Question

A thin layer of colourless oil on water surface looks to be coloured in the sun light, why?

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Answer 1

$\blue{\textbf{Answer}}$:-

➮The thin layer of oil looks to be coloured due to❝ $\blue{\texttt{Interference}}$❞. When sun light falls on the layer of oil, it is reflected from both the upper layer and lower layer of surface.

➮The path difference between the two reflected rays is given by ,

$\blue{\texttt{P}}$ ➭ 2μt cosθ

➮where,

u=$\blue{\texttt{refractive index of layer}}$

t=$\blue{\texttt{thickness of layer}}$

θ = $\blue{\texttt{angle of refraction}}$

If , p ➭n (λ/ 2)

➮Where,

$\textbf{n}$➭ ➋,➍ ,➏ , ...

then layer will appear dark .

and ,if $\textbf{n}$➭ ➊,❸, ❺,➐ , ..

the layer will appear bright.

❝The colours present in the reflected light will depend on the $\blue{\texttt{thickness of the layer}}$.❞

Answer 2

$\huge \ \blue{ \bf{answer}}$

When petrol is dropped on the road during a rainy day, a thin layer of oil will appear on the water surface.

Both the top and bottom surfaces of this oil film can reflect light. If the path difference between two light rays is an integral times of the wavelength, there will be constructive interference (Fig .1). A light ray will pass through different thickness of oil when the angle of reflection varies.

The wavelength corresponding to the constructive interference also differs and this causes the reflected light to have various colours. As a result, a rainbow-like colour pattern is shown on the oil surface and this phenomenon is called thin-film interference.

$\green{ \bf{hope \: this \: helps}}$