Virtually all metal detectors used at airports are a type of pulse induction system. These pulse about 100 times per second and create a momentary magnetic field.
When a metal body enters the metal detector this causes an opposite magnetic field. It works in a way similar to hearing echoes and the detector picks up these magnetic type echoes and the alarms go off.
Many people mistakenly think that when they pass though the metal detector they are passing through an x-ray machine. This is not the case as it is only a metal detector.
As passengers walk through the metal detector their hand luggage will pass through the x-ray machine. The x-rays penetrate nearly all objects.
The computer reads the x-ray signals and represents these on monitors as images in different colors depending on whether they are organic, inorganic or metal.
Airport security officers are trained to search for anything suspicious in the images. Organic material is always displayed in orange on computer screens to make it easy to see. Most improvised explosives are made out of organic material.
Many photographers worry that the hand luggage x-ray machines will damage their film. The hand luggage x-ray machines are lower in radiation and are not powerful enough to damage photographic film. However hold luggage has to pass through much stronger x-ray machines which can damage film. So it is always best to carry photographic film with you on a flight.