Question

A film of soapy water (n = 1.33) on top of a sheet of Plexiglas (n = 1.51) has a thickness of 266 nm. What wavelength is most strongly reflected if it is illuminated perpendicular to its surface? (This is the apparent color of the soapy film.) nm

Answer 1

Answer: Wavelength of 1420nm

Explanation: When Ray's are incident perpendicularly the path lenght(difference) = 2t

Where t is the thickness of film given as 266nm.

For reflection to take place and be seen there must be constructive interference. Condition for this is

2t = (wavelength/n of soapy

water)/2

Therefore we have,

4t = wavelength/1.33

Making wavelength subject of formula,

Wavelength = 4*266nm*1.33

= 1.42*EXP{-6} m

= 1420*EXP{-9}m

Wavelength reflected strongly

Is 1420nm.

NOTE: EXP represent 10^