How do x-rays work? Amber, Blackboys School. Answered by Jonathan Brown.

8/27/2015

Jonathan Brown

8/31/2015 06:08:52 am

Hi Amber,

Let’s start with a little background. What are X-rays themselves?

X-rays are simply a frequency on what we call the electro-magnetic spectrum. This is the spectrum of all the frequencies of radiation energy that are emitted by objects (such as the Sun). Part of the electromagnetic spectrum contains the visible light we see with our eyes, however, this visible light is only a tiny part of the complete spectrum of electro-magnetic radiation energy.

The length of the light waves we see ranges from about 400 nanometres to 700 nanometres (a nanometre is one billionth of a metre in length – pretty small!). The length of the waves on the complete electro-magnetic spectrum range from about a picometre (a thousandth of a nanometre) up to about a hundred-thousand kilometres in length, so as you can see there is much more of it outside the range we can visibly see than inside the range. We can also measure the frequency of these waves and we find that the very short waves have a very high frequency, the very long waves have a very low frequency (the frequency of the waves is how many times the wave passes in a measure of time. We usually measure frequency in “waves per second”, known as Hertz). The big waves (low frequency) below the visible light are the micro and radio waves – we use these to broadcast radio, TV, etc. The small waves (high frequency) above the visible light are the high energy rays – X-rays & gamma rays. The shorter the wavelength (and higher the frequency) the greater the amount of energy the wave contains, hence why gamma rays at the top of the spectrum are so dangerous (gamma radiation is the damaging radiation we talk about in nuclear power stations for example)

Ok, that’s the background of what X-rays are, they are, in effect, very high frequency light, for want of a better explanation. However, we can’t see this light and it contains much, much more energy than the light we can see (that is why we should not be exposed to a lot of X-rays, the high energy in them can damage our bodies). When someone takes an X-ray of a part of your body, a sensor is placed one side of the subject to be X-rayed and an X-ray lamp the other. This sensor used to be similar to photographic film that would have to be developed by a chemical process, like old photography. These days, much like cameras are all digital and don’t need film anymore, X-ray machines also use digital sensors and don’t need film either. When the X-ray lamp is switched on (only for a very short time, like the flash in a camera) as the X-rays pass through the subject (your body) from the lamp on one side to the sensor on the other the energy in the rays is absorbed or reflected in different amounts by the various materials it passes through. For example it’s mostly reflected or absorbed by bones, therefore any bones between the lamp and the sensor cast a shadow on the sensor, so the sensor won’t receive any X-rays in this area.

Different other materials in the body will reflect/absorb by greater or lesser amounts, varying the amount of X-rays that reach the sensor at any location, just like when you take a photograph the amount of light is focused onto the sensor by a lens to a greater or lesser amount in different areas. When you then look at the image on the sensor, you will be able to see the more solid parts of the subject (such as bones) compared to the softer material.

Jonathan Brown

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