Question

White light is incident on a soap film (n = 1.32) in air. The reflected light looks bluish because the red light (λ = 670 nm ) is absent in the reflection. What is the minimum thickness of the soap film? t=____nm

Answer 1

Incident light undergoes a phase change at one interface with air but not the other, thus if the soap film is 1/2 λ thick the reflected light will be out of phase from the incident light

T = λ / 2 thick

λf / λ /1.32 = 6.70E-7 m / 1.32 = 5.08E-7 wavelength in film

T = 5.08E-7 m / 2 = 2.54E-7 m for thickness of film