# 3 questions on physical optics

shemer77

## Homework Statement

Hi guys, I have 3 questions on physical optics, I really don't know where to start or what equations to use. Sorry for the horrible drawings by the way

http://yfrog.com/0dfigure1p
1) Coherent monochromatic light of wavelength, lambda, in air is incident on two narrow slits, the centers of which are 2.0mm apart, as shown above. the interference pattern observed on a screen 5.0m away is represented in the figure by the graph of light intensity I as a function of position x on the screen.
a) what property of light does this interference experiment demonstrate?

b) at point P in the diagram, there is a minimum in the interference pattern. Determine the path difference between the light arriving at this point from the two slits.

c) determine the wavelength, lambda, of the light.

d) briefly and qualitatively describe how the interference pattern would change under reach of the following separate modifications and explain your reasoning.
I. the experiment is performed in water, which has an index of refraction greater than 1.

II. one of the slits is covered.

III. the slits are moved farther apart.

2) Glass lenses in cameras and other optical instruments are usually coated with one or more transparent thin films, such as magnesium fluoride to reduce or eliminate unwanted reflection. Carl zeiss developed this method; his first coating was 1 x 10^2nm thick. Using n = 1.38 for magnesium fluoride, what visible wavelength would be eliminated by destructive interference in the reflected light?

3) Semiconductors such as silicon are used to fabricate solar cells - devices that generate electricity when exposed to sunlight. solar cells are often coated with a transparent thin film, such as silicon monoxide (n=1.45) to minimize reflective losses. a silicon solar cell ( n=3.5) is coated with a thin film of silicon monoxide for this purpose. determine the minimum thickness of the film that will produce the least reflection at a wavelength of 552nm.
http://yfrog.com/05figure2p

## The Attempt at a Solution

1a) is it the principle of linear superposition?
1B) i really have no idea, because I am looking through my notes, and i don't see anything that can help me with this problem
1c) there are a couple of equations i can use, but I am not sure, is this similar to youngs double-slit experiment?

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## Answers and Replies

Science Advisor
Homework Helper
1) Coherent monochromatic light of wavelength, lambda, in air is incident on two narrow slits, the centers of which are 2.0mm apart, as shown above. the interference pattern observed on a screen 5.0m away is represented in the figure by the graph of light intensity I as a function of position x on the screen.
a) what property of light does this interference experiment demonstrate?

Hi shemer77!

Show us what you've tried, and where you're stuck, and then we'll know how to help!

Start with 1a).

shemer77
1a) is it the principle of linear superposition?
1B) i really have no idea, because I am looking through my notes, and i don't see anything that can help me with this problem
1c) there are a couple of equations i can use, but I am not sure, is this similar to youngs double-slit experiment?

Staff Emeritus
Science Advisor
Homework Helper
1a) Let's come back to this later.

1b) You can use geometry to figure out the path difference. Note that point P, and the locations of the 2 slits, determine a triangle.

1c) Yes, it's similar to Young's double slit experiment.

Homework Helper
1a) is it the principle of linear superposition?
1B) i really have no idea, because I am looking through my notes, and i don't see anything that can help me with this problem
1c) there are a couple of equations i can use, but I am not sure, is this similar to youngs double-slit experiment?

(b)
I have looked at your diagram several times but I just can't find point P ?