State the conditions for dispersion without deviation.

Explanation

If the angles of two prisms are so adjusted that the deviation produced for the mean ray by the first prism is equal and opposite to that produced by the second prism, then the emergent mean ray will be parallel to the incident ray. In this case the combination of such prisms produces dispersion without deviation. The net deviation of the mean ray from the two prisms is zero. The refracting edges of two prisms will be placed opposite to each other. The two prisms are made up of different materials and have different reflacting angles.