Transcript Dual Nature of Light - Red Hook Central School Dst
Dual Nature of Light
Is light a wave or a particle?
Wave Properties
• • •
-Diffraction -Interference -Polarization
Diffraction
Constructive & Destructive Interference
Polarization
Energy
Wave E increases with A 2 /I 2 .
Studies of Wave E
• •
Planck – color (f,
l
) vs. T.
As T inc. , f inc, (
l
decr)
Radiation & Temperature Hot Objects Emit Waves
Problem
:
Classical physics could not accurately predict f vs. Temperature
Max Planck related f to T. Light (EM) E, is quantized--it can only take on certain whole number values. E comes in little "chunks" of f x a constant now called Planck's constant, h:
• • • •
EM radiation waves chunks quanta photons
Can calculate E in EM waves units quanta or photons based on frequency.
E = hf.
h is Plank’s constant 6.63 x 10 -34 Js.
E is energy in Joules f is frequency of radiation
Show that if E = hf, E = hc.
l • • • • • •
For waves, v = f
l
. Rearrange f = v/
l
.
Vacuum/air EM v = c
(3 x 10 8 m/s).
f = c/
l
.
E = hf f = c/
l
.
E = hc.
l
Ex 1. Each photon of a certain color light has an energy of 2.5 eV. What is the frequency of and color of the light?
Solution: E = hf f = E/h convert eV to Joules.
(2.5 eV)(1.6 x 10 -19 J/eV) = 6.03 x 10 14 6.626 x 10 -34 J s Hz Green Light
2. The energy of a certain photon is 2.9 eV. What type of wave is it? Be specific.
•
2.9 eV x 1.6 x 10 -19 J = 4.64 x 10 -19 J. eV E = hf (4.64 x 10 -19 J) = (6.63 x 10 -34 Js) f f = 7 x 10 14 Hz Violet Light
Finish Ex Sheet
•
Hwk : Text Read 830 – 833 Do pg 833 #1-4 and 839 #2, & 856 # 2-4, 9.
Do Now.
• •
Plank’s Formula Sheet from yesterday.
Solve problem 2. Show work
Do Now: A photon of light has energy = 2.072 eV. What color is it?
• • • • • •
2.072 eV (1.6 x 10 3.3152 x 10 -19 E = hf.
(3.3152 x 10 -19 J) = (6.63 x 10 -34 f = 5.00 x 10 14 Orange J Hz.
-19 J/eV).
Js)f
So Energy of EM Radiation comes in chunks, E = hf, maybe it’s not waves.
•
All objects above 0 K radiate EM waves as E.
•
Hotter = more total E = higher freq. (different color)
•
Energy quantized, E = hf (J).
Other evidence:
• Photoelectric effect.
http://phet.colorado.edu/en/simulation/photoelectric
More evidence for E particles Photoelectric Effect When EM waves shine on a metal surface, the E in wave may be absorbed by e- in metal. It may have enough E to kick out surface e-. Materials that emit e- are photoemissive. The ejected e- are called photoelectrons.
Phet observations
• • • • •
Use higher Amplitude/Intensity/brightness – more e- fly off w same speed. Current increases (A, C/s) Increased f e- fly off faster w higher KE.
http://phet.colorado.edu/en/simulation/photoelectric
Classical Mechanics cannot explain why increasing A or exposure time does not increase photoemission. After all:
•
Boat would be tossed higher & faster with increased wave amplitude.
•
But ejected e- not faster.
Einstein: EM wave E is quantized– photons.
•
The collision of a photon with e- causes e- ejection.
•
Increasing f, increases E (p) of each photon, so photoelectron has more KE (faster)
•
Increasing Intensity (A) increases number of photons hitting more e- so more fly out – higher current!
• •
Envision EM as little chucks. High f are heavier.
http://phet.colorado.edu/en/simulation/photoelectric
Photoemission only works with metals with weakly bound e-.
Photo-emissive metals have:
• •
Threshold Frequency f
o
.
Work Function, W
o
.
Threshold frequency f
o
an e-.
= lowest f that will free Light frequencies below the f
o
eject no e-, no matter how intense or bright the light.
Light frequencies above the f
o
eject e-, no matter how low the A (how dim).
A metal has a threshold frequency f
o
blue light range.
in the 1. What will happen if very bright red light is shone upon the metal?
a) No e- will be emitted b) more e- will be emitted c) The emitted e- will have less energy.
High f vs. Low f.
Einstein confirmed EM waves/photons have E =hf. Very high f give e- more KE. e- flies out faster.
*2. A metal has a threshold frequency f o the blue light range.
in Predict what will happen to e- if UV light is shone upon the metal?
a) nothing b) the emitted e- will have more energy (KE) c) more e- will be emitted with the same energy.
Increasing the I/A/brightness, increases the number of photons increases rate of e- emission - the current; more e- ejected, but each e- won’t gain any extra E/speed.
A metal has a threshold frequency in the blue light range.
3. What will happen to photo e- if the blue light is made twice as bright?
a) nothing b) the emitted e- will have more energy (KE) c) more e- will be emitted with the same energy.
Energy & Frequency
EM waves can be described as quanta or photons. The E carried by photons is: E photon = h
f
or E photon = hc/ l .
(for photon traveling at speed of light).
This E can be absorbed by photo-emissive materials.
The min. frequency to free e- is f o .
The min energy needed to free an e is called work function W o , or
F
.
Metals have low W o .
W
o
= hf
o
.
If photon f is higher than f
o
.
•
E photon greater than W o .
•
Any photon E left over after the work function, goes into KE of e-.
4. A certain metal has a work function (W o ) of 1.7 eV. If photons of energy 3.0 eV are absorbed by the metal:
•
a) No e- will be emitted at that energy.
•
b) More e- will be emitted than would be at the W o .
•
c) Higher KE e- will be emitted than would be at W o .
•
Classical (wave) vs. Modern (particle) theory different predictions Wave
•
Photon Theory
•
Metal needs time to absorb energy (like boiling water on a stove), eventually e- will be ejected.
•
Higher amplitude/intensity waves (brighter), will give photo e- more E.
• • •
Photons are particles that collide with e- so no time needed for e- to absorb E.
High f photons have more E, ejected e- come out faster – more KE.
High amplitude/brighter = more photons of EM so can eject more e- but with same E.
• • • • • • • • • •
Experiment –
f
o W o Summary: EM waves as chunks of energy/photons travel at c.
Calculate the Energy J E = hc/
l
.
Evidence for photons – from Photoelectric Effect f not A responsible for KE of ejected e-. High f = high E, photon.
E = hf, or High A = high number of photons.
Photo-emissive materials have: = min f to eject e- (Hz) = min E to eject e- (J)
Read Txt 834-837 Photoelectric Effect Questions
Graph of Photoelectric Experiment
• KE of photoelectron vs. frequency.
max KE of photo e- vs. f for metal. As f of EM wave increases, KE increases, slope = h.
F
(work function), is minimum energy needed to eject e-.
Work function
State the work function & threshold frequency of this metal eV 0.4
0.0
0.4
0.8
1.0
2 3 4 5 x 10 14 Hz 6 7 8
5. A particular metal has a threshold frequency f o , of 5 x 10 14 Hz. What is its work function W o in J & eV?
W o = hf o .
3.3 x 10 -19 J 2.07 eV
•
E
photon
= hf is the total E available.
•
Absorbed photon E splits between W o e-, so total E of absorbed by e- is: & KE photo
•
E
pho
= W o + KE.
• •
The maximum KE of ejected e- is: KE
elc
= E
pho
– W o .
•
Don’t forget W o = hf o .
6: Photoelectric Effect: Light having f = 1 x 10 15 hz shines on a sodium surface. The photoelectrons have a maximum KE of 3 x 10 -19 J.
Find the threshold frequency for sodium.
Photon Photoelectron.
E tot = W o + KE.
E
tot
– KE = W o .
hf – KE = hf
o
.
f
o = (hf
photon
(h) – KE
max
)
change eV to Joules: (1.86 eV) (1.6 x 10 -19 J/eV) = 2.85 x 10 -19 J
f
o = (hf
photon
– KE
max
)/(h) (6.63 x 10 -34 Js)(1 x 10 15 (6.63 x 10 -34 Js) hz) - (2.85 x 10 -19 J) f o = 5.5 x 10 14 Hz.
Below this frequency no electrons will be ejected.
In 1913-1914, R.A. Millikan did a series of extremely careful experiments involving the photoelectric effect. He found that all of his results agreed exactly with Einstein's predictions about photons, not with the wave theory. Einstein actually won the Nobel Prize for his work on the photoelectric effect, not for his more famous theory of relativity.
Some experimental results, like this one, seem to prove that light consists of particles; others insist, that it's waves. We can only conclude that light is somehow both a wave and a particle--or that it's something else we can't quite visualize, which appears to us as one or the other depending on how we look at it.
Reg Hwk Intro Photoelectric Effect Prac Packet
•
Hwk Text 834 – 837
•
Finish photo elec packet
•
Do Regents Packet
Light Fantastic BBC part 3 58 min
• http://www.youtube.com/watch?v=VuGjo9 oNqao
•
Review of photoelec w german accent 4.11
http://www.youtube.com/watch?v=GpcWc5 KLVRo • • Photoelectric Effect Explained 6 min http://www.youtube.com/watch?v=0qKrOF -gJZ4
Particle Properties of Waves extend to conservation of energy and momentum.
Photons may give up all or part of their energy in collisions, but the sum of the momentums and energy before must equal the sum after.
Compton Effect
If light behaves like a particle, then a collision btw photon & e- should be similar to billiard balls colliding. Photons must have momentum (p), & energy. In collision of photons with particles (like e-), conservation of energy & conservation of momentum apply.
If the photon gives only part of its energy & momentum to an e-, its momentum decreases after the collision by the same amount as absorbed by the electron. Therefore, the frequency or energy of the photon decreases. The wavelength increases.
p before = p after .
E photon before hf
i
= KE = KE elc after elc after + hf . + E photon after f photon after
p
photon
= hf/c = h/
l
. The wavelength of the photon increases after collision.
Matter has wave-like properties.
1924 Louis DeBroglie suggested that since waves had particle properties, matter might have wave properties.
It turns out that matter does have wave properties which are inversely related to the momentum of the particle.
For matter:
l
=h/p
l
= h/mv. or Since the mass of most objects is so large, the wavelengths would be very small & not measurable.
Electrons, however, do show diffraction & other wave characteristics.