Light Interference Problem, the hair between the slides...

[hellocubed]

http://imgur.com/wfwXk

A.) I only
B.) I and II only
C.) I and III only
D.) I and II and III


I really do no understand this scenario.

So... light is being shined UP perpendicular to the slide, and then it is bent by the n difference between the glass and the air, and then bent again by the n difference of the air and the glass, then bent Again by the n difference of the glass and the air.

So why is there an interference effect? I would understand if the light was being reflected back, because then you have 2 different rays:
1.) the light reflecting off the top
2.) the light reflecting off the inner surface (no in or out of phase.)


When you are not considering reflection, the entire body of the wave just changes phase together and there should be no effect.

 

[Dasypus]

You've got one crucial thing wrong in your description: light is being shone *down* onto the apparatus, and you're looking down onto it as well.

Check out the diagram here: http://en.wikipedia.org/wiki/Interference_%28wave_propagation%29#Optical_interference

It seems to be basically the same as this problem. Any glass slide will have some reflection, and 'light' and 'dark' are relative.

What answer is given in the text, anyhow?

 

[hellocubed]

Oh WOW, that explains quite a lot

The answer is C

 

[Morsetlis]

Decreasing wavelength will give smaller fringes.

If you move the hair closer in, the distance between the two slides increase. The bigger the distance, the more wavelengths can fit in that distance -> more fringes. More fringes -> smaller fringes.