# Homework Help: Thin Film Interference in a Soap Bubble

1. Oct 6, 2009

### calvert11

1. The problem statement, all variables and given/known data
Question:

A soap bubble of index of refraction 1.48 strongly reflects both the red and the green
components of white light. What film thickness allows this to happen? (In air, the wavelength of red light is 716 nm, of green light 511.429 nm.)

n = 1.48
$$\lambda$$(r) = 716 nm
$$\lambda$$(g) = 511.429 nm

2. Relevant equations
2L = (m + 1/2)*(λ/n)

3. The attempt at a solution

I used the equation for thin film interference (maxima): 2L = (m + 1/2)*(λ/n)

Since the light strongly reflects, I assume m = 0.

I end up with 2 equations giving thickness: one for red light, one for green light:

L(r) = 716e-9/(4*1.48)
L(g) = 511.429e-9/(4*1.48)

But I don't know how to proceed from there.