mrmackenzie

x-rays

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X-rays are a form of electromagnetic radiation.  They have a much higher frequency than visible light or ultraviolet.  The diagram below, taken from Wikipedia, shows where x-rays fit into the electromagnetic spectrum.

image by Materialscientist

Wilhelm Röntgen discovered x-rays and the image below is the first x-ray image ever taken.  It shows Mrs. Röntgen’s hand and wedding ring.  The x-ray source used by Röntgen was quite weak, so his wife had to hold her hand still for about 15 minutes to expose the film.  Can you imagine waiting that long nowadays?

This was the first time anyone had seen inside a human body without cutting it open.  Poor Mrs. Röntgen was so alarmed by the sight of the image made by her husband that she cried out “I have seen my death!” Or, since she was in Germany, it might have been “Ich habe meinen Tod gesehen!” that she actually said.

Röntgen continued to work on x-rays until he was able to produce better images. The x-ray below was taken about a year after the first x-ray and you can see the improvements in quality.

 

Notice that these early x-rays are the opposite of what we would expect to see today. They show dark bones on a lighter background while we are used to seeing white bones on a dark background, such as the x-ray shown below.  The difference is due to the processing the film has received after being exposed to x-rays.

In hospitals, x-rays expose a film which is then developed and viewed with bright light.  X-rays are able to travel through soft body tissue and the film behind receives a large exposure.  The x-rays darken the film. More dense structures such as bone, metal fillings in teeth, artificial hip/knee joints, etc. block the path of x-rays and prevent them from reaching the film.  Unexposed regions of the film remain light in colour.

Röntgen’s x-ray films would have involved additional processing steps.  The exposed films were developed and used to create a positive.  In creating a positive, light areas become dark and dark areas become light.  So the light and dark areas in Röntgen’s x-rays are the opposite of what we see today.  Our modern method makes it easier to detect issues in the bones as they are the lighter areas.

Röntgen was awarded the first ever Nobel Prize for Physics in 1901 for his pioneering work in this field of physics.

Medical imaging has come a long way since Röntgen’s discovery of x-rays.  This promotional video from German company Siemens outlines the advances that have been made since the early 20th century.

ultraviolet

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We spent some time looking at the physics of ultraviolet radiation last week.

image courtesy of sonrisaelectrica

The section of the electromagnetic spectrum with wavelengths ranging from 10nm to 400nm is called ultraviolet radiation (uv for short).  Sunlight contains uv rays and it’s those uv rays that are responsible for the suntan you get during the summer holidays.  This Australian animation shows how the ultraviolet in sunlight causes our skin to tan and explains why too much uv will damage our skin.  The SunSmart page has loads of information on staying safe in the sun.

The damage that uv can do to cells is put to good use in some sterilisation equipment, such as this bottle for safe drinking water and the toothbrush sanitiser shown below.

The Nobel Prize for Medicine was awarded to Niels Rydberg Finsen in 1903 for his research into the effects of ultraviolet on the bacteria that cause tuberculosis.

We used uv banknote checkers in class to view some of the security features built into British banknotes. This image of a Clydesdale Bank £10 note shows part of the pattern that can only be seen under uv light.

image from Science Photo Library

There is a Bank of England leaflet (pdf) with further information on the security features in our banknotes.

Remember that whenever something glows under a uv light, we’re not seeing the uv radiation itself because our eyes can’t detect ultraviolet.  Instead, we see the fluoresence; visible light given out in response to the uv falling on the material.

Kuba demonstrated fluorescence in class with some uv hair gel.  Here he is with some of the gel in his hair.

but we don’t see anything until we turn on the uv light.

Cool, eh?

You can even buy genetically modified tropical fish that glow under uv light.

drawing ray diagrams

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We can use ray diagrams to predict whether or not a lens will produce a useful image of an object.  The diagram is also able to predict the relative size and orientation of the image, compared to the object.

Watch these short video guides to drawing ray diagrams.

This second video shows an object more than two focal lengths away from the lens.

Finally, for an object within one focal length of the lens.

Once the ray diagram is complete, we need to describe the image that has been formed.  The description must tell us about the size, orientation and type of image that is formed.

Size
If the image is larger then the original object, the image is magnified
If the image is smaller than the original object, the image is diminished.

Orientation
If the image is the same way up as the object, it is upright.
If the image is upside down compared to the object, it as inverted.

Type
If the object and image are on opposite sides of the lens, it is a real image.
If the object and image are on the same side of the lens, it is a virtual image.