Spark image

Experimental measurement of Planck's constant

The experiment shown in the following diagram can be used to show the stopping potential for a photo cathode surface when it is illuminated by radiation of different wavelengths.

Light is shone through a coloured filter from a white light source onto the photo cathode of a photoelectric cell. The lamp is powered by an a.c source and so with no p.d on the cathode an alternating trace can be seen on the oscilloscope.

For each coloured filter the positive potential on the cathode is increased until the oscilloscope trace is flattened - this means that no photoelectrons are reaching the detector.
A graph can then be plotted of stopping potential (electron energy) against frequency and the results are shown in the next diagram.

Using Einstein's equation you should see that the gradient of the line is h/e and so this experiment can be used to measure Planck's constant h if the electron charge is known.
The intercept on the frequency axis is fo.

If you could change the sensitive surface to another metal with a higher work function then since the gradient of the line is constant (h/e) you would simply get a second line parallel to the first but shifted to the right.

This simple experiment is very good evidence that light sometimes behaves like a stream of particles.

Uses of the photoelectric effect:- TV camera tubes, light meters and solar cells.

© Keith Gibbs 2011