If a plane glass slab is placed on letters of different colours, then red coloured letter appears to be raised minimum, why ?

Assertion : If a plane glass slab is placed on the letters of different colours all the letters appear to be raised up to the same height. Reason : Different colours have different wavelengths.

When a rectangular glass slab is kept on a few letter of different colours, the letter of violet colour appears to be raised more. What is the reason behind it?

A plane glass slab is placed over various coloured letters. The letter which appears to be raised the least is

A plane glass slab is placed over various coloured letters. The letter which appears to be raised the least is

A plane glass slab is placed over various coloured letters. The letter which appears to be raised the least is