refraction: tir and dispersion ap physics: m. blachly light and optics

Refraction:TIR and Dispersion

AP Physics: M. Blachly

Light and Optics

Nature of Light

What is light?

• Light is a wave

• But what is the medium?

• Applet

Light is also a particle

• The particle is called a photon.

• The particle nature of light was first proposed by Einstein to explain the photoelectric effect.

Reflection

A mirror and a piece of white paper both reflect essentially all light that hits them. What is the difference?

Reflection

Mirror is example of specular reflection

m irro r s u rface

Reflection

Diffuse reflection from a rough surface

Reflection

Microscopic view of a sheet of paper

Reflections

The Law of Reflection

Reflection: Headlights

Why do oncoming headlights seem so bright?

Why don’t headlights work in the rain?

Transmission

Light can enter an optically transparent material.

The light slows down as it is absorbed and emitted by the atoms

Speed of Light

n the called the index of refraction

it can be thought of as the “slow down” factor for lightc

nv

Index of Refraction

0%20%40%60%80%

100%V

acuu

m Air

Ice

Wat

er

EtO

H

glas

s

Dia

mon

d

Index of Refraction

Material Index of RefractionVacuum 1.000

Air 1.00029Ice 1.310

Water 1.330EtOH 1.360glass 1.520

Diamond 2.417

Are you awake?

Material Index of RefractionVacuum 1.000

Air 1.00029Ice 1.310

Water 1.330EtOH 1.360glass 1.520

Diamond 2.417

What is the speed of light in Water ?

Refraction

What happens if a wave of light approached an interface where it must slow down?

Refraction

RefractionRefraction is the bending of a wave when it travels from one medium to another.

Animation showing how light bends

Refraction Examples

Slice your finger example

The bent straw

Spear fishing

Refraction Examples

Catching fish

n = 1

n = 1 .5

Refraction Terminology

Refraction

Light bends towards the normal when it enters a material with a slower speed (higher n)

n = 1

n = 1 .5

Refraction

Light bends away the normal when it enters a material with a higher speed (lower n)

n = 1

n = 1 .5

Refraction: Snell’s Law

The angle of refraction depends on the indices of refraction, and is given by Snell’s law:

Example Problem

Light, traveling in glass, strikes an optical interface at an angle of 50 as measured from the normal. What is the angle of refraction?

n = 1 .3 3

n = 1 .5 0

Refraction of Sound

Dispersion

Dispersion is the separation of light into colors by refraction

Animation

Dispersion

The index of refraction of a material varies somewhat with the wavelength of the light.

Dispersion

Dispersion

Most famous example of dispersion is the rainbow

Rainbow

Rainbow

Total Internal Reflection

Simulator

When going from a slower to a faster medium, light cannot escape if the angle is too big.

Applications

Diamonds

Applications

Light Pipes

Are you conscious still?

Light, traveling in plastic, strikes an optical interface at an angle of 65. as measured from the normal. What is the angle of refraction?

n = 1 .7 8

n = 1 .0 0

If light passes into a medium with a smaller index of refraction, the angle of refraction is larger. There is an angle of incidence for which the angle of refraction will be 90°; this is called the critical angle:

(23-5)

Total Internal Reflection

If the angle of incidence is larger than this, no transmission occurs. This is called total internal reflection.

Total Internal Reflection

Example

The index of refraction for glass is 1.33. What is the critical angle for TIR for a beam of light traveling in glass and striking an optical boundary with air ( n = 1 )?

Binoculars often use total internal reflection; this gives true 100% reflection, which even the best mirror cannot do.

Total Internal Reflection

Total internal reflection is also the principle behind fiber optics. Light will be transmitted along the fiber even if it is not straight. An image can be formed using multiple small fibers.

Total Internal Reflection

Dispersion

Wavelengths of visible light: 400 nm to 750 nm

Shorter wavelengths are ultraviolet; longer are infrared

This variation in refractive index is why a prism will split visible light into a rainbow of colors.

Dispersion

Actual rainbows are created by dispersion in tiny drops of water.

Dispersion