Reflection & Refractionof

Electromagnetic Radiation

ByEmin Menachekanian, PhD

What Is Light?

Wave Particle

ɤ

Electromagnetic Spectrum

The Speed of Light Is Finite, But Fast!

Consider the round-trip distance from here to UCLA and back: 30 mi ≈ 60 km

How much time is involved in the blink of an eye?

≈ 0.3 seconds!In that time, light would have made

an astonishing 1500 round trips!!

The Speed of Light Is Finite, But Fast!

Voyager 1, launched in 1977, took about 30 years to reach the edge of our solar system.

Light would take about 12 hours to do the same thing!!

Understanding How Light Reflects & Transmits

Use:Fermat’s Principle of Least Time

Pierre de Fermat

Fermat’s Principle of Least Time

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

time of its journey.

In Empty Space:

What About for Reflecting from a Mirror?

Mirror

Mirror Image

What’s the Correct Reflection?

Mirror

A

B

Equidistant to Mirror

B’

This path not only minimizes the distance, but it also minimizes the time!

Because speed of incident ray is the same as the speed of reflected ray.

Law of Reflection

Mirror

A

BNormal to

Mirror

θi θr

θi = θr

Why Does He Look Like This?

Simple Example

Mirror

Simple Example

Mirror

Simple Example

Mirror

Transmission of Light

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

Speed of light in medium Speed of light in empty space

Index of Refraction

Indices of Refraction for Various Materials

Which One Is The Shortest Distance?

ABC

50 m

50 m

85 m

25 m

Which One Takes The Least Time?

ABC

Running Speed Is 10 m/s

Swimming Speed Is 2.5 m/s

5 s

20 s

8.5 s

10 s

Which One Takes The Least Time?

ABC

Fast

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.

Refraction Simulator

http://interactagram.com/physics/optics/refraction/

The “Bent” Pencil

The “Bent” Pencil

Water

Actual Straw

Image of Straw

Full Reflection from Medium Interface!

Total Internal Reflection

Only occurs when light is going FROM a medium with a HIGHER index of refraction INTO a medium with a

LOWER index of refraction

http://interactagram.com/physics/optics/refraction/

The Critical Angle

The angle (with respect to the normal) at which the light totally internally

reflects is known as the critical angle.

The larger the index of refraction of the medium in which the light totally internally reflects, the smaller the

critical angle.

Critical Angles (Surrounding Medium: Air)

Internal Medium Refractive Index (n) Critical Angle [Deg.]

Water 1.33 48.8

Ethyl Alcohol 1.36 47.3

Quartz, Fused 1.46 43.2

Lucite 1.50 41.8

Glass, Crown 1.52 41.1

Glass, Flint 1.66 37.0

Zircon 1.92 31.4

Diamond 2.42 24.4

45° Glass Prisms> 41° (TIR)

Binoculars

Fiber-Optic Cables