Light Transmission

Amplitude of excitation is low enough to not cause collisions between neighboring molecules. The beam then is transmitted consecutively across the material.

Glass Is Opaque to UV & IR

•IR & UV:•Excitation amplitude is large → Lots of collisions!•Initial energy from light lost to thermal energy

•Visible:•Excitation amplitude is small → Collisions less likely•Absorbed light beam gets re-emitted through material

Glass Protects you From Sunburn!

Pierre de Fermat

Theorem Proved!

Fermat’s Principle of Least Time

Light will always take the path from one point to another that minimizes the

time of its journey.

What About for Reflecting from a Mirror?

Mirror

Quantum mechanically, they all (including all the possibilities that have not been drawn) happen!

What’s the One That’s Seen?

Mirror

A

B

Equidistant to Mirror

B’

Law of Reflection

Mirror

A

BNormal to

Mirror

θi θr

θi = θr

Mirror Image

I’m Floating!!

Transmission (Revisited)

Visible light can pass through certain materials (e.g., glass).

Question:Does the speed of light change in going

from one transparent medium into another?

Transmission (Revisited)

It takes time for the light to be re-emitted when captured → Slows down!

Speed of light in medium Speed of light in vacuum

Index of Refraction

Indices of Refraction for Various Materials

How Slow Can Light Go?

In ultracold sodium (at temperatures around 1 nK = 1 x 10 -9 K):

v = 17 m/sThat’s about 38 miles/hour!

Which One Is The Shortest Distance?

ABC

Which One Takes The Least Time?

ABC

Running → Fast

Swimming→ Slow

Light Behaves the Same Way!

•As light enters from a medium with a LOW index of refraction to one with a HIGH index of refraction, the ray bends TOWARDS the normal.

•As light enters from a medium with a HIGH index of refraction to one with a LOW index of refraction, the ray bends AWAY FROM the normal.