# Design Realization lecture 25Page 2

## Refractive index

High-quality optical glass is engineered to have a constant refractive index across the visible spectrum.

Deviations are still possible. Such deviations are called chromatic aberration.

Slide 13

## Refractive indices

Water is approximately 1.33

Normal glass and acrylic plastic is about 1.5

Highest optical plastic index is 1.66

Bismuth glass is over 2

Diamond is 2.42

Slide 14

## Internal reflection

Across a refractive index drop, there is an angle beyond which ray exit is impossible:

Slide 15

## Total internal reflection (TIR)

The critical angle is where the refracted ray would have 90 incidence.

The internal reflection angle is therefore:

For glass/acrylic, this is 42

For diamond, it is 24 - light will make many internal reflections before leaving, creating the “fire” in the diamond.

Slide 16

## Penta-prisms

Penta-prisms are used in SLR cameras to rotate an image without inverting it.

They are equivalent to two conventional mirrors, and cause a 90 rotation of the image, without inversion.

An even number of mirrors produce a non- inverted rotated image of the object.

Slide 17

## Retro-reflection: Corner reflectors

In 2D, two mirrors at right angles will retro-reflect light rays, i.e. send them back in the direction they came from.

Slide 18

## Retro-reflection: Corner reflectors

In 3D, you need 3 mirrors to do this:

Analysis: each mirror inverts one of X,Y,Z

Slide 19

## Retro-reflection: TIR spheres

Consider a sphere and an incoming ray.

Incoming and refracted ray angles are , .

For the ray to hit the centerline,  = 2.

For retro-reflection, we want  = sin  /sin 

For small angles,  = 2 gives good results.

Slide 20

## Retro-reflective sheets

Inexpensive retro-reflective tapes are available that use tiny corner reflectors or spheres embedded in clear plastic (3M Scotchlite)

They come in many colors, including black.

Slide 21

Go to page:
1  2  3