Carrying out the experiment
Set up the projector and prism as described above, and position the clock movement so that the minute hand with thermistor attached passes through the spectrum when 3 o'clock coincides with yellow.
Start Picoscope:
Settings > Timebase > 50 s/div x 1
Settings > Channels > Trace A, Channel Volts, Range Volts, Multiplier x 10
Start a Meter:
View > New Meter > Settings > Parameters > Channel, Volts > Function, DC Signal
Interpose a piece of card to block the light from the thermistor.
Run Picoscope and adjust the scale so the trace is near the bottom of the screen.
Turn the minute hand anticlockwise to say 1 o’clock well above the red.
Remove the card and observe the scope trace as the thermistor moves through the spectrum.
Make a note of the time in seconds as the thermistor reaches the centre of each colour, red, orange, yellow, green, blue, violet.
Print out the graph. Using the times you recorded subtract these times from the running time on the PicoScope graph and draw vertical lines to signify the position of the colours in the spectrum, and label or colour the areas with the appropriate colours.
Questions and discussion of the results
Where in the spectrum is the voltage (temperature) highest?
Is there any heat beyond the violet end of the spectrum? (block the light again by interposing the card)
Compare your graph with William Herschel’s. If you found that the temperature was highest beyond the red end of the spectrum, you have rediscovered infrared radiation.
Further study
If you are lucky enough to have good sunny weather, why not try using sunlight like William Herschel? He went on to try the heating effect from other sources, an oil lamp, a black cast iron stove, etc. Herschel demonstrated that infrared could be focused by a lens in the same way as visible light. He also tried the effect of different coloured glasses, and found that some let through heat but no light. Try putting the heat absorbing glass from the projector back in the optical path. Where does the heat go?
Results & teachers' notes
View results and additional information for teachers
Credits, comments and further info
This experiment was written by Michael Tabb - a committee member of the William Herschel Society in Bath. The society was formed to save the house in Bath where Herschel was living in 1781 when he discovered the planet Uranus with a telescope he had made to survey the entire night sky. This was the only planet to be discovered with a telescope. Before that all the planets had been known since antiquity. Neptune was discovered by mathematical calculation, confirmed by telescopic observation, Pluto again by mathematics, confirmed by photography.
King George III asked Herschel to move near to Windsor so that he could show the royal family the heavens. At Slough, he built the largest telescope in the world, laid the foundations of the science we now call cosmology, and also discovered infrared.
Herschel’s house in Bath is now a museum to his life and work as musician and astronomer. To find out more about him go to www.williamherschel.org.uk