1. “Ray Optics: Reflection, Refraction, Polarization”
Physics 117B02 — March 19
“Ray Optics: Reflection, Refraction, Polarization”
9/19/2018
PHYS117B.02 JV

2. Previous 2 lectures: Chapter 32
Major theme: electromagnetic waves
Predicted by Maxwell, verified by Hertz
Radiation from accelerated charges (e.g., dipoles)
Harmonic (sinusoidal) disturbance of E and B fields
Travel with speed c in vacuum, c/n in matter
in vacuum, in matter
EM waves transport energy and momentum
Poynting vector gives power/unit area
Applications
Communications, solar heating, laser cutting, welding, surgery
Radiation pressure (e.g., solar “wind”)
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PHYS117B.02 JV

3. Today’s lecture: Chapter 33
Major themes: reflection, refraction, polarization
Rays and wavefronts: Huygens’ principle
Reflection and refraction from interfaces
Dispersion and total internal reflection (rainbows)
Polarization effects due to reflection and scattering
Scattering
Applications
Why objects look shifted under water
Telecommunication through optical fibers
Medical endoscopes and fiberscopes
What causes rainbows
How do polarization glasses work
How to make 3d movies using polarization of light
Why is the sky blue and the sunset - red
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PHYS117B.02 JV

4. Terminology Light rays travel perpendicular to phase fronts
Each point on wave front acts as source of secondary spherical or circular “wavelets” … and
Phase fronts are the tangents to those spheres or circles (Huygens’ principle)
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PHYS117B.02 JV

5. Limits of ray optics
When observing EM waves at great distances from their source, it is useful to assume that light travels in a straight line (perpendicular to the wavefronts) unless it encounters an obstacle.
9/19/2018
PHYS117B.02 JV

6. Specular vs diffuse
Light incident on smooth reflecting surfaces is reflected at a specular angle; if the surface is rough, the light scatters in many directions from the asperities in the surface, and there is both specular and diffuse reflection.
9/19/2018
PHYS117B.02 JV

7. Fermat’s principle: reflection
Fermat’s idea was that light follows the geometrical path that minimizes the transit time. Consider the case of reflection from a surface in air. The total transit time from point A to point B is
The minimum travel time is found by taking the derivative with respect to y and setting it equal to zero:
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Problem 33.52
PHYS117B.02 JV

8. The critical angle for TIR
Consider light incident from glass (n=1.5) to air (n=1.0) at several different angles (see figure).
Snell’s law relates the angle of incidence to the angle of refraction. J1 J2 0˚
30˚
40˚
45˚
???
At the critical angle …
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PHYS117B.02 JV

9. Application of TIR: prisms
Total internal reflection in glass or crystalline quartz prisms are frequently used in optical systems instead of mirrors. The prisms below all are cut at angles of 45˚ and 90˚. Why?
EXAMPLES
Cameras
Periscopes
High-power laser beams
Can you think of others?
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PHYS117B.02 JV

10. Optical fibers Why communicate with light? (in a word, bandwidth!)
Added advantage: not subject to electrical interference
Fiber transmission works because of total internal reflection at core-cladding interface
Fibers can support multiple transmission paths (multi-mode) or only one (single-mode)
9/19/2018
PHYS117B.02 JV

11. Polarization: Polarizers work by preferred direction of absorption
Malus’s law for polarized
light passing through a
polarizer-analyzer pair
Polarizers work by preferred direction of absorption
Long, stringy molecules work for visible light
If you have owned a pair of polarizing sunglasses …
Have you noticed a change in brightness of sky when you put them on? What does this mean?
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PHYS117B.02 JV

12. Polarization by reflection
The Brewster angle …
Reflections can polarize light that was initially not polarized … (Brewster’s angle)
Since scattering is a form of reflection, what happens to polarized light scattered from a rough surface?
Then why is it smart to wear polarizing sunglasses when driving in bright sunlight?
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PHYS117B.02 JV

13. Summing up …
Light rays travel in straight lines until they run into something
Even when they bump into something, light rays travel the path that takes the shortest time
In material media (e.g., glass) the speed of light depends on its color (dispersion)
When light is reflected from, scattered from or transmitted through a surface, it may be polarized
The degree of polarization caused by reflection depends on the angle of incidence
9/19/2018
PHYS117B.02 JV