They are high frequency, short wavelength electromagnetic waves. Their wavelength is roughly the same size as an atom.
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Name 2 materials X-rays are absorbed by
Dense materials such as bone and metal
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What is a charged-couple device?
Silicon chip the same size as a postage stamp, divided into a grid of millions of identical pixels. They detect X-rays and produce electronic signals which are used to form high resolution images.
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How do CT scans form images of the body?
They use X-rays to produce high resolution images of soft and hard tissue. The patient is put inside a cylindrical scanner and an X-ray beam is fired through the body and picked up on detectors.
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How can X-rays be used to treat cancer?
X-rays can cause ionisation - they can kill living cells in high doses. The X-rays have to be carefully focussed and at the right dosage to kill cancer cells without damaging too many normal cells.
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What precautions can radiographers take to minimise their radiation dose?
They wear lead aprons, stand behind a lead screen or leave the room (lead absorbs X-rays)
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What is ultrasound?
A sound with a higher frequency than we can hear (20,000Hz+)
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What is ultrasound used instead of X-rays to take a picture of a fetus?
Ultrasound is non-ionising and is therefore safe. X-rays are ionising (cause cancer) so are not safe for use on developing babies.
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What are the advantages of using CT scans over ultrasound scans?
Ultrasound is usually fuzzy, which can make it hard to diagnose some conditions. CT scans produce detailed images and can be 3D which can make it easier to work out the problem and plan complicated surgery.
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What is refraction?
Refraction is when waves change direction as they enter a different medium. This is caused by a change in density from one medium to another which changes the speed of the waves.
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What is the formula for refractive index?
Refractive index (n) = sin i/sin r
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What is meant by the principal focus of a lens?
On a converging lens it is where rays parallel to the axis all meet. On a diverging lens it is the point where rays hitting the lens parallel to the axis appear to all come from. There is a principal focus on each side of the lens
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What type of lenses are used to create magnifying glasses?
Converging lenses
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What is a converging lens?
Convex. It bulges outward
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What is a diverging lens?
Concave. It caves inward.
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What affects the focal length of a lens?
The more powerful the lens, the more strongly it converges rays of light, so the shorter the focal length
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What is the far point of vision?
The furthest point that the eye can see comfortably. For normally-sighted people that's infinity.
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What is the near point of vision? Give an approximate value of the near point for adults.
The closest point that the eye can focus on. For adults the near point is approximately 25cm.
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What are 2 causes of short sight and long sight?
Short sight is caused by the eyeball being too long or by the cornea or lens system being too powerful. Long sight is caused by the eyeball being too short or the cornea and lens being too weak.
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How can short and long sight be corrected by lenses?
To correct short sight you need to put a diverging lens in front of the eye. This diverges light before it enters the eye which means the eye can focus it on the retina. Long sight can be corrected by a converging lens. The light is refracted.
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How can lasers be used to correct vision problems?
A laser is used to vaporise some of the cornea to change its shape which changes its focussing ability. This can increase or decrease the power of the retina.
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What is total internal reflection?
When the angle of incidence is greater than the critical angle and no light comes out. (all internally reflected)
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What happens if the angle of incidence is less than the critical angle?
Most of the light passes out but a little bit of it is internally reflected
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What is a refractive index?
The refractivce index of a medium is the ratio of speed of light in a vacuum to speed of light in that medium
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What happens if the angle of incidence is more than the critical angle?
No light comes out. It's all internally reflected (total internal reflection)
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What is an endoscope?
A thin tube containing optical fibres that lets surgeons examine inside the body.
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Name one medical technique made possible by endoscopes
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