A layer of oil 3cm thick is floating on a layer of coloured water 5cm thick. Refractive index of coloured water is 5/3 and the apparent depth of the two liquids appears to be 36/7cm. Find the refractive index of oil.

1.4

Apparent depth (AI) = $\frac{t_1}{m_1}+\frac{t_2}{m_2}$

$\frac{36}{7}=\frac{5}{5/3}+\frac{3}{m_2}$

$\frac{3}{m_2}=\frac{36}{7}-3=\frac{15}{7}$

$m_2=\frac{7}{5}=1.4$