Download Physics Laboratory Last update: 2003.5.26 Experiment 8. Where

Physics Laboratory
Last update: 2003.5.26
Experiment 8. Where there is light
Caution for TA
Purpose of Experiment
Light is a kind of electromagnetic wave. This is expected by Maxwell’s theory of
electromagnetism and verified by Hertz’s electromagnetic wave generation experiment.
Therefore as a kind of electromagnetic wave, light is a wave so that the shape of electric field(or
magnetic field)’s oscillation propagates as time passes. But light(electromagnetic wave) is a
peculiar wave in that it does not need medium. In other words, it propagate through free space
where no materials exist. The sunlight’s arrival to the earth is a good example.
-From E. Hecht’s general physics book
Solar energy that is vital for life activity is transferred as a form of electromagnetic waves. The
picture is the sun’s shape pictured in short wavelength of X-ray. From this, the range of
electromagnetic waves from the sun is wide so that it contains not only ultraviolet, visible light,
infrared, but also X-ray ranges. Recently, from the ultraviolet light detecting using space probes,
it is known that the sun’s activity is very active and irregular. For humankind, the sun is a very
mysterious object as the origin of life is mysterious.
Light propagate through medium too. But in the interface between the two different media, the
direction of propagation of light changes, which is called reflection and refraction of light. In the
reflection and refraction of light, snell’s law can be applied, and these also can be explained by
Fermat’s principle which means that light take the path that takes least time to propagate
between the two points. And electromagnetic wave is transverse wave. Ie, the direction of
oscillation is perpendicular to the direction of propagation. The direction of electric field’s
oscillation is called polarization. In this experiment, reflection, refraction, polarization of light is
the main subjects. Also, especially, total reflection and Brewster’s angle too are subjects those
are applicable to optical communication and laser.
If the obstacle’s size is similar to the wavelength of light, instead of reflection and refraction, an
unique phenomena of light, diffraction occurs. And the diffracted waves from each obstacles
interference each other. As the intensity of interference of waves is determined by path
difference of the two waves and the ratio of wavelengths, the position of maximum intensity of
interference pattern depends on the wavelength of light. Ie, light can be separated by the
wavelength. A diffraction gating, that uses this characteristic of diffracted waves, is made by
drawing fine lines with same interval on a glass plate or a reflective thin film or by depositing
thin reflective films with same interlayer distance on a glass plate. Using this, we can separate a
monochromatic wave from white light or find out the wavelength of light.
Interference in compact disks – from Halliday & Resnick ‘s general physics book
CD(compact disks)s are good example of diffraction grating. In the case of audio CD, it records
data using well shaped grooves in a spiral track, and the distance between tracks is 1.6 μm, each
groove has 0.2 μm depth and width. Therefore CD is a diffraction grating with 1.6 μm inter-slit
distances. Observe color changes as the angle changes in a CD after exposing it in light. What
can you know from this?
Outline of Experiment
•Investigate how to generate electromagnetic waves, principles of electromagnetic wave
generation, propagation processes and propagation velocity.
• Using a laser pointer as a straight light source, experiment characteristics of reflection,
refraction and polarization of light
-
How can you make experimental set up?
What will you measure?
• Study structures, types, functions of a diffraction grating.
-
What is the most important part in the structure of a diffraction grating?
Find out the relation between structures and properties of a diffraction grating.
• Experiment interference of light using a diffraction grating.
-
Using a laser pointer, find out the propagation properties of light passing through a
diffraction grating.
-
Estimate the interval of the diffraction grating by using diffracted angles and check that it
is coinside with known interval distance.
-
Estimate the wavelength of laser by using interval distance of the diffraction grating.
Other than these, learn about kinds of lasers, emitting mechanism of each diffraction grating,
structures and properties of a photo diode that is used as a photo detector, and a DC amplifier
that amplifies converted small electric signals.
Experimental Methods
These equipments are prepared in the laboratory. (Parentheses mean the number of them.)
A laser source(a pointer) (1)
A photo detector and a DC amplifier (1)
A polarizer(1)
A transparent acrylic rod(1)
A half sphere shape plastic bowl(1)
A polar coordinate graph paper (a little)
A protractor(1)
A diffraction grating 600lines/mm (1)
A safety goggle(1)
Tissue (prepare for yourself)
Compact disk(if possible, prepare for yourself)
If you need more equipments, inquire to your teaching assistant or the experiment preparation
room (19-114), or prepare them yourself. And you need to know about a laser source and a photo
detectors.
Bellows are the recommended and standard experimental methods
1) Investigate properties of light reflecting in the interface of two different media.
[video]
① Point laser source light to a acrylic rod at a incident angle θi.[This experiment can be
conducted in the way that you pour water in a half sphere plastic bowl and measure
the reflection at the plane.]
② Measure the reflected angle θr'of reflected at the surface of the acrylic rod.
③ Repeat this measurement for more than 5 other incident angles θi.
④ Obtain the relation between reflection angle and incident angle from the graph.
2) Investigate the properties of light refracting at the interface of two different media.
[video]
① Pour water in a half sphere shape plastic bowl and place it at the center of a polar
coordinate graph paper.
② Shoot laser source light in the center of plane of the plastic bowl with the incident
angle of θi
③ Measure the refracted angle θr'of light refracted by water in the plastic bowl.[caution:
be careful not to see laser light directly. Especially, when you observe a light rays,
make sure to use safety goggles.
④ Repeat this measurement for more than 5 other incident angles θi.
⑤ Obtain the relation between reflection angle and incident angle from the graph.
⑥ Obtain the refractive index of water [nH2O] .
⑦ Discuss about what the plastic wall can affect the measurement.
3) Observe total reflection of light.
[video]
.
① Pour water in a half sphere shape plastic bowl and place it at the center of a polar
coordinate graph paper.
② Shoot laser source light at the center of plane of the plastic bowl through the curved
surface with the incident angle of θi
③ Measure the refracted angle θr'of light refracted by water at the plane of the plastic
bowl. [caution: be careful not to see laser light directly. Especially, when you observe
a light rays, make sure to use safety goggles.]
④ Repeat this measurement for other incident angles θi to obtain incident angle θc that
make θr' = 90o
⑤ Using the refractive index obtained by 2) compare this with that of calculated by using
theoretical formula 3).
4) Investigate the polarization of light produced by a laser source(a pointer) and the
propagation properties of polarized light passed through a polarizer.
[video]
¨ç
Shoot light from a laser source to a polarizer and detect the intensity of the polarized
light by the photo detector. [caution: be careful not to see laser light directly. Especially,
when you observe a light rays, make sure to use safety goggles.]
② By rotating the polarizer, repeat the measurement for more than 10 other angles.
③ Plot the result and compare it with the angle dependence of polarization in formula 4)
and find out the polarization of the light source.
5) Investigate Brewster’s angle and the polarization of light reflected at Brewster’s angle.
[video]
① Pour water in a half sphere shape plastic bowl and place it at the center of a polar
coordinate graph paper.
② Shoot laser source light at the center of plane of the plastic bowl through the plane of
the bowl with the incident angle of θi This time, using polarization direction
obtained by experiment 4), rotate the laser source so that the polarization of incident
light is parallel to incident plane.
③ Measure reflection angle θr of light reflected by the plane and the refraction angle θr'
of the light refracted by the plane. [caution: be careful not to see laser light directly.
Especially, when you observe a light rays, make sure to use safety goggles.]
④ Repeat this measurement for other incident angles θi to obtain incident angle θB
where the intensity of reflected light is zero.
⑤ Compare the refractive index obtained by 2) with the calculated value from theory
formula 6).
[Caution: make sure to turn off the switch of the photo detector]
6)
By using laser pointer, observe interference of monochromatic waves passed through a
diffraction grating.
① Obtain the angle θ1 where the maximum intensity of order n=1 is occurs and by
using formular 7) and wavelength of the laser λ = 6700 Å, calculate the distance d in
the diffraction grating. [ Caution: Do not see the laser light directly. Be careful not to
shoot laser to other’s eyes. Observe the laser rays in the paper, as the intensity of the
laser reflected by the paper too is strong, be careful not to incident the laser to your
eyes and not to see the pointing point for a long time. The obserber should ware
glasses.] Check that the obtained value is comparable with real distance d. If not,
find out the reasons.
②
Use θ1 of ① and given diffraction grating interval d = (1/600) mm, obtain the
wavelength λ of the laser source.
③
Observe the interference pheonomena of monochromatic waves passed through other
diffraction gratings to calculate the inter window distance of the diffraction grating.
Other than that, if possible, investigate the interference of laser incident to the prepared compact
disk. During the experiment, the observer should wear safety goggles. Estimate the line interval
of the diffraction grating made by a gradation of the tiled ruler, and check that this value is
consistent with the observed interference patterns. Estimate the mean inter groove distance of the
CD. Can these make problems in an audio CD player or a computer CD ROM?
Below is a recommended format for the laboratory report.
(Investigation of characteristics of reflection of light)
Incident angle θi (o)
reflected angle θr (o)
-------------------------------------
------------------------------------<θr/θi>AV =
±
-----------------------------
(investigation of characteristics of refraction of light)
Incident angle θi (o)
refracted angle θr' (o)
-------------------------------------------------------
sinθi/sinθr
------------------------------------------------------Refractive index nH2O =
----------------------------
±
(investigation of characteristics of total reflection of light)
Critical angle θc =
----------------------------
o
±
o
(investigation of characteristics of polarization of light)
Angle of the polarizer θ(o)
voltage of the photo detector V (mV)
----------------------------------------------------
----------------------------------------------------
(investigation of Brewster’s angle)
Brewster’s angle θB =
-------------------------------
o
±
o
(measurement of interference of a diffraction grating using semiconductor laser)
Interwindow distance of the grationg: d = 1/600 mm
---------------------------------Wavelength of the laser : λ = 6600 Å ~ 6800 Å [주]
--------------------------------------[주: The wavelength of a semiconductor laser can be changed due to the difference of
the material used and it’s condition of operation.]
o
o
Diffracted angle θ1 =
±
-------------------------------Interwindow distance of the diffraction grating : d =
-------------------------------Wavelength of the laser : λ =
Å
mm
-------------------------------
Background Theory
The general way of generating electromagnetic waves are
① To flow a AC current in a antenna
② To exelerate(or decelerate) a charged particle
③ To excite an atom or a molercule to transite between energy levels
The way of flowing AC current is mainly used when we want to generate electromagnetic
waves that have longer wavelength than visible light such as radio waves, microwaves.
- from the general physics book of E. Hecht
In the FM radio station antenna on the left or microwave dish shape antenna on the right, AC
current is used to generate electromagnetic waves. The polarization of the ectromagnetic waves
radiated from these antenna is determined by the shape of the antenna.
When a charged particle, an electron for example, moves circular orbits or scattered by some
object, it radiates electromagnetic waves that have short wavelength such as x-rays. The circular
accelerator is, based on this principle, is used to obtain monochromatic electromagnetic waves in
the short wavelength range such as x-ray region. In the cathode ray tube which is used in a
television monitor or a computer monitor, electron flux is accelerated to collide in the screen
where fluorescent materials are painted so that it radiates light. During the collision, x-ray
electromagnetic waves are generated by suddenly decelerated electrons. Though most of them
are absorbed in the material painted in the screen, as considerable amount of electromagnetic
waves can be emitted, it is recommended not to use a TV or a computer monitor too closely. Xray equipments which are used in a hospital to diagnose diseases use this effect.
- from general physics book of E. Hecht
In a x-ray tube an accelerated electron collides to a target object to generate electromagnetic
waves. The spectrum of generated electromagnetic wave is consists of gentle continuous
spectrum and sharp line spectrum. Line spectrum is due to the exciting of target atoms and
continuous spectrum is emitted from the decelerated electron.
Visible lays are generated by using exited states of atoms. First of all, by using mechanical
methods(collision) or electric methods(discharging) or light, apply energy to an atom to be
excited state. The atom in a excited state soon goes back to it’s ground state(least energy state)
and emits light that have the same energy as difference of the excited state and the ground state.
- from general physics book of E. Hecht
The picture on the left shows the generation of light from an atom. (a) An atom excited by
absorbing light or by mechanical(kinetic energy transfer by collision) method (b) emits light with
the energy of the energy difference(c) to go back to it’s ground state. By this mechanism, all
objects emits (or absorb) electromagnetic waves. The picture on the right shows that an object
with the temperature of body emits long wavelength electromagnetic waves(infrared rays)
though it is not visible to the eyes. If you use infrared rays, you can easily detect people or etc
even at night.
Light emitted by the excitation of atoms, in the case that the light source is consists of many
number of atoms such as the sun or a filament of a light bulb, has many kind of colors and not
coherent. Incoherence means that photons are not related each other. By the way, thought a laser
is based on the excitation of atoms, by the method of stimulated emission, it accumulates
monochromatic coherent waves to emit plane waves. By this reason, it is used not only for
experiments in laboratories but for medical, industrial purposes or even for office works, for
example laser pointers, laser printers, Fax, CD.
- from general physics book of E. Hecht
The picture on the left represents induced emission where light emitted by spontaneous emission
induces emission of other excited atoms. The induced light have same frequency and same phase
as that of spontaneously emitted light. In other words, it is coherent light. In a laser, (a) first of
all, excited atoms emit light spontaneously and this light induces other atom’s emission.(b) The
amplitude of this light increases as it travels back and forth repeatedly between two mirrors. (c)
It emits parts of this increased light outside, which is laser rays.
The speed of light is vaccum is c(≒ 3 x 108 m/s) that is constant. In other media, the speed of
light become slower, and the ratio between c and the speed of light in the media v is called the
refractive index of the media. When light propagate and meet a material that has different
refractive index, the reflection or refraction occurs in the interface of the two media. Each of this
has regularity, which is called law of reflection, law of refraction or Snell’s law.
1. Law of reflection
1) The incident light and the reflected light lies in the same plane(incident plane)
2) The incident angle is always the same as reflected angle.
- from general physics book of E. Hecht
2. Law of refraction
1) The incident light and the refracted light lies in the same plane(incident plane)
2) The relation written below is always valid between the incident angle θi and
the refracted angle θr'
relative refraction index n12 = n2/n1 = sinθi/sinθr'
(1)
Where n12 is the relative refraction index when light is incident from medium 1 to
medium 2, and is defined by the ratio of the speed of light in each medium.
- from general physics book of E. Hecht
By the way, there exist a critical (incident) angle that when light propagate from large
refractive index medium to a small refractive index medium, as the incident angle
becomes larger, the incident light can’t penetrate and reflected totally in the interface.
This is called the tatal reflection, and the optical fiber which is the core of modern
express information communication uses this characteristic to prevent the light from
penetrating the surface of the fiber not to lose light flowing inside.
- from general physics book of F. J. Keller
The critical angle θc of total reflection is the incident angle where the refracted angle is 90
o
, from
sin90o/sinθc = n
(2)
the critical angle is
θc = sin-1(1/n)
(3)
.
Like other waves, in the case of light there are oscillating physical quantities as time
varies, these are electric field and magnetic field. In the electromagnetic waves like light,
as the intensity of electric field is much higher than that of magnetic waves and the
electric field and the magnetic field is correlated by Maxwell’s equations, the direction of
electric field’s oscillation is called polarization. Generally speaking, light from light
source can be polarized in a specific direction or not. The former is called polarized light,
and the latter is called unpolarized light. The polarized light can be divided by linearly
polarized and circularly polarized light according to their direction of polarization. The
former has constant direction and the latter has rotating polarization direction. If linear
polarized and circularly polarized light is mixed, it is called elliptically polarized.
Among the transparent materials, due to the electric anisotropy of molecules consisting
the material, there are some materials that passes light polarized in a certain direction and
does not passes light polarized perpendicular to the certain direction. A polarizer uses this
characteristic.
If an unpolarized waves pass through a polarizer, among the all direction of polarization,
the polarization component that is polarized in polarizer direction passes out. If linearly
polarized light with the intensity Io is passed through a polarizer that is rotated an angle θ
with respect to the polarization direction of the incident rays, the intensity of transmitted
light is reduced to
Iθ = Io cos2θ
(4)
,which is explained by the fact that the magnitude of electric field is reduced to the factor
of cosθ and the intensity of the light is proportional to the square of the magnitude of the
electric field. In He-Ne lasers, it uses the characteristic that when light is incident at the
Brewster’s angle by the method of linearly polarizing the light, only the polarization
component that is perpendicular to the plane of incidence is reflected.
- from Halliday & Resnick’s general physics book
This phenomena occurs when the reflected rays are perpendicular to the refracted rays, if
the angle of reflection is θr, the angle of refraction θr', the refractive index of the medium
n, and suppose that the light is incident from air(or vacuum), Brewster’s angle θB is
θB = θr
θr + θr' = 90o
sinθB/sinθr' = n
(5a)
(5b)
so
θB = tan-1n
(6)
A diffraction grating can be thought as small open windows with same interval d(that is
comparable with wavelength of light to make diffraction). When two rays passed through
adjacent windows meet at a point P in a screen, the case that the two rays interfere to
make the maximum intensity occurs when the path difference is multiple of the
wavelength.
Ie, the condition of maximum intensity is
dsinθn = nλ
(n = 1,2,3,...)
(7)
Therefore, the maximum intensity occurs outer position(ie, high θn ) as the wavelength of
the rays is increases and if white rays that include many different monochromatic waves
passes through the grating, a color spectrum occurs with respect to the wavelengths. If the
wavelength is not long, the maximum intensity of high n(n>1) can occur.
References
• laser source and a photodetector
• Analysis method of measured data
• Analysis method via graph
• Christiaan Huygens- a pioneer of waves who invented pendulum clock
• Thomas Young – interference of light(waves)
• Carl Friedrich Gauss-A genius who counted numbers before he learn how to say.
• Niels Bohr – a hero of atomic model who explained line spectrum