Combining Prisms

Two prisms of different glass, placed point to base, can split colours without bending the beam, or bend the beam without splitting the colours. The second trick is the key to colour-free lenses.

Two opposed thin prisms#

Put two thin prisms in contact with their refracting angles A and A′ pointing opposite ways; their dispersive powers are ω and ω′. They deviate light in opposite directions, so for any ray

Using μᵥ − μᵣ = ω(μᵧ − 1), the net angular dispersion and the net mean deviation are

Dispersion without average deviation#

For the yellow ray to go straight on, δᵧ = 0:

Average deviation without dispersion#

For the colours to come out together, δᵥ − δᵣ = 0:

Both tricks need ω ≠ ω′, so the two prisms must be made of different glasses, typically crown and flint.