1. Reflection of Light Waves
Refleted wave momentum
Incident wave momentum
i
r
What direction will the reflected wave be?
No sideways force acts on the light beam
Parallel momentum will be conserved
Total energy is conserved
Perpendicular momentum must be equal and opposite

2. Double Reflections C First beam comes in 10 degrees away from vertical
Mirrors are at right angles B A
Which direction is the twice-reflected beam?
10o
Therefore 10o from vertical
First reflection beam is 100 from vertical
10o
90o
Reflected at 80o from horizontal
Right triangle angles add up to 180o
90o
80o
Beam comes in at 800 from horizontal
10o
Therefore parallel to incoming

3. Refraction and Reflection
If light moves from one region to the another with a different index of refraction, it will be bent – refracted
Some of the light will normally also be reflected
1
2
1
We can “see” glass mostly because it partly reflects light
If you match the index of refraction, all the light is transmitted

4. Light inside a medium Light interacts with particles inside materials
This slows down the light
The speed of light inside a medium is less than c
n is called the index of refraction
It is always bigger than 1
Material n
Air
Water
Glass 1.6
Diamond 2.419

5. Snell’s Law Snell’s Law
What happens to an EM wave that comes in at an angle? d
2
1
Consider where the wave crests are impacting the glass

6. Snell’s Law 2 1 index n2 index n1
When light goes from low index medium to high index medium, the angle gets smaller
When light goes from high index medium to low index medium, the angle gets bigger

7. Quiz A) B) C)

8. Quiz

9. Quiz 1 2 5 3 4 Which rays are reflected and which rays refracted?
Rays 2 and 3 are reflected, 4 and 5 refracted
Rays 2 and 4 are reflected, 3 and 5 refracted
Rays 2 and 5 are reflected, 3 and 4 refracted
Rays 3 and 4 are reflected, 2 and 5 refracted

10. Multiple Refractions Material n Air 1.000 Water 1.333 F Glass 1.6
Diamond 2.419
F
g
g
d
d
w
w
370
A light beam starting in air enters a series of parallel sheets, passing through water, diamond, and then glass. If the initial angle is 37o, what is the final angle F ?
37o B) Less than 37o C) More than 37o