Year 12 physics - thin films question

[ash]

I'm really stuck on this question:

A soap film is 100 nm thick. What wavelength of light will be most strongly reflected by this film; that is, what colour will it appear? (n soap = 1.33).

Any help you could provide would be greatly appreciated.

 

[lektor]

um.. would this be asking which would refract the most?

If so i would say red..

 

[kevinalm]

I know you need to adjust for the wavelength change due to n, but I can't remember if it's a 1/2 wave or a full wave coating. One is minimum reflection and the other maximum. Either

100nm x 1.33 x 2 or 100nm x 1.33

 

[Chi Meson]

there is 1/2 wave inversion at the top surface, but no inversion at the bottom surface. The "most reflected" wavelength will experience constructive interference from both reflections. To do so, the second ray will travel a total distance through the film equal to 1/2 its wavelength in the film.

Can you take it from there?

 

[kevinalm]

just to refresh my memory, what determines the 1/2 wave inversion or not? Is it the sense of the the transition low index > high index vs. high > low, or something else?

 

[vincentchan]

low index > high index----reflect with 180 degree phase shift
high > low----reflect with no phase shift

the above two conditions + interference = answer of this problem