Solving for Visible Light Reflection/Transmission: A Homework Help Guide

[Feodalherren]

Homework Statement

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 the light in the visible spectrum
most strongly reflected and (b) the color of the
light in the spectrum most strongly transmitted. Explain
your reasoning.

Homework Equations

2t=m(λ/n)

Where t is thickness and m is the order number.

The Attempt at a Solution

I don't understand part b. I must have dozed off in class (it's an 8am class, don't judge!). How exactly would I go about determining this?

 

[Simon Bridge]

Does that mean you can do part (a)?
Part (b) uses the same principle ... how do you get resonant reflection off a thin film?
What happens when you don't have resonant reflection?

 

[Feodalherren]

Yes I could do part a).

What I'm confused about is the physics behind it. The first question was pretty straight forward plug and chug - I just needed to look for constructive interference patterns.

I guess you get it if you get constructive interference? But that would be the same question... Hmm I have no idea what happens. I must have completely missed this in class or the professor forgot to mention.

 

[Simon Bridge]

The first question was pretty straight forward plug and chug - I just needed to look for constructive interference patterns.

I guess you get it if you get constructive interference? But that would be the same question... Hmm I have no idea what happens. I must have completely missed this in class or the professor forgot to mention.

... OK. Potted lecture:

You light coming back at you from the top surface consists of waves that have been reflected off the top surface added to waves that have been reflected off the bottom surface of the film.

The wavelength where these waves constructively interfere is the dominant color reflected.

The wavelength where these waves destructively interfere is the dominant color transmitted.

http://farside.ph.utexas.edu/teaching/302l/lectures/node152.html

 

[Feodalherren]

You're a God among men Simon! Thanks a bunch! Those links were really helpful and ringed lots of bells for me.

I still don't understand how destructive interference could possibly be related to the dominant color. That makes no intuitive sense to me at all. I thought destructive interference meant that the light got wiped out and we wouldn't see it.

I guess what I'm really saying is I don't understand the difference between the two questions.

 

[Simon Bridge]

The links I gave you tend to focus on reflection.
Sketch the situation for transmission - the light that goes through the film.
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/soapfilm.html