Answer:
Explanation:
There is reflection ( two times ) from upper and lower surface of the film . In both cases , reflection is from low to high density medium so there is change in phase of 180 twice .
So for constructive interference
2μd = n λ₁ , d is thickness required , λ is wavelength n₁ is order of bright fringe
For destructive interference ( minimum light )
2μd = (2n+1) λ₂/2
n λ₁ =(2n+1) λ₂/2
(2n+1) / 2n = λ₁ / λ₂
= 650 / 505
= 5 / 4 ( approx )
2n = 4
n = 2
2μd = n λ₁
2 x 1.33 x d = 2 x 650 nm
d = 488.72 nm
