The Nature of Light and the Laws of Geometric Optics

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If the ray originated at B, it would follow the line AB to reach point A.

Section 35.5

Slide 27

Following the Reflected and Refracted Rays

Ray  is the incident ray.

Ray  is the reflected ray.

Ray  is refracted into the lucite.

Ray  is internally reflected in the lucite.

Ray  is refracted as it enters the air from the lucite.

Section 35.5

Slide 28

Refraction Details, 1

Light may refract into a material where its speed is lower.

The angle of refraction is less than the angle of incidence.

The ray bends toward the normal.

Section 35.5

Slide 29

Refraction Details, 2

Light may refract into a material where its speed is higher.

The angle of refraction is greater than the angle of incidence.

The ray bends away from the normal.

Section 35.5

Slide 30

Light in a Medium

The light enters from the left.

The light may encounter an electron.

The electron may absorb the light, oscillate, and reradiate the light.

The absorption and radiation cause the average speed of the light moving through the material to decrease.

Section 35.5

Slide 31

The Index of Refraction

The speed of light in any material is less than its speed in vacuum.

The index of refraction, n, of a medium can be defined as

Section 35.5

Slide 32

Index of Refraction, cont.

For a vacuum, n = 1

We assume n = 1 for air also

For other media, n > 1

n is a dimensionless number greater than unity.

n is not necessarily an integer.

Section 35.5

Slide 33

Some Indices of Refraction

Section 35.5

Slide 34

Frequency Between Media

As light travels from one medium to another, its frequency does not change.

Both the wave speed and the wavelength do change.

The wavefronts do not pile up, nor are they created or destroyed at the boundary, so ƒ must stay the same.

Section 35.5

Slide 35

Index of Refraction Extended

The frequency stays the same as the wave travels from one medium to the other.

v = ƒλ

ƒ1 = ƒ2 but v1  v2 so λ1  λ2