00:01
So in this question we have a layer of soap film, that is n equals 1 .34, on top of glass that is in 0 .1 .52.
00:15
So we're asked about how sick is a film if it reflects the red light more strongly when illuminated normally by white light.
00:25
So that means it experiences constructive interference for red light.
00:33
And if we write down the end and we remember that a phase shift of pi will occur every time the light goes from, is reflected as a boundary of when it encounters a media of high refractive index, then we know that it will experience face shift at both of these boundaries.
00:54
And that will cancel each other.
00:57
So in order to have constructive interference, we have 2 t equals n times lambda, and lambda n, because it's lambda inside the media, instead of lambda in the air.
01:13
So this question, it does, there could, m could be any value, but since this question does not specify which m it wants, i'm going to assume it's the lowest m.
01:25
So that is to say it, we want the thinness, the smallest sickness of the film, because you can have m equals 2, m equals 3, m equals infinity.
01:40
All of these are going to work, but i'm just going to find the smallest m.
01:45
So usually when we try to find the smallest value of m, we start from 0 and we go 0, 1, 2, 3, 3, etc.
01:58
But zero does not work in this case.
02:00
Why? because if it is zero that lose, there is barely any coating.
02:07
And in that case, the coating will reflect any light equally because for t equals zero and m could be anything...