Answer:
λ = 800 10⁻⁹ m
Explanation:
The interference of a thin film has a contribution of pi / 2 in the reflection on the first surface and a contribution by the change of the ode length within the medium, therefore the expression for constructive interference is
2t = (m + ½) λ
m = 0, 1,2, ...
Let's clear
λ = 2t / (m + ½)
To have the highest value of the wavelength m = 0
λ = 2t / (0+ ½)
λ = 4 t
Let's calculate
λ= 4 200 10⁻⁹
λ = 800 10⁻⁹ m