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.
Water is approximately 1.33
Normal glass and acrylic plastic is about 1.5
Polycarbonate is about 1.56
Highest optical plastic index is 1.66
Bismuth glass is over 2
Diamond is 2.42
Across a refractive index drop, there is an angle beyond which ray exit is impossible:
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.
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.
In 2D, two mirrors at right angles will retro-reflect light rays, i.e. send them back in the direction they came from.
In 3D, you need 3 mirrors to do this:
Analysis: each mirror inverts one of X,Y,Z
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.
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.