On the hairy nature of light—redux

A while ago I reported on an experiment. Using my laser beam I demonstrated the wave nature of light, by measuring its diffraction. One of the referees, for reasons of poetry, wanted to see the experiment repeated with a different wavelength laser.

So, like a good little peer-reviewed scientist I toddled off and repeated the experiment with a red laser beam that very evening. And here, finally, are the results. No surprises: the red laser was refracted more than the green one. About 20% more, which is what I’d have predicted given the difference in rated wavelengths.

But here are the pictures, and the maths. The working for the green laser is given in my earlier post.

First, here’s the experimental set up:

Note the high-tech use of a peg and Scotch Tape to hold the hair (human, brunette) in place. There’s also a slightly fuzzy photo, taken under ambient light (one of those dreadful energy-saving bulbs) in which you can see the beam itself thanks to Rayleigh scattering and the dust in the spare room.

Measuring the green laser

For the red laser, I had to turn off the lights (it only being rated at 1 mW); I also bounced it off a mirror to give me a longer path length and therefore a better node measurement.

Using this method, I measured the third maximum at 13 cm from the central spot. I measured the path length at 6.75 m. Given

nλ = xd/L
where

n = # of maximum
x = distance of maximum
d = slit distance (i.e. the hair width–approximately 100 microns)
L = length of beam

then

nλ = (0.13 m 100 x 10^-6 m) / 6.75 m

so

λ = (1.9259 x 10^-6 m) / 3

therefore the wavelength is 642 nm.

QE, as they say, D.