Question

what is the thickness of a coating of material of refractive index n=1.40 which when applied to flint glass lense n=1.65 will cause distraction of yellow light (5.9*10^-7)

Answer 1

Here, we have to find the thickness of the coating of a material which is applied to glass lens which will cause distraction of yellow light.

Now we have to find the thickness of coating such that destructive interference occurs when yellow light reflected from air-coating interference and from Coating and glass interference.

Let say the thickness of coating be (t) so the total distance traveled by a wave in the coating -glass is 2t. this 2t is equaled the half the wavelength of the coating.

2t = coating / 2 ......................(1)

Now, coating = air / n Where n = refractive index of coating (1.40) , air = 5.9*10^-7 m

coating = 5.9*10^-7/1.40

coating = 4.2*10^-7

Inserting the value of coating in eq (1)

2t = 4.2*10^-7 /2

t = 4.2*10^-7 / 4

t = 1.05 * 10^-7 m or 105 nm (Thickness)