2) A soap buble (n=1.33) is floating in air. If the thickness of the buble wall is 115 nm, what is
the wavelength of light that is most strongly reflected?
Light reflecting from the first surface suffers phase reversal. Light reflecting from the second
surface does not because n2>nair. However, light reflecting from the second surface passes
twice through the thickness, t of the film.
For constructive interference, we require
1 

2t   m   .
2 n

For m=0,   4nt  4.1, 33.115  612nm
3) An oil film (n=1.45) floating on water is illuminated by white light at normal incidence. The
film is 280 nm thick. Find a) the color of light in the visible spectrum most strongly reflected
and b) the color of light in the visible spectrum most strongly transmitted. (nwater=1.33)
air
oil
water
Since 1<1.45, the light reflected from the top of the oil film undergoes phase reversal. The light
reflected from the bottom undergoes no reversal because 1.45>1.33.
a) For constructive interference - Reflection
Substituting for m gives, m=0, λ0=1620 nm (infrared)
m=1, λ1=541 nm (green)
m=2, λ2=325 nm (ultraviolet)
Both infrared and ultraviolet light are invisible to human eye, so the dominant color in reflected
light is green.
b) For destructive interference - Transmission
Substituting for m gives, m=1, λ1=812 nm (near infrared)
m=2, λ2=406 nm (violet)
m=3, λ3=271 nm (ultraviolet)
The dominant color visible to human eye is violet.