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Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of...

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Lecture 15: Electromagnetic Radiation • Reading: Zumdahl 12.1, 12.2 • Outline – The nature of electromagnetic radiation. – Light as energy. – The workfunction of metals.
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Page 1: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

Lecture 15: Electromagnetic Radiation

• Reading: Zumdahl 12.1, 12.2

• Outline– The nature of electromagnetic radiation.– Light as energy.– The workfunction of metals.

Page 2: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

Electromagnetic Radiation• Electromagnetic radiation or “light” is a

form of energy.

• Characterized by:–Wavelength ()–Amplitude (A)

• Has both electric (E) and magnetic (H) components.

Page 3: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

Electromagnetic Radiation (cont.)

QuickTime™ and aCinepak Codec by Radius decompressorare needed to see this picture.

Page 4: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

Electromagnetic Radiation (cont.)

• Wavelength (): The distance between two consecutive peaks in the wave.

Increasing Wavelength

1 > 2 > 3

Unit: length (m)

Page 5: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

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)

Page 6: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

Electromagnetic Radiation (cont.)

• The product of wavelength () and frequency () is a constant.

()() = c

Speed of light

c = 3 x 108 m/s

Page 7: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

Electromagnetic Radiation (cont.)

• We classify electromagnetic radiation by wavelength.

• Visible radiation takes up only a small part of the electromagnetic spectrum.

Page 8: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

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.

Page 9: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

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.

Page 10: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

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?

Page 11: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

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”

Page 12: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

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

Page 13: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

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

Page 14: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

Which type of photon will have the largest energy?

A. Ultraviolet

B. X-Ray

C. Microwave

D. Visible

Page 15: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

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.

Page 16: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

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

Page 17: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

The Photoelectric Effect (cont.)

Page 18: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

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 )

Page 19: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

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( ) 3x108m /s( )

4.4x10−19J( )

= 4.52 x 10-7 m = 452 nm

00

(Frequency )

Page 20: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

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

Page 21: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

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.

Page 22: Lecture 15: Electromagnetic Radiation Reading: Zumdahl 12.1, 12.2 Outline –The nature of electromagnetic radiation. –Light as energy. –The workfunction.

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!


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