Question

The oil layer on the surface of water appears coloured, due to interference . For this effect to be visible the thickness of oil layers will be

• A. $1mm$
• B. $1cm$
• C. $100A^o$
• D. $1000A^o$

Answer 1

The correct option is D $1000A^o$

A phase difference of $\pi$ occurs after reflection from the oil surface. But for the second beam, this does not happen when reflected from water surface,

This gives the condition for constructive interference as-

$2{\mu }_{oil}d=\frac{{\lambda }_{air}}{2}$

Hence, $\lambda =4{\mu }_{oil}d$

For $400nm<\lambda <700nm$

$400nm<4{\mu }_{oil}d<700nm$

${\mu }_{oil}=1.45$

Hence $689.65A^{\circ }<d<1206.8A^{\circ }$