Snell’s LawSnell’s Law describes refraction as

light strikes the boundary between two media

n1 sin q1 = n2 sin q2

The index of refraction of a pure vacuum and of air is n = 1.

The index of refraction of every other substance is greater than 1.

incidence reflected

refracted

q q

q

Example:

Light traveling through air enters a block of glass at an angle of 30° and refracts at an angle of 22°.

What is the index of refraction of the glass?

2

11sinsinnqq

incidence reflected

refracted

q q

q2211 sinnsinn qq

2

112 sin

sinnnqq

33.122sin30sin1n2

Different frequencies (colors) refract slightly different amounts.

This means that the index of refraction, “n”, for blue light is slightly different than “n” for red light.

This results in a dispersions of colors as seen in a prism or a rainbow.

Blue Bends Best!(ok, actually violet refracts the

most…)

Rainbows!

Sunlight refracts as it enters a raindrop.

Different colors refract different amounts.

This spreads out the colors.The light reflects off the back of the

raindrop.The light refracts again, spreading out

the colors even more.We see the rainbow!

The Critical Angle and Total Internal Reflection

q

inci

denc

e

refracted

When light passes from a material that is MORE dense to one that is LESS dense, its refracts AWAY from the Normal line.

As the angle of incidence increase, the angle of refraction also increases.

The Critical Angle and Total Internal Reflection

At some Critical Angle of incidence, the angle of refraction is 90°.

Beyond that critical angle, no light that is refracted!

All of the light is reflected back into the original medium.

This is called Total Internal Reflection

inciden

ce

reflected

qcritical

1

2critical n

nsin q

n1

n2

)190(sinsinsin 2211 qq nn

The most useful application of the phenomenon of

Total Internal Reflection is in

Fiber Optics

When wavefronts pass through a narrow slit they spread out. This effect is called diffraction.

The amount of diffraction depends upon the size of the slit. If the slit is comparable in size to the wavelength of the wave then maximum diffraction occurs.

• The number of slit openings also determines what the diffraction pattern looks like.

• Thomas Young, in1801, first established that

light was a wave by demonstrating that light

diffracted.• He also provided the first

measurement of the wavelength of light.

Thomas Young’s Double-Slit Experiment

• He allowed sunlight to fall on two slits.• He knew that if light was a wave, it would diffract as it

passed through the slits.• The diffracted waves would have areas of both

constructive and destructive interference.• This interference would produce bright and dark areas

on a screen.

If the wavelength of light is longer, the pattern on the screen is more spread out.

(700 nm- 400 nm) Red light spreads out more than violet.If the screen is farther, the pattern on the screen is more spread out.If the slits are CLOSER to each other, the pattern on the screen is more spread out.

Optical diffraction effects can be seen with eye - in fact most of us when children have noticed it, but ignored it

when becoming adults.

Look through a narrow slit between your fingers. If you look carefully you should see the objects behind are distorted and that blackish bands parallel to the slit

appear in the gap. The bands are diffraction patterns.

• In the atmosphere, diffracted light is actually bent around atmospheric particles -- most commonly, the atmospheric particles are tiny water droplets found in clouds.

• An optical effect that results from the diffraction of light is the silver lining sometimes found around the edges of clouds

• The pattern of bright and dark fringes did appear on a screen.

• The brightest area, in the center, he called the “central bright spot”.

• He was able to mathematically determine the wavelength by measuring the distance from the central bright spot to each fringe.

ml = d(x ÷ L) = dsinqm- “order” (m = 0 is the central bright spot)l- wavelength of lightd- distance between the slitsx- distance from central bright spot to another

bright fringeL- distance from the slits to the screenq- the angle between the line to the central bright

spot and the observed bright fringe.

Different frequencies (colors) of lightdiffract by different amounts

• The more slits there are, the narrower the fringes become.

• The fringes on top are from two slits.• The fringes on bottom are from eight slits.• A “diffraction grating” has hundreds of slits per

millimeter.