PHY ICS - The Institute of Physics in Ireland

Estimating the speed of light from some
measurements with a laser diode
GREECE:
L I G H T
We can verify an order of magnitude for the speed of light by measuring the
wavelength of red light from a laser pointer and the striking voltage of the laserdiode.
The author made a diffraction grating of 0.16mm line spacing, by using a
drawing package on a computer and printing out the result to make a 35mm slide.
We found that a diffraction grating of 300 lines per mm worked very well. Even
a pair of homemade Young’s slits worked. A 9V battery is connected across a
potentiometer. The variable output is measured on a voltmeter and supplied to
the laserdiode. The minimum voltage at which the laser strikes (giving a very faint
light intensity) is recorded. The energy of an electron passing through this potential
difference gives the approximate enery of the photons emitted. This is divided by
Planck’s constant to give the frequency of the red light. Also the spacing between
the interference fringes produced by the grating is recorded and this is used to
calculate the wavelength of the red light. The product f x λ is our approximation
of the speed light.
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