Lecture 16: Electromanetic Radiation

1. Lecture 16: Electromanetic Radiation • Reading: Zumdahl 12.1, 12.2 • Outline • The nature of electromagnetic radiation. • Light as energy. • The workfunction of metals.

2. Electromagnetic Radiation • Electromagnetic radiation or “light” is a form of energy. • Has both electric (E) and magnetic (H) components. • Characterized by: • Wavelength (l) • Amplitude (A)

3. Electromagnetic Radiation (cont.)

4. Electromagnetic Radiation (cont.) • Wavelength (l): The distance between two consecutive peaks in the wave. Increasing Wavelength l1 > l2 > l3 Unit: length (m)

5. Electromagnetic Radiation (cont.) • Frequency (n): The number of waves (or cycles) that pass a given point in space per second. Decreasing Frequency n1 < n2 < n3 Units: 1/time (1/sec)

6. Electromagnetic Radiation (cont.) • The product of wavelength (l) and frequency (n) is a constant. (l)(n) = c Speed of light c = 3 x 108 m/s

7. Electromagnetic Radiation (cont.) • We classify electromagnetic radiation by wavelength. • Visible radiation takes up only a small part of the electromagnetic spectrum.

8. 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.

9. 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?

10. Light as Energy (cont.) • Comparison of experiment to the “classical” prediction: Classical prediction is for significantly higher intensity as smaller wavelengths than what is observed. “The Ultraviolet Catastrophe”

11. 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: DE = nhn frequency Energy Change n = 1, 2, 3 (integers) h = Planck’s constant = 6.626 x 10-34 J.s

12. Light as Energy (cont.) • In general the relationship between frequency and “photon” energy is Ephoton = hn • Example: What is the energy of a 500 nm photon? n = c/l = (3x108 m/s)/(5.0 x 10-7 m) n = 6 x 1014 1/s E = h n =(6.626 x 10-34 J.s)(6 x 1014 1/s) = 4 x 10-19 J

13. 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.

14. 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 (“no”). • Also find that for n ≥ no, number of electrons increases linearly with light intensity .

15. The Photoelectric Effect (cont.)

16. The Photoelectric Effect (cont.) • Finally, notice that as frequency of incident light is increased, kinetic energy of emitted e- increases linearly. F = energy needed to release e- • Light apparently behaves as a particle.

17. The Photoelectric Effect (cont.) • For Na with F = 4.4 x 10-19 J, what wavelength corresponds to no? 0 hn = F = 4.4 x 10-19 J hc/l = 4.4 x 10-19 J l = 4.52 x 10-7 m = 452 nm

18. 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.

19. 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!