Lecture 15: Electromagnetic 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.)

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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.

What statement is true when comparing red light to blue light?

A. Red light travels at a greater speed than blue light.

B. Blue light travels at greater speed than red light.

C. The wavelength of blue light is longer.

D. The wavelength of red light is longer.

Light as Energy

• For times <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

Which type of photon will have the largest energy?

A. Ultraviolet

B. X-Ray

C. Microwave

D. Visible

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.)

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

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=hc

4.4x10−19J=

6.626x10−34 J.s( ) 3x108m /s( )

4.4x10−19J( )

= 4.52 x 10-7 m = 452 nm

00

(Frequency )

In a workfunction experiment using 300 nm light, the electrons ejected from Potassium (K) have a greater velocity that those ejected from Sodium (Na). Therefore:

A. Na > K

B. K > Na

C. K = Na

D. K = 0

metal

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!