Question

What is the minimum thickness of a soap bubble needed for constructive interference in reflected light if the light incident on the film is 900 nm? Refractive index for the film is µ = 1.5–

• A. 100 nm
• B. 150 nm
• C. 200 nm
• D. 250 nm

Answer 1

Answer: (B)

150 nm

Answer 2

2µt cos r = (2n – 1) $\frac{\lambda}{2}$

t = (2n – 1) $\frac{\lambda}{2\mu\cos r}$

cos r ® 1.

\ t = (2n _ 1) $\frac{\lambda}{4\mu}$.

n = 1

t = $\frac{\lambda}{4\mu} = \frac{900}{4 \times 1.5}$ nm

\ t = 150 nm