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(D) For destructive interference in the reflected light from a thin film,the condition is given by $2 \mu t \cos r = n \lambda$,where $n = 1, 2, 3, \d

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All of these

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(D) For destructive interference in the reflected light from a thin film,the condition is given by $2 \mu t \cos r = n \lambda$,where $n = 1, 2, 3, \dots$.Given: $\mu = 1.4$,$t = 10^{-4} \,cm = 10^{-6} \,m = 10^{-4} 	imes 10^{10} \, \mathring{A} = 10^6 \, \mathring{A}$.For normal incidence,$r = 0^{\circ}$,so $\cos r = 1$.The condition becomes $2 	imes 1.4 	imes 10^{-4} \,cm = n \lambda$.$2.8 	imes 10^{-4} \,cm = n \lambda$.Converting to $\mathring{A}$s: $2.8 	imes 10^{-4} 	imes 10^8 \, \mathring{A} = n \lambda$.$28000 \, \mathring{A} = n \lambda$.Thus,$\lambda = \frac{28000}{n} \, \mathring{A}$.For $n = 4$,$\lambda = \frac{28000}{4} = 7000 \, \mathring{A}$.For $n = 5$,$\lambda = \frac{28000}{5} = 5600 \, \mathring{A}$.For $n = 7$,$\lambda = \frac{28000}{7} = 4000 \, \mathring{A}$.Since all the given options satisfy the condition for destructive interference,they will not be seen in the reflected light.

When white light is incident normally on an oil film of thickness $10^{-4} \,cm$ and refractive index $1.4$,the wavelength which will not be seen in the reflected system of light is ........... $\mathring{A}$.

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Assertion: Thin films such as a soap bubble or a thin layer of oil on water show beautiful colours when illuminated by white light.
Reason: It happens due to the interference of light reflected from the upper surface of the thin film.

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