# Design Realization lecture 25Page 1

Slide 1

## This time

Reflection, Scattering

Refraction, TIR

Retro-reflection

Lenses

Slide 2

## Wavefronts and Rays

EM waves propagate normal to the wavefront surface, and vice-versa.

The ray description is most useful for describing the geometry of images.

Slide 3

## Reflection

Most metals are excellent conductors.

They reduce the E field to zero at the surface, causing reflection.

If I, R, N unit vectors:

IN = RN

I(N  R) = 0

Slide 4

## Ray-tracing

By tracing rays back from the viewer, we can estimate what a reflected object would look like. Follow at least two rays at extremes of the object.

Slide 5

## Lambertian scattering

For most non-metallic objects, the apparent brightness depends on surface orientation relative to the light source but not the viewer.

i.e. brightness is proportional to IN

Slide 6

## Refraction – wave representation

In transparent materials (plastic, glass), light propagates slower than in air.

At the boundary, wavefronts bend:

Slide 7

## Refractive index

Refractive index measures how fast light propagates through a medium.

Such media must be poor conductors and are usually called dielectric media.

The refractive index of a dielectric medium is where c is the speed of light in vacuum, and v is the speed in the medium. Note that  > 1.

Slide 8

## Refraction – Snell’s law

Incident and refracted rays satisfy:

Slide 9

## Refraction – ray representation

In terms of rays, light bends toward the normal in the slower material.

Slide 10

## Refraction in triangular prisms

For most media, refractive index varies with wavelength. This gives the familiar rainbow spectrum with white light in glass or water.

Slide 11

## Refractive index

Refractive index as a function of wavelength for glass and water

Slide 12

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