Recombination of white light can be done through glass slab

Explanation:

Recombination of spectrum of colours

A spectrum of seven colours of lights can be recombined to form back white light. This can be done by placing two glass prisms side by side. But, place the second glass prism in an inverted position. When white light passes through first prism it disperses the light into seven colours and when this beam of light enters the second prism which is placed in an inverted position, a white light is obtained when it comes out of the second prism.

The seven coloured lights recombine to form white light due to second glass prism which is placed in a reversed position.

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