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A glass of refractive index 1.5 is coated with a thin layer of thickness of  of refractive index 1.8. light of wavelength  travelling in air is incident normally on the layer. It is partly reflected at the upper and the lower surfaces of the layer and the two reflected rays interfere. If  nm, obtain the least value of  (in  m) for which the rays interfere constructively



Question ID - 154123 | SaraNextGen Top Answer

A glass of refractive index 1.5 is coated with a thin layer of thickness of  of refractive index 1.8. light of wavelength  travelling in air is incident normally on the layer. It is partly reflected at the upper and the lower surfaces of the layer and the two reflected rays interfere. If  nm, obtain the least value of  (in  m) for which the rays interfere constructively

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Answer Key / Explanation : (90) -

(90)

Path difference between rays reflected from upper and lower faces of layer  (for normal incidence). But there is abrupt change in path of  of light at upper surface. So actual path difference is

For constructive interference

. For least thickness

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127