1. The Wave Nature of Light
AP Physics
Chapter 24

2. The Wave Nature of Light
24.1 Waves Versus Particles

3. 24.1 Waves Versus Particles
Huygen’s principle – every point on a wave front can be considered as a source of tiny wavelets that spread out in the forward direction at the speed of the wave itself.
24.1

4. 24.1 Waves Versus Particles
Particularly useful for analyzing when a wave hits an obstacle
For example reflection and refraction
Or diffraction – the bending
of waves behind
obstacles
Diffraction
24.1

5. 24.1 Waves Versus Particles
Ray model of light can not account for diffraction
24.1

6. The Wave Nature of Light
24.3 Interference – Young’s Double-Slit Experiment

7. 24.3 Young’s Double-Slit Experiment
Thomas Young (1801)
Double Slit Experiment
Light from a single
source (sun)
Sent through a
thin slit first (so
one point source
of light
3. Allowed to pass through two small slits
24.3

8. 24.3 Young’s Double-Slit Experiment
If light is a ray we would expect two slits of light
The actual pattern looked like this
Caused by constructive and
destructive interference
24.3

9. 24.3 Young’s Double-Slit Experiment
The geometry of the
path of two rays of
light
Constructive interference occurs when the path difference, equals a whole number of wavelengths Or
24.3

10. 24.3 Young’s Double-Slit Experiment
For destructive interference the difference in distance has to be by some half wavelength So
m is called the order
Constructive interference – bright fringe
Destructive interference – dark fringe
24.3

11. 24.3 Young’s Double-Slit Experiment
The distance between slits will depend on how far the screen is from the slits
24.3

12. S-106
A monochromatic light with a frequency of 5.35x1014 Hz is shot through a set of double slits that are m apart and the resulting interference pattern is placed on a screen 1.5 m away. What will be the angle of the 3rd order maxima?

13. S-107
A monochromatic light with a frequency of 5.35x1014 Hz is shot through a set of double slits that are m apart and the resulting interference pattern is placed on a screen 1.5 m away. What is the minimum path difference between two waves that will allow a dark fringe to be produced?

14. 24.3 Young’s Double-Slit Experiment
Brightness fades as distance increases from the 0th order maxima (central maxima)
Applet
24.3

15. 24.3 Young’s Double-Slit Experiment
If white light is incident on slits, the central maxima is white, but rainbows occur at higher order maxima
Because different colors have different wavelengths
24.3

16. The Wave Nature of Light
24.4 The Visible Spectrum and Dispersion

17. 24.4 The Visible Spectrum and Dispersion
Ranges from 750 nm (red) to 400 nm (violet)
Dispersion - the spreading of white light into a spectrum
24.4

18. 24.4 The Visible Spectrum and Dispersion
Prism
The index of refraction for each frequency is slightly different, so the angle of refraction differs
24.4

19. 24.4 The Visible Spectrum and Dispersion
Rainbow
Caused by refraction
and internal
reflection
24.4

20. The Wave Nature of Light
24.5 Diffraction by a Single Slit of Disk

21. 24.5 Diffraction by a Single Slit of Disk
If light is a wave, then light from a point source will diffract around a disk and create a constructive interference point in the middle of the shadow
A single slit will produce a pattern for the same reason
Applet
24.5

22. The Wave Nature of Light
24.6 Diffraction Grating

23. Diffraction Grating – a large number of equally spaced parallel slits
The maxima are sharper
and brighter for a
diffraction grating
Equations are the same
24.6

24. S-109
A rabid raccoon shoots a laser through a diffraction grating. The frequency of the light is 4.11x1014 Hz. The result is a interference pattern on a wall 2.5 m from the diffraction grating. If the 1st order minima is located 12.4 cm above the center, how many slits per cm are on the diffraction grating?

25. The Wave Nature of Light
24.8 Interference by Thin Films

26. 24.8 Interference by Thin Films
Occur when films are just a few wavelengths thick
Soap Bubbles
Oil
24.8

27. 24.8 Interference by Thin Films
Light strikes a surface
Some of the light
reflects
refracts, then
reflects off the other boundary
If the path difference ABC = nl, then constructive interference
If the path difference ABC = (n+½)l, the destructive interference
24.8

28. 24.8 Interference by Thin Films
The wavelength of light in a medium is given as
24.8

29. The Wave Nature of Light
24.10 Polarization

30. Light is Polarized when it only vibrates in one plane
24.10 Polarization
Light is Polarized when it only vibrates in one plane
Demo
24.10

31. S-110
Light with a wavelength of 482 nm is incident on a thin layer of oil (n=1.45). The light strikes at an angle of 38o from the normal. What is the minimum thickness of oil that would cause an interference pattern to form?

32. S-112
Light strikes a block of plastic (n=1.4) at an angle of 28o to the normal. Using the diagram below calculate the angle that it will exit the block.
random walrus
q=42o

33. S-108
Happy Test Day!