lecture 16: electromanetic radiation reading: zumdahl 12.1, 12.2 outline –the nature of...
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Lecture 16: Electromanetic
Radiation• Reading: Zumdahl 12.1, 12.2
• Outline– The nature of electromagnetic radiation.
– Light as energy.– The workfunction of metals.
Electromagnetic Radiation
• Electromagnetic radiation or “light” is a form of energy.
• Characterized by:–Wavelength ()–Amplitude (A)
• Has both electric (E) and magnetic (H) components.
Electromagnetic Radiation (cont.)
QuickTime™ and aCinepak Codec by Radius decompressorare needed to see this picture.
Electromagnetic Radiation (cont.)
• Wavelength (): The distance between two consecutive peaks in the wave.
Increasing Wavelength
1 > 2 > 3
Unit: length (m)
Electromagnetic Radiation (cont.)
• Frequency (): The number of waves (or cycles) that pass a given point in space per second.
Decreasing Frequency
1 < 2 < 3
Units: 1/time (1/sec)
Electromagnetic Radiation (cont.)
• The product of wavelength () and frequency () is a constant.
()() = c
Speed of light
c = 3 x 108 m/s
Electromagnetic Radiation (cont.)
• We classify electromagnetic radiation by wavelength.
• Visible radiation takes up only a small part of the electromagnetic spectrum.
Light as Energy
• Before 1900, it was assumed that energy and matter were not the same.
• The interaction of light with matter was one of the first examples where the separation of energy and matter fell apart.
Light as Energy (cont.)
• Planck’s experiments on light emitted from a solid heated to “incandescence”.
As body is heated, intensity increases, and peak wavelength shifts to smaller wavelengths.
Can “classical” physics reproduce this observation?
Light as Energy (cont.)
• Comparison of experiment to the “classical” prediction:
Classical prediction isfor significantly higherintensity as smaller wavelengths than what is observed.
“The Ultraviolet Catastrophe”
Light as Energy (cont.)
• Planck found that in order to model this behavior, one has to envision that energy (in the form of light) is lost in integer values according to:
E = nh
Energy Change
n = 1, 2, 3 (integers)
frequency
h = Planck’s constant = 6.626 x 10-34 J.s
Light as Energy (cont.)
• In general the relationship between frequency and “photon” energy is
Ephoton = h
• Example: What is the energy of a 500 nm photon?
= c/ = (3x108 m/s)/(5.0 x 10-7 m)
= 6 x 1014 1/s
E = h =(6.626 x 10-34 J.s)(6 x 1014 1/s) = 4 x 10-19 J
Waves vs. Particles• We began our discussion by defining light in terms of wave-like properties.
• But Planck’s relationships suggest that light can be thought of as a series of energy “packets” or photons.
The Photoelectric Effect
• Shine light on a metal and observe electrons that are released.
• Find that one needs a minimum amount of photon energy to see electrons (“o”).
• Also find that for ≥ o, number of electrons increases linearly with light intensity .
metal
The Photoelectric Effect (cont.)
• Finally, notice that as frequency of incident light is increased, kinetic energy of emitted e-
increases linearly.
€
1
2meν
2 = hν photon −Φ
= energy needed to release e-
• Light apparently behaves as a particle.
00
(Frequency )
The Photoelectric Effect (cont.)
• For Na with = 4.4 x 10-19 J, what wavelength corresponds to o?
€
1
2meν
2 = hν photon −Φ0
h = = 4.4 x 10-19 J
hc = 4.4 x 10-19 J
€
=hc
4.4x10−19J=6.626x10−34 J.s( ) 3x10
8m /s( )
4.4x10−19J( )
= 4.52 x 10-7 m = 452 nm
00
(Frequency )
Interference of Light
• Shine light through a crystal and look at pattern of scattering.
• Diffraction can only be explained by treating light as a wave instead of a particle.
Summary• We have seen experimental examples where
light behaves both as a particle and as a wave.• This is referred to as “wave-particle” duality.
• Wave-particle duality is not limited to light! All matter demonstrates this behavior.
• Need something more than classical physics to describe such behavior….quantum mechanics!