Why don't I see interference when I try the two slit experiment with my flashlight?

Question

It seems like I heard somewhere before it was because it had to be just one frequency of light, but I tried it with my red led light and still no interference.

Answer 1

I did this Quantum Eraser experiment from Scientific American.

http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=DD39218F-E7F2-99DF-39D45DA3DD2602A1
http://www.sciam.com/slideshow.cfm?articleid=DD39218F-E7F2-99DF-39D45DA3DD2602A1&slide=1

Its easy to do at home, all you need is a laser pointer, some thin wire, and some polarized film...very nifty to do and shows very good interference patterns...

I'm not addressing your question because the others have :)

Answer 2

You need coherent light...like laser light. Even colored light, like your red, has many many frequencies rather than the few coherent light has. And, of course, white light from a flashlight, for example, has just about all the visible light frequencies in the spectrum of visible light.

Coherent light is essentially a single frequency, with its harmonics. And it's that single frequency that interfers with each other as it passes through the two slits and projects onto a screen. That's why you see bands of constructive and destructive interference on the screen when coherent light is used.

The hardware stores sell laser light beam level markers...for putting up picture frames on the wall for example. They are not too expensive and they can provide that coherent light you need to do the slit experiment. Pointer beams for pointing at screens while giving talks is another source for coherent light.

Answer 3

You don't really need monochromatic (single-frequency) light, although it helps. The original double-slit experiment, performed by Thomas Young in 1801, actually used unfiltered sunlight (which is definitely not monochromatic).

But you do need to meet the following conditions:

1. The light needs to come from a "point source." This is usually simulated by letting the light fall initially onto a screen in which a single pinhole (or single slit) has been cut. (From there, it continues on to the double-slit screen).

2. The distance between the two slits needs to be small compare to the the distance between the double-slit screen and the final screen.

3. (This is probably the hardest)--The width of the slits has to be very small--comparable to one wavelength of light. Otherwise the light won't diffract (spread out), but will just essentially go "straight through" the slits, and produce just a couple of slit-shaped lines of light on the screen, which is probably what you're seeing.

Answer 4

This is quite fiddly experiment to set up if you are not using the coherent light from a laser.

You need to use Young's original system.
A bright light falling on a *very* narrow single slit as your light source. This is then allowed to fall on a pair of closely spaced narrow slits.

In the days when we used to make our own kit, we used to make our own slits by pinning up a big sheet of paper, drawing lines with a black felt tip pen, photographing the sheet in 35 mm black and white, (over) developing the film and using the mounted negative as the slit.

When lab lasers became available life became a whole lot easier all round!

Answer 5

i think u need a very precise laser beam to do tht....... u cant use ur normal toy laser guns(the 1's tht use 3 miniature batteries and a LED).... i had performed an experiment on interference in my coll practicals...... and they had used a well collimated.... actual laser gun which was almost as big as a small size fire extinguisher..... use a good laser......u might get what u are lookin for..... btw using flashlight wont help........ it doesnt occur with white light..... flashlight is almost shimilar to white light

Answer 6

The light needs to be spatially coherent, which means it needs to be a plane wave. This can be obtained by a collimating lens in front of a point which allows light through.

If light is spatially coherent, but has lots of wavelengths, each wavelength will form its own diffraction pattern, which will be a different spacing for each different wavelength.