Download PHYSICS CHAPTER 12 NOTES THE NATURE OF LIGHT

PHYSICS CHAPTER 12 NOTES THE NATURE OF LIGHT
THE PARTICLE-WAVE DUALITY OF LIGHT
1.
The Corpuscular Theory-- (particle)--this theory supported by Newton & Einstein.
a.
Rectilinear Propagation--a ball thrown into space follows a curved path, but this path becomes straighter as the
velocity of the ball increases.
b.
Reflection--is easily explainable by a ball bouncing on the floor.
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R
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c.
2.
Refraction--can't be explained by the particle theory because as a ball goes down an incline its path is bent toward
the normal as it should be but the velocity of the ball increases instead of decreases as we know it should. (Light
moves slower in dense medium, refraction towards the normal.
The Wave Theory--supported by Huygens & Young.
a.
Huygens Principle--each point on a wave front may be regarded as a new source of disturbance.
b.
The wave theory treats light as a train of waves having the wave fronts perpendicular to the paths of the light rays.
This method can easily explain the reflection and refraction but has trouble explaining rectilinear propagation.
THE ELECTROMAGNETIC THEORY FOLLOWED BY THE QUANTUM THEORY
1.
The Electromagnetic Theory--deals with the propagation of heat, light and electrical energy in free space. This theory fails in
the second and third law of photoelectric emission.
a.
Electromagnetic Wave--is a periodic disturbance involving electric and magnetic forces. The energy of
electromagnetic waves is equally divided between and electric field and a magnetic field, each perpendicular to the
other, and both perpendicular to the propagation of the wave.
b.
Electromagnetic Energy--can be converted to other energy forms such as thermal, electric, kinetic, potential,
chemical.
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c.
All electromagnetic waves travel at the speed of light (c) or 3 X 10 m/s. The wavelength ranges from 3 X 10 to 3
-17
X 10 meters.
-10
d.
Angstrom = 10 meters
e.
The Photoelectric Effect--is the emission of electrons by a substance when illuminated by electromagnetic radiation.
1.
Photoelectrons--are the electrons that are ejected form the metal surface when illuminated by the electromagnetic
spectrum.
2.
The First Law of Photoelectric Emission--The rate of emission of photoelectrons is directly proportional to the
intensity of the incident light.
3.
The cut-off potential--as the collector plate is made slightly negative with respect to the emitter plate, this will
regulate the amount of emission from the emitter plate. (Explain This)
a.
The cut-off potential depends only on the frequency of the incident light.
4.
As cut-off is approached the maximum kinetic energy of photoelectrons can be measured because only the highest
KE electrons will be allowed to escape. At cut-off even these will be stopped.
a.
Maximum kinetic energy of photoelectrons increases with the frequency of the light illuminating the
emitter.
5.
The Second Law of Photoelectric Emission--The KE of photoelectrons is independent of the intensity of the incident
light.
6.
The Third Law of Photoelectric Emission--The maximum kinetic energy of photoelectrons varies directly with the
difference between the frequency of the incident light and the cut-off frequency.
2.
The Quantum Theory--assumes that the transfer of energy between light radiation and matter occurs in discrete units called
quanta, (photons) the magnitude of which depends on the frequency of the radiation.
A.
Energy emitted by the radiating source to be an integral multiple of a fundamental quantity hf. Where h is known as
Planks Constant, and f is the frequency of radiation.
E=h f
where h = planks constant=6.63 × 10−34 Js
f = frequency
Maximum kinetic energy possessed by ejected photoelectron
1
2
mV 2 = h f − w
Where KE = 12 mV 2 w = work function of material
E = h f cut off = w
MODERN VIEW OF THE NATURE OF LIGHT RECOGNIZES THIS DUAL CHARACTER: RADIANT ENERGY IS
TRANSPORTED IN PHOTONS THAT ARE GUIDED ALONG THEIR PATH BY A WAVE FIELD.
PHYSICS CHAPTER 12 NOTES THE NATURE OF LIGHT
QUANTUM MECHANICAL MODEL
1.
Energy in the form of photons excites electrons upon absorption. The electron jumps to a higher energy orbit; this is called a
quantum leap. When the electron becomes unstable in this orbit it is said to fall back to the original orbit and emitting a precise
amount of energy photon. In this way elements have specific photon emissions (due to the fact that each element has a specific Bohr
model) which are a precise signature of that element. This signature is known as the bright line spectra.
a.
The uncertainty principal--there is some uncertainty to where exactly the electron is in relation to the nucleus of the
atom and the precise amount of momentum it contains.
\ h f = mc2
E = h f & E = mc2
hf
= mc
Where c = V = f l
c
hf
h
h
= mc fi
= mc fi l =
fl
mc
l
Where momentum = P = mV
b.
devide both sides by c
fi
l=
h
mV
The higher the frequency of the electric radiation, the more energy is transferred by the photons, therefore the
particles of matter associated with photon units of energy have more momentum.
f = mV and P = mV
COHERENT LIGHT
1.
Incoherent--waves differ in frequency or phase or both.
a.
Polychromatic--light that contains many frequencies or colors. (White light).
2.
Coherent--waves that have the same frequencies and a constant phase relationship.
a.
Monochromatic--light that contains one frequency or color. (Laser light)
hf
Momentum of photon ⇒ P = m c =
THE PRESSURE OF LIGHT
λ
ILLUMINATION
Photon Energy ( h f ) , if it is totally reflected the pressure or change in
1.
The energy of thermal radiation emitted by hot
2h f
bodies depends upon the temperature of the body and the
momentum should be ⇒ 2m c =
c
nature of its surface.
2.
Luminous--is an object that gives off light because of the energy of its accelerated particles.
3.
Light energy hitting a medium can a. Be reflected, transmitted or absorbed
4.
Transparent--are substances that readily transmit light.
5.
Translucent--are substances that transmit light but scatter or diffuse it so objects seen through them cannot be identified.
6.
Opaque--are substances that do not transmit light at all.
7.
Ray--a single line of light from a luminous point.
8.
Beam--is a group of closely space rays. Small beams are referred to as pencils.
9.
Diverging Light--(spreading out) when several rays of light come form a point.
10.
Converging Light--(coming together) when several rays of light proceed toward a point.
11.
Focus--the point at which the light comes together. Focal Point is the point at which the light through a lens comes together.
12.
Photometry--is the quantitative study of light.
a.
Luminous Intensity-- (I) Unit is the candle.
1.
Candle--is 1/16 of the luminous intensity of a square centimeter of a black-body radiator maintained at the
temperature of freezing platinum.
40 watt lamp----35 candles
100 watt lamp---130 candles
2.
Luminous Flux--the rate of flow of visible radiation. Luminous flux is represented by the Greek letter Phi.
The unit of luminous flux is the lumen (lm).
a.
Lumen--is the luminous flux on a unit surface all points of which are a unit distance from a point
source of one candle. It is a rate at which luminous energy is being emitted, transmitted, or
received.
3.
Illumination-- (E)--is the density of the luminous flux on a surface. Illumination (e) is in lumens per square
meter.
a.
The inverse square law--is used to calculate the illumination form an individual point source. The
illumination decreases as the square of the distance from the source.
b.
The illumination of a surface varies inversely with the square of the distance form the luminous
source and directly with the cosine of the angle between the luminous flux and the normal to the
surface.