To the left you can see three pictures taken with three different types of camera: visible, infrared and millimetre wave. The infrared and millimetre wave cameras show heat coming from the body which you can't see with normal ‘visible’ light

Nearly all objects and materials emit a range of wavelengths of light in a way which depends on their temperature.

Objects have to be ‘red hot’ (about 500ºC) before they visibly glow. Objects at room temperature emit most of their heat in the infrared part of the spectrum, but some of the heat is also emitted at longer wavelengths

Infrared wavelengths are longer than those of visible light so the image is more coarse - in other words it has less RESOLUTION. The millimetre wave image is even coarser because the wavelength is longer still

But you can also see through things with millimetre waves because they behave more like radio signals. The hair and beard appear cold in the IR since they are insulating but the millimetre wave image doesn’t see the hair and beard at all - the heat passes straight through!