Infra-red radiation from the human body
Practical Activity for 14-16
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."
- Set up the apparatus
- Cover the horn window with metal foil. Zero the galvanometer and carefully switch it to the most sensitive range.
- Remove the foil and point the horn at the body, at a distance of a few centimetres. There will be a noticeable deflection.
- Students may be surprised to discover that they emit infra-red radiation. Thermal imaging systems used by the military and by emergency workers (e.g. seeking people trapped in burning or collapsed buildings) detect this infra-red radiation.
- You can show that the detector is responding to infra-red radiation by placing a simple aluminium reflector, painted black, between the radiation source (human body) and detector. The detector will show no effect. Infra-red photons are absorbed by the black coating; any microwaves noise will be reflected without any loss.
- The experiment can also be used when discussing radio telescopes. While gathering radio waves emitted from astronomical objects, radio telescopes also detect ‘noise’ in the form of infra-red 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 a temperature of 3 K.
- Electromagnetic radiation will be detected from the head, body, limbs, etc. and also from a plastic bucket of hot water. This will mainly be infra-red radiation 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 radiators of infra-red radiation.
- 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 noise consisting of infra-red radiation. But as wavelength increases, the surface brightness decreases. These two effects counteract each other, so the noise power per bandwidth interval is uniform across the...
- Electromagnetic radiation gathered will warm the telescope’s detector, producing ‘Johnson noise’, random motions of electrons in a metal conductor. Johnson noise power, P , in watts, given by P = 4 kT Δ f , where k is Boltzmann's constant in joules per kelvin, T is the conductor temperature in kelvins, and Δ f 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.