Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Beyond the visible spectrum

Demonstration

This experiment can be used when introducing the electromagnetic spectrum. It shows that there is a form of radiation beyond the visible.

Apparatus and materials

Prism, high-dispersion

Light source, compact

Positive lens, large

Screen, white

Infra-red detector, metered output

Power supply, low voltage, continuously variable.

Health & Safety and Technical notes


Read our standard health & safety guidance

Radiation transistor behind the slitFor an infra-red detector, make a potential divider using a photo-diode in series with a 100kΩ resistor, and a 5 V supply across the pair. Connecting a digital voltmeter across the resistor will give the required metered output. 

Compact light source: 100 W at 12 volts 

The power supply should supply 8 amps.

 

Procedure


a Set up the compact light source. The lamp filament should be small enough that no slit is needed. 

b Place the lens about 20 cm from the lamp. (If the lens is plano-convex, its plane face should be towards the lamp.) Move it to make an image of the filament on a white screen, 2 or 3 metres away. 
 
c Place the prism just beyond the lens and move the screen round to catch the spectrum at the same distance from the lens as before but in the new direction. 
 
The spectrum will be pure enough for this demonstration if the prism is turned to minimum deviation. To make the spectrum longer, twist the screen to catch it obliquely. 
 
d Move the detector across the spectrum near the screen and observe the output readings. 


Teaching notes


The radiation emitted by a hot filament has its maximum not in the visible spectrum, but beyond the red - in the 'infra-red' part of the spectrum. Light is just one (small) part of a family of radiations called the electromagnetic spectrum. 

This experiment was safety-checked in February 2006