thin film interference
drawing shows how light behaves to get to your eye and from this can determine what you see
when light goes from one medium to next, it refracts at the interface (snell's law)
it also reflects at the interface.
if the index of refraction of the new medium is greater than the index of refraction of the
original medium, the reflection picks up a 1/2 wavelength shift
if the index of refraction of the new medium is less than the index of refraction of the
original medium, the reflection does not pick up a 1/2 wavelength shift
combining the effect of the reflection with the path of ray 2 being twice the thickness
of the film longer than the path of ray 1, we get a relationship between the thickness
of the film, the wavelength of the light
to simplify the math, assume the incidence is perpendicular to the interface so we do not need snell's law
then 2t +(1/2 wavelength shift, if needed) = integer number of wavelengths of light in the medium gives constructive interference
while 2t +(1/2 wavelength shift, if needed) = (integer +1/2) number of wavelengths of light in the medium gives destructive interference
the different colors are therefore related to the thickness of the film and any underlying material
some colors are constructively interfered, so we see that color
some colors are destructively interfered, so we see do not that color