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Science P3 Topic 1
GCSE edexcel further additional physics: radiation in treatment and medicine
| Question | Answer |
|---|---|
| What is radiation? | Any form of energy emitted from a source, including both waves and particles |
| What is intensity of radiation affected by? | The distance from the source (intensity decreases and distance increases), and the density of the medium through which the radiation is travelling (intensity is lower in a dense medium) |
| How do you calculate intensity? | Intensity (W/m2) = power of incident radiation (W) / area over which it is spread (m2) |
| How is ionising radiation used in medicine and what is it? | Ionising radiation has enough energy to knock electrons out of their shells in an atom causing it to become an ion- they can be used in medicine to damage and destroy cancer cells e.g. X rays, gamma rays |
| How is an image produced and how does this get to the brain? | Light rays pass through the pupil, the cornea and the lens and converge on the retina, where an image is formed. Cells in the retina convert the image into electrical impulses which are carried to the brain by neurones, where they are interpreted |
| What is a near point? | A near point is the closest you can see an un-blurred image |
| What is the function of the ciliary muscles? | They change the shape of the lens to change how much the light rays are refracted: - they relax when the light source is distant, pulling the lens into a thinner shape - they contract when the light source is near, so the lens goes into a fatter shape |
| What is refraction? | Refraction is the change in direction of a wave due to a change in wave speed at the interfaced between two different transparent media |
| How do converging lenses refract light? | Converging lenses refract parallel rays so that they meet at a focal point |
| How do diverging lenses refract light? | Diverging lenses refract parallel rays so that they spread out - the focal point is the point that the rays appear to be coming from after passing through the lens |
| Which lenses produce virtual and real images - what are they? | Converging lenses produce real images which can be shown on a screen & have positive image distances (shown by plus sign). Diverging lenses produce virtual images which can’t be focused on a screen & have negative image distances (shown by minus sign) |
| What determines the strength of a lens? | A lens with more sharply curved faces will be more powerful as it will refract the rays more, so the focal length is shorter |
| How do you calculate the power of a lens? | Power of lens (dioptre, D) = 1 / focal length (metre, m) |
| What is the lens equation and when is it used? | The lens equation is used when the incident rays are not parallel 1/f = 1/u + 1/v (f is the focal length, u is the object distance, and v is the image distance) |
| What is short-sightedness and what is it caused by? | Objects a short distance away are clear but far-away objects are blurred. Caused by the eyeball being too long or the cornea being curved too sharply. It causes the image to focus infront of the retina. |
| How can short-sightnedness be corrected? | By placing a diverging lens in front of the eye as it bends the incoming rays apart so that they can be focussed correctly onto the retina |
| What is long-sightedness and what is it caused by? | Objects a short distance away are blurred but far-away objects are clear. Caused by the eyeball being too short or the lens not being thick or curved enough (ciliary muscles are taut but lens can’t bend light enough.). Light focused behind the retina |
| How can long-sightedness be corrected? | It can be corrected by placing a converging lens in front the eye as it refracts the light rays so that they converge at an earlier point |
| What are the advantages and disadvantages of treatments for short sight and long sight? | Glasses- no operation, can easily be changed. initial expense, temporary solution. Contact lenses- no operation, can easily be changed. infections, temporary solution, can irritate eyes. Laser correction- permanent solution. expensive, medical risk |
| What is the law of reflection? | the angle of incidence = the angle of reflection |
| What is snell's law? | Snell's law links the angle of incidence and the angle of refraction: sin i / sin r = constant = nr / ni (n is the refractive index of the medium) |
| What is the critical angle? | The critical angle is the smallest angle of incidence when a light ray is travelling into a less dense medium at which the angle of refraction is 90º |
| What happens if a light ray is travelling from glass to air at an angle less than the critical angle? | At angles less than the critical angle, the wave’s energy is partially reflected and refracted- as the angle of incidence decreases, there is more refraction and less reflection |
| What happens if a light ray is travelling from glass to air at an angle greater than the critical angle? | At angles greater than the critical angle, there is total internal reflection so all the wave energy is reflected |
| What happens if a light ray is travelling from glass to air at the critical angle? | If a light ray enters a less dense medium at the critical angle it will travel along the boundary between the two media |
| How can critical angle be calculated with Snell's law? | sin i = 1 / ni (n = refractive index) |
| What is an optical fibre and how does it work? | A tight bundle of glass fibres - transmits light from one end to the other by total internal reflection. TIR occurs because the light ray reaches the boundary with air (less dense) at an angle greater than the critical angle. Interface acts as a mirror |
| Where are optical fibres used? | Optical fibres transmit infrared or visible light signals for data communications e.g. TV and telephones. They are also used in endoscopes for keyhole surgery. |
| How do endoscopes work? | They consist of optical fibres in a flexible rod that is inserted into the body. The light carried by the optical fibres reflects off the features in the body and is gathered by an eyepiece lens to form an image |
| What is ultrasound and some features of it? | Ultrasound waves are at frequencies above the human hearing range. Ultrasound waves travel through solid objects but are partly reflected when they enter a different medium |
| What are some advantages of using ultrasound in medicine? | •Ultrasound waves can be focused •Intensity can be controlled |
| What are some uses of ultrasound in medicine? | *In medical scanning the reflected waves are detected & converted into an image on a screen - used for locating kidney stones *Absorption of high intensity ultrasound energy breaks kidney stones *Absorption of ultrasound energy treats injured muscles |
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