Explaining the Rainbow Colors in Thin Films: Interference Homework Problem

[physgrl]

Homework Statement

When light is shined on thin films the interference pattern results in:

a. there is no interference pattern

*b. rainbow like colors

c. light and dark pattern with no separation of colors

d. a mirror quality reflection

e. polarization of the light

The Attempt at a Solution

I thought the answer was c because I thought all interference was seen this way but the answer key says the answer is b. Can someone explain why?
Thanks!

 

[NewtonianAlch]

Thin-film interference

Interference between light waves is the reason that thin films, such as soap bubbles, show colorful patterns. This is known as thin-film interference, because it is the interference of light waves reflecting off the top surface of a film with the waves reflecting from the bottom surface. To obtain a nice colored pattern, the thickness of the film has to be similar to the wavelength of light.

An important consideration in determining whether these waves interfere constructively or destructively is the fact that whenever light reflects off a surface of higher index of refraction, the wave is inverted. Peaks become troughs, and troughs become peaks. This is referred to as a 180° phase shift in the wave, but the easiest way to think of it is as an effective shift in the wave by half a wavelength.

http://physics.bu.edu/~duffy/py106/Diffraction.html

 

[physgrl]

but why does it happen? i understand the bright and dark fringes due to constructive and destructive interference but i don't get how we achieve the colors through that process

 

[gneill]

but why does it happen? i understand the bright and dark fringes due to constructive and destructive interference but i don't get how we achieve the colors through that process

The question is not well phrased because they don't specify the nature of the light being shined on the film. Does the observed pattern depend upon the wavelength of the light? What happens if the light is white light?

 

[physgrl]

it should not depend on the wavelenght right because its always going to be 1/2 a wavelength out of phase. ohhh only if its white it will be a black and white pattern so the color/colors of the light will change the pattern right?

 

[gneill]

it should not depend on the wavelenght right because its always going to be 1/2 a wavelength out of phase.

Are you saying that all light has the same wavelength? So every color of light will produce the same pattern in exactly the same locations?

ohhh only if its white it will be a black and white pattern so the color/colors of the light will change the pattern right?

What color is white light?

 

[physgrl]

no that's not what i meant...im saying the constructive and destructive interferences will happen in the same relative locations

all colors :)

 

[gneill]

no that's not what i meant...im saying the constructive and destructive interferences will happen in the same relative locations

all colors :)

I do believe you've got it

 

[physgrl]

thanks! :)