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Appendix 3: References
Electrolysis of sodium silicate in glass
Adlam GHJ 1931 The Science Masters’ Book Series 1 Part 1 (London: John
Murray), pp207-217
Mak, Se-yuen 2006 Neon bulbs test photoelectricity, Physics Education (UK),
May 2006, pp213-216
History of photoelectric effect
http://www.colorado.edu/physics/2000/quantumsone/photoelectric.html
http://www.britannica.com/nobel/micro/465_57.html
http://www.galileo.phys.virginia.edu/classes/252/photoelectric_effect.html
UV diode
http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=40230679
http://www.brl.ntt.co.jp/E/activities/file/report99/E/qe/qe2.htm
Work function of Metals
McGraw-Hill Encyclopedia of Science and Technology
Glow discharge of neon bulb
http://en.wikipedia.org/wiki/Neon_lamp
Manufacturing neon bulbs
http://en.wikipedia.org/wiki/Neon_sign
http://www.madehow.com/Volume-2/Neon-Sign.html
P. 21
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Appendix 4: Lesson Plan
Activity 1: Nature of light
Time
Intended Learning Outcomes
 Understand the difference between laws and
theories in science.
70 – 80 min
Teaching Flow
 Teacher introduces the activity by telling how the ancient
scholars thought about the nature of light. (PPT P.1-3)
 Understand the importance of evidence, not the  Teacher introduces the debate between Newton and
status of scientists.
Huygens. Students are told that people in 17th century
knew light can perform reflection and refraction already.
Teacher reviews law of refraction and law of reflection
 Understand that developing scientific
with student. (PPT P.4-7)
knowledge requires hard work building on the
hard work of others.
 Teacher presents how Newton used his “particle theory of
light” to explain the characteristics of light to students.
 Understand that scientific knowledge may be
(PPT P.8-13)
durable but is subject to change. Changes may
sometimes be revolutionary.
 Teacher presents how Huygens used his “wave theory of
light” to explain the characteristics of light to students.
(PPT P.14-18)
Remarks: Click to play the animations.
Materials
Remarks
 Worksheets.
Teachers can flexibly decide
whether the discussion
 PowerPoint File questions in the PowerPoint are
discussed in the form of whole(P.1-41)
class discussion or group
discussions. (Group
discussions are often
productive of deeper learning.)
 Teacher probes the students’ understanding of scientific
laws and theories and points out their misconception. The
correct concepts of scientific laws and theories are
introduced to students. Emphasis is put on clearing the
wrong concept: “A theory will turn into a law.”
(PPT P.19-23)
 Teacher asked students whether they support Newton or
Huygens if they were people living in 17th century.
(PPT P.24)
P. 22
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Activity 1: Nature of light (continued)
Time
Intended Learning Outcomes
Teaching Flow
Materials
 Success of particle theory of light (for a while): Teacher
 Worksheets.
explained the reasons why Newton’s “particle theory of light
“could defeat Huygens’ “wave theory of light” in the 17th
 PowerPoint File
century. The two main reasons were the greater explanatory
(P.1-41)
power of Newton’s theory and the high reputation of
Newton. (PPT P.25-29)
 Teacher summarizes the aspects of nature of science students
have come across from PPT P.1 to P.28. (PPT P.30)
Remarks
Teachers can flexibly decide
whether the discussion
questions in the PowerPoint are
discussed in the form of wholeclass discussion or group
discussions. (Group
discussions are often
productive of deeper learning.)
 Wave theory of light re-examined: Teacher introduces the
Young’s double slit experiment designed by Thomas Young.
The experiment can demonstrate the interference of light.
(PPT P.31-32)
 Particle theory of light refuted wave theory of light again:
Teacher introduces how Albert Einstein demonstrated the
particle nature of light by the photoelectric experiment.
(PPT P.33-37)
 Light is both particles and waves: Teacher tells students
that another scientist Louis de Broglie confirmed light has
both the nature of particles and waves. (PPT P.38-39)
 Teacher summarizes the nature of science aspects students
have come across in the whole story about the nature of light.
(PPT P.40-41)
P. 23
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.
Senior Form Physics – Nature of Light
Activity 2 – 4: Photoelectric effect (Revision and experiments)
P. 24
70-80 min
 History of photoelectric effect.
(Activity 2)
70-80 min
 Generating a current by UV radiation.
(Activity 3)
70-80 min
 Generating a current by visible light and
determination of Planck’s constant.
(Activity 4)
 Teacher can refer to the procedures described in the
worksheets to demonstrate the experiments in class.
 Worksheets.
 Experiment
apparatus and
materials.
© 2007 The University of Hong Kong – Faculty of Education. All rights reserved.