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

Thermal radiation from the human body

Demonstration

This experiment shows that electromagnetic radiation in the infrared region is emitted from warm objects such as the human body.

Apparatus and materials

Mirror galvanometer, with sensitivity of about 20 mm per μA

X-band microwave detector with its horn

Health & Safety and Technical notes


Do not use any source of power on the diode detector. Do not use a Gunn diode source.

Do not use a transmitter. Do not apply any source of power. 

Note: even under good conditions the galvanometer, with a sensitivity in μV, will have a deflection of only about 5% fsd. 
 
David Sumner says: I used a diode detector, Unilab 045.674, which comes complete with a horn. This detector has enormous bandwidth. Any similar X-band receiver can be used. 

Procedure


a Set up the apparatus

b Cover the horn window with metal foil. Zero the galvanometer and carefully switch it to the most sensitive range.

c Remove the foil and point the horn at the body, at a distance of a few centimetres. There will be a noticeable deflection. 

Teaching notes


1 Students may be surprised to discover that they emit thermal radiation. Thermal imaging systems used by the military and by emergency workers (e.g. seeking people trapped in burning or collapsed buildings) detect this radiation. 

You can show that the detector is responding to infrared by placing a simple aluminium reflector, painted black, between the radiation source (human body) and detector. The detector will show no effect. Infrared photons are absorbed by the black coating; any microwaves noise will be reflected without any loss. 
 
2 The experiment can also be used when discussing radio telescopes. While gathering radio emissions from astronomical objects, radio telescopes also detect ‘noise’ in the form of thermal radiation from Earth’s horizon, the atmosphere and the antenna itself. 
 
The operation of a radio telescope involves identifying noise power and improving the signal-to-noise ratio. Radio astronomers think of the various contributions to noise in terms of system noise ‘temperature’. Nobel prize-winners Wilson and Penzias were studying just such effects when they identified cosmic microwave background radiation, corresponding to a black body radiator at temperature 3 K. 
 
3 Electromagnetic radiation will be detected from the head, body, limbs, etc. and also from a plastic bucket of hot water. This will mainly be infrared but may also include some from the microwave region (depending on the detector used). Radiation will not be detected from a metal container, since reflective surfaces are poor thermal radiators. 
 
4 The long wavelength portion of the electromagnetic spectrum gathered by a radio telescope is referred to as the Rayleigh-Jeans region. In this region, as wavelength increases, the solid angle of the beam that an antenna collects also increases, meaning it sees a greater surface emitting thermal noise. But as wavelength increases, the surface brightness decreases. These two effects counteract each other, so the thermal noise power per bandwidth interval is uniform across the Rayleigh-Jeans region
 
5 Electromagnetic radiation gathered will warm the telescope’s detector, producing ‘Johnson noise’, random thermal motions of electrons in a metal conductor. Johnson noise power, P, in watts, given by P = 4kTΔf , where k is Boltzmann's constant in joules per kelvin, T is the conductor temperature in kelvins, and Δf is the bandwidth in hertz. 
 
Some astronomical detectors are cooled by liquid helium to reduce Johnson noise. 

This experiment was originally submitted by David Sumner, a Science Technician at Glebelands School in Surrey. It now incorporates improvements suggested by microwave engineer Jiri Polivka, of Santa Barbara, California.