Vice
I have a question from a past paper, based on one of my weakest areas - waves. The question says that a soap film is formed in a rectangular wire, and positioned so it stands vertically in front of the lamp - which emits light with a wavelength of 4.4*10^(-7). It also shows an image of light entering the soap film and passing straight through, and light entering and being reflected twice, once at each side of the film, and passing through (like a 'Z' shape). I got the simpler questions about path difference due to the 'Z' etc., but the latter 3 confuse me.

The first asks why a student sees alternate bright and horizontal stripes on the film (of thickness 2.22 *10^(-7)). I had a guess at light travelling different distances to the film from the lamp, although I'm not sure this is right.

The second question asks why the whole pattern of dark and bright gradually moves downwards.

The third asks 'In fact the thickness of the film increases from the top of the film to the bottom Suggest a reason for this'. Having read this, this could be the answer previous to the two above (bad photocopying) which could lead the answer to be related to path differences due to the thickness of the film rather than the distance between the sodium lamp and the film. I presumed it was due to gravity pulling the film downwards.

Just noticed a fourth right at the bottom saying to add another path of light to figure 2, where it shows light moving through straight, and one moving like a 'Z'. I put the light reflecting in the alternate direction, so hitting the far end of the film and moving left rather than right.
Help would be appreciated.
Trippy
QUOTE (Vice+Jan 1 2008, 08:02 AM)
I have a question from a past paper, based on one of my weakest areas - waves. The question says that a soap film is formed in a rectangular wire, and positioned so it stands vertically in front of the lamp - which emits light with a wavelength of 4.4*10^(-7). It also shows an image of light entering the soap film and passing straight through, and light entering and being reflected twice, once at each side of the film, and passing through (like a 'Z' shape). I got the simpler questions about path difference due to the 'Z' etc., but the latter 3 confuse me.

The first asks why a student sees alternate bright and horizontal stripes on the film (of thickness 2.22 *10^(-7)). I had a guess at light travelling different distances to the film from the lamp, although I'm not sure this is right.

The second question asks why the whole pattern of dark and bright gradually moves downwards.

The third asks 'In fact the thickness of the film increases from the top of the film to the bottom Suggest a reason for this'. Having read this, this could be the answer previous to the two above (bad photocopying) which could lead the answer to be related to path differences due to the thickness of the film rather than the distance between the sodium lamp and the film.    I presumed it was due to gravity pulling the film downwards.

Just noticed a fourth right at the bottom saying to add another path of light to figure 2, where it shows light moving through straight, and one moving like a 'Z'. I put the light reflecting in the alternate direction, so hitting the far end of the film and moving left rather than right.
Help would be appreciated.

It's called thin film interference.

You can acheive the same effects with a quartz wedge - in fact, they use the standard quartz wedge in mineagraphy as a galibration for polarized light microscopy.

First off, you're using a monochromatic light source, a sodium vapour lamp.

Thin film interference occurs whenever you have a film that has a thickness comparable to the wavelength of the light.

http://physics.bu.edu/py106/notes/Thinfilm.html

Basically, the light being relflected from the top surface undergoes a phase change, but the light traveling through the medium and reflected off the bottom surface travels a greater distance (twice the thickness of the medium being passed through) so if the thickness of the film is 1/4 the wavelength of the light, the light constructively interfers with itself, but if the film is half the wavelength of the light, this becomes destructive interference.

I seem to recall that you can calculate the thickness by counting the number of bands- in Geology we use polychromatic light, so we get 1st, 2nd, 3rd, and some times 4th order colours

My recollection is that part of the answer you may be looking for has something to do with higher order reflections - light that's been bounced back and forth two or three times. I also have an inkling that if you're dealing with light from a point source (in many respects a good approximation) then light that enters from some angles will be end up travelling (for example) 3λ/4 total distance resulting in a bright band - so you're on the right track, it's about different distances, but it's what's going on inside the film that's important.

As far as the downards movement goes., the film starts off with the two surfaces being paralell, but then the soap/water mixture begins to accelerate downwards under the influence of gravity, this results in the top of the film becoming thinner, and the bottom becoming wider. As this process progresses, the majority of the mass of the original film becomes concentrated in the bottom, which will eventually lead us to a film that approaches zero thickness almost everywhere except right at the bottom. This entire process results in the apparent migration of the stripes down the film. Essentially what happens is that the first stripe becomes thicker.

I hope this all helps.

edit:
Further elaboration of specific examples, including soap film: http://hyperphysics.phy-astr.gsu.edu/Hbase...t/thinfilm.html

A Java (I think) applet that includes an interactive illustration of what happens (somewhat styilized)
http://webphysics.davidson.edu/physlet_res...6_thinfilm.html

And a shockwave flash that illustrates how anti reflective coatings work.
http://mysite.verizon.net/vzeoacw1/thinfilm.html
Sapo