The Physics of Rainbows!

Reflection, Refraction, and

Dispersion

Energy of Light

Ray Diagrams of Light

Light travels in a straight line until it encounters a boundary.

In which case, light can either reflect or transmit through the

boundary. Sometimes it does both.

Wave front is the plane where all the light is traveling at the

same speed and are in phase.

Reflection

In the case of specular reflection, light “bounces off ” the

surface similar to how a basket ball bounces off a smooth

surface.

In the case of diffuse reflection, light “bounces off ” the same

way as above but this time the surface has large rocks on it.

θi= θr where i stands for incident and r stands for reflected.

Reflection

Refraction

Refraction

n1= 1.00

n2= 1.33

From a lower to higher index of

refraction, transmitted light bends

toward normal

Refraction

n2= 1.00

n1= 1.33

From a higher to lower index of

refraction, transmitted light bends

away from normal

These are the Equations Used to Calculate

how much Light is Transmitted and Reflected

Dispersion

Dispersion is the dependence of wavelength for the index of

refraction.

Colors of light have different wavelength and thus index of

refraction. Red moves the fastest with purple moving the

slowest.

ROYGBIV

Red’s wavelength is the longest and Violet’s is the shortest.

Happens in refraction as shown by prism in book and at

http://phet.colorado.edu/en/simulation/bending-light

This allows us to see the visible spectrum.

Huygen’s Principle

Essentially: A new wave front is tangent to wavelets. What

this is used for is to prove Snell’s Law.

http://phet.colorado.edu/en/simulation/bending-light

Total Internal Reflection

TIR

n2= 1.00

n1= 1.33

From a higher to lower index of

refraction it is possible to have TIR

occur if the angle is right.

Fiber Optics

If TIR didn’t happen then fiber optics would be impossible.

What do you suspect the design of fiber optics to be?

Rainbow

What causes the rainbow is the combination of reflection,

refraction, dispersion, and TIR. With the shape of the

raindrop this can only occur at a very select angle of entry to

the raindrop. That is why you usually only see one rainbow.

But sometimes it is possible to see a faint Double Rainbow.