The Nature of Light and the Laws of Geometric Optics

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The Rainbow

A ray of light strikes a drop of water in the atmosphere.

It undergoes both reflection and refraction.

First refraction at the front of the drop

Violet light will deviate the most.

Red light will deviate the least.

Section 35.7

Slide 51

The Rainbow, cont.

At the back surface the light is reflected.

It is refracted again as it returns to the front surface and moves into the air.

The rays leave the drop at various angles.

The angle between the white light and the most intense violet ray is 40°.

The angle between the white light and the most intense red ray is 42°.

Section 35.7

Slide 52

Observing the Rainbow

If a raindrop high in the sky is observed, the red ray is seen.

A drop lower in the sky would direct violet light to the observer.

The other colors of the spectra lie in between the red and the violet.

Section 35.7

Slide 53

Double Rainbow

The secondary rainbow is fainter than the primary.

The colors are reversed.

The secondary rainbow arises from light that makes two reflections from the interior surface before exiting the raindrop.

Higher-order rainbows are possible, but their intensity is low.

Section 35.7

Slide 54

Total Internal Reflection

A phenomenon called total internal reflection can occur when light is directed from a medium having a given index of refraction toward one having a lower index of refraction.

Section 35.8

Slide 55

Possible Beam Directions

Possible directions of the beam are indicated by rays numbered 1 through 5.

The refracted rays are bent away from the normal since n1 > n2.

Section 35.8

Slide 56

Critical Angle

There is a particular angle of incidence that will result in an angle of refraction of 90°.

This angle of incidence is called the critical angle, θC.

Section 35.8

Slide 57

Critical Angle, cont.

For angles of incidence greater than the critical angle, the beam is entirely reflected at the boundary.

This ray obeys the law of reflection at the boundary.

Total internal reflection occurs only when light is directed from a medium of a given index of refraction toward a medium of lower index of refraction.