Science Buddies: "Ask an Expert"

Hey, I got a science project for summer vacations, after dropping "solid motor rockets" as an idea due to time limitations I decided to pursue Thomas Young's double-slit experiment to prove light exhibits wave-like properties, so uh, I did my bit of research and trying to find more, to replicate it, I have the required materials but I don't know how to set the apparatus up correctly, I mean what should be the ideal distance from the light source and the slits, the two slits and the slits from the screen, can someone help me regarding this?

It's pretty tricky to set up a two slit experiment with light because of the very short wavelengths involved. To work like the textbook presentation you need slits that are less than a wavelength wide separated by 10-100 wavelengths; since the optical wavelength will be of order 1e-6 m, the experiment would involve fabrication with dimensions in the range of, very roughly, 1e-5 cm to 1e-2 cm -- too small for convenient fabrication. Practical 2 slit interferometers are typically fabricated with larger slit spacings that result in very finely spaced interference patterns that must be observed with a magnifying lens system. Also, temperature changes, vibration, and air currents can all influence optical interference patterns. The nicest solution to these problems that I have seen for college physics instructional lab setups is to use microwaves instead. Microwaves can be readily made with wavelengths of a few cm, which brings the scale of the apparatus to a convenient size. The magnetrons used in microwave ovens produce a wavelength of around 13 cm which would be fairly convenient. They generate enough power (~1 kW!) that a very simple detector can be used -- just a microwave diode and a multimeter plus a half-wave (~6 cm) dipole antenna made from two lengths of solid copper wire, e.g. paperclips. Yet another way to go would be sound waves, 3.4 cm wavelength at 10 kHz, or ~1 cm at 30 kHz, which has the advantage of being inaudible (the 10 kHz whine could become rather tiresome at the lower frequency). Many speakers use "tweeters" that work all the way up to 30 kHz, as do the common electret or piezoelectric microphones.

harry14,

John Dreher is certainly right that fabricating a two-slit experiment for light is difficult because the wavelength of light is so small.

You can find some ideas about how to do it at http://www.physicsforums.com/showthread.php?t=589073.

Please consider that microwave ovens cook things. If you ripped the magnetron microwave generator out of an oven and operated it in the open, it might cook you!

I like the idea of using sound waves since you can build a double slit experiement on a centimeter scale or larger. You could make detailed measurements of the sound intensity with a small microphone. Blocking out sound leaking around your setup would be difficult.

A simple way to observe diffraction is to use prefabricated diffractors. See https://www.sciencebuddies.org/science- ... p011.shtml.

Good luck, WW

Thank you! I get the idea now, though I am not familiar with the microwave idea, I will give it a try too, I have one more question, the interference pattern or the linear pattern produced in any slit case would be too small on the screen, how can I create a magnifying lens system such that I can display it somewhere else? (not digital).

harry14,

A small interference pattern could be magnified with an inexpensive microscope such as those sold for home experimenting. A search for "microscope" at Amazon.com will show you many suitable ones.

Remember to be extremely careful if you try anything with microwaves.

WW

Thank you, I already own a microscope, I mean how can you see a pattern placed on a vertical screen through a microscope, sorry maybe I'm being foolish but it seems pretty strange to me, can you please answer the question?
Also would sodium lamps work finer as a monochromatic light source than laser light if in case I do not use microwaves?

harry14,

You have several options to couple your microscope to a vertically oriented diffraction pattern.

Your microscope doesn't have to sit on a level table. If you provide some way to clamp it to a vertical surface, it works just fine. Of course, this will require you to sit or lie so that you can look horizontally into the eyepiece.

Another option is to build your double slit setup so that the light shines vertically onto a horizontal screen. Then you can use the microscope in the normal orientation.

A third option would be to place a mirror at 45 degrees to reflect the diffraction fringes from horizontal to vertical.

You may think of other arrangements.

Sodium light has most of its energy in an orange-colored, nearly-monochromatic emission, so it should work just fine. It needs to be very bright because you get very little of the emitted light through the slits. A laser not only emits monochromatic light, but it also emits most of the light in just one direction. The total light emitted doesn't have to be nearly as strong since you can aim it at the slits.

Keep us posted on your progress. WW

Hey guys, I have successfully built my project, it's working fine, the only problem being the fringes are very less visible due to illumination, I used a laser light, a glass slide, covered it with metal foil and made two sharp narrow slits with a razor, and a convex and concave lens for magnifying the pattern.
My presentation is due tomorrow and I'm collecting all information I can about the whole thing today, one thing I wanted to ask is, to find the distance between successive fringes (bright or dark), there's this equation 2*pi*L/k*d where 2*pi is for finding distance between successive bright fringes, k is the wavenumber, L is the distance between slit screen and projection screen and d is the distance between the two slits, it is derived using three figures, I found the derivation on a site, but I am confused as to how a step was done, here's d link http://skullsinthestars.com/2009/03/28/ ... xperiment/
you can scroll down a little and find the equations, so what I'm not getting is how does he end up with sinθ is approximately equal to x/L, that's the only thing bugging me, if anyone could explain?

harry14,

The answer to your question is fairly simple, but it is explained best by referring to one of the figures in your reference. I have put the answer with figures here.

I will check back in a few hours to see if you have any additional questions.

Glad to hear you've made good progress, WW