# Homework Help: Questions about Young's Double-Slit experiment and light waves intereference

1. Oct 18, 2005

### insertnamehere

Hello. I just have some questions about Young's Double-Slit experiment and light wave interference. I think i got the right answers, but i just want to know if i grasped the concept correctly.
1-If we perform young's double slit experiment under water, how would the interference pattern be affected?
-well, since this is performed under water, we would be dealing with water waves, so they can't really diffract once they pass the slits, right? Because they're mechanical waves and lack that property, therefore there would not be any intereference pattern on the screen.

2- As a soap bubble evaporates, it appears black just before it breaks. Explain this phenomenon in terms of the phase changes that occur on reflection from the two surfaces of the soap film.
-i think it turns black before it breaks because at that point, the beams interfered destructively (which explains the darkness). but i'm not really sure...it turns black where the film of the soap bubble is thinnest since color also depends on the thickness of film.

3-A certain oil film on water appears brightest at the outer region, where it is the thinnest. From this information, what can you say about the index of refraction of oil relative to that of water?
-well, since it is brightest at the outer region, constructive interference occurs there, so would the index of refraction of oil be lower than that of water, since if it were larger, the two reflections would be out of phase and interfere destructively.

Did i get the right concept? Please, I need to move on with my assignment, but I can't do so unless I know whether or not I truly understand this. Thank you very much for your time.

2. Oct 18, 2005

### insertnamehere

PLEASE! I need feedback or else i won't be able to go on with my assignement! I just want to know if i got the right idea!

3. Oct 18, 2005

### Staff: Mentor

I'm sure they mean the usual Young's double slit experiment--with light, not water waves!--just performed underwater. What changes when light travels through water?
The reflections destructively interfere, but why? (What's the phase change on reflection at each surface?)
Right. The reflections constructively interfere, which can only happen if they are in phase--which implies that the index of refraction of the oil is less than that of water. (Compare this with question 2.)

4. Oct 18, 2005

### insertnamehere

Thank you thank you thank you!

5. Oct 18, 2005

### insertnamehere

Sorry, i just need to clarify one thing.
For my first question, when light travels through water, it's speed would obviously decrease, but the interference pattern shouldn't be affected as the beams themselves would remain in phase, etc... Am I right?

6. Oct 19, 2005

### Staff: Mentor

No, not right. If the speed of the light changes, what other characteristic of the light must also change? (Hint: What determines the spacing of the fringes in the interference pattern?)

7. Oct 19, 2005

### insertnamehere

so if we change the speed of the light, it's wavelength would be affected, therefore it would diffract differently from the slits, thus giving us different interference patterns on the screen (from what we would obtain "on land") But then how would it be different, would there be larger spacing between the fringes since the wavelength would decrease (due to change in medium)?

8. Oct 19, 2005

### Staff: Mentor

Exactly.
You tell me: How does the fringe spacing depend on the wavelength? (Figure it out or look it up!)

9. Sep 3, 2011

### ohmuaphysics

fringe separation is directly proportional to wavelength,hence the diffraction becomes more concave?