# Physics Unit 3 Revision Cards

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what are CAT scans?
using x-rays which are absorbed by some materials but not others. a negative image is produced
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what are ultrasounds?
high frequency sound waves (above 20 000 Hz) reflect off features inside the body to form an image
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what are endoscopes?
using visible light where light reflects off features to form an image
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creates reactive ions in the body, which can cause damage, e.g. gamma rays and x-rays are used to detect and treat cancer cells
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(e.g. light, ultrasound), is used in treatments. lazers are used in eye surgery to correct vision defects etc.
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what is an example of radiation?
visible light
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energy carried by waves or particles from a source.
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light and other types of radiation are used to identify and teat medical problems
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what will happen to the intensity of radiation?
the intensity of the radiation decreases further away from the source
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how is intensity affected?
by the medium the radiation is travelling through, the denser the medium, the weaker the radiation
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what is the equation to find intensity?
I = P / A
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what are these elements measured in?
P = watts (w) , A = metres squared (m^2), I = watts per square metre (w/m^2)
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what is the characteristic of a converging lens?
it joins the light rays together
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what is the characteristic of a diverging lens?
the light rays move apart
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what affect does shape have on power?
the fatter/more curved the lens, the more powerful it is
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where is the focal point on a converging lens?
after the lens (carrys on)
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where is the focal point on a diverging lens?
before the lens (taken back)
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what is the equation for the power of a lens?
power = 1 / focal length
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what are these elements measured in?
power = dioptre, D focal length = metre, m
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what is a virtual image?
image produced by a diverging lens, the image on the retina is real but the point where the rays appear to come from is a virtual image
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what is a real image?
a focused image from a converging lens, shown on a screen, having positive image distances
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what is an exception to these definitions?
there can also be a virtual image produced by a converging lens. virtual images cannot be focused to a point on a screen and have negative image distances
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what is the lens equation?
1/f = 1/u + 1/v
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what do these letters stand for?
f = focal length, u = object distance, v = image distance
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what are all of these 'lengths' measured in?
metres, m
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what are the parts of the eye?
optic nerve, retina, ciliary muscles, pupil, lens, iris, cornea
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how is light focused?
it is focused on the retina, by the work of the cornea and lens, refracting the light so that it focuses perfectly
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what is the average near point of a human?
25cm
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what is the average far point of a human?
infintity
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what are the symptoms of short sight?
far away objects are blurred but close objects are clear
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what are the causes of short sight?
a 'long' eyeball, cornea too sharply curved
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what does short sightedness mean
the rays from a distant object are focused infront of the retina
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what are the symptoms of long sight?
near objects are blurred, distant objects are clear
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what are the causes of long sight?
short eyeball, lens not thick enough
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what does long sightedness mean?
the ciliary muscles are taught and cannot bend the light enough, the rays focus past the retina
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what are the treatments for bad sight?
simple lenses (spectacles), contact lenses, laser correction
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what do 'simple lenses' entail?
wearing glasses with converging or diverging lenses, light rays are focused to the correct point
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what do 'contact lenses' entail?
same corrections, can be used in sports, are more discreet but must be kept clean as they can cause eye infections or be uncomfortable to wear
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what does 'lazer correction' entail?
under local anaesthetic, lasers are used to make small precise cuts in the cornea that permanently change the shape making them focus as required
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what is reflection?
a ray of light is reflected by a different medium, and goes off in the same 180° it came in on
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what is the law of reflection?
angle i = angle r
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what is refraction?
where the speed of the ray is changed as it goes through a different medium, causing it to change direction
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what is total internal reflection?
when an ray hits a boundary at an angle greater than the critical angle, it is not refracted, it is totally internally reflected
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what is the critical angle?
the angle at which a ray can hit a boundary and neither be reflected nor refracted, it runs along the boundary
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what is the law of refraction?
the law of refraction is used to calculate the critical angle, this is snell's law
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what is snells law?
n = sin i / sin r .............. sin c = 1/n
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how can you explain refraction in terms of speed?
refraction happens because light waves travel at different speeds in different materials. E.G. light waves travel more slowly in glass than they do in air
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what is one use of total internal reflection?
in optical fibres
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what are optical fibres?
thin glass fibres that are used to transmit digital data in the form of pulses or light, the light travels along the fibres by total internal reflection
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how does this work?
rays of light meeting the boundary between the cladding and the core are totally internally reflected
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what are the two conditions necessary?
the cladding must have a smaller refractive index
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## Other cards in this set

### Card 2

#### Front

what are ultrasounds?

#### Back

high frequency sound waves (above 20 000 Hz) reflect off features inside the body to form an image

### Card 3

#### Front

what are endoscopes?