GCSE Physics Edexcel P3 Topic 1

These are revision notes I made myself. I got A* in the Physics GCSE exam and 80 UMS (full marks) in the P3 exam using only these notes.

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P3 Topic 1: Radiation in treatment and medicine
Radiation in Medicine
Both ionising and non-ionising radiation is used in medicine. Their uses aren't always straightforward and
sometimes compromises have to be made.
For example, ionising radiation makes a clear image of the inside of the body but can damage the patient's
Radiation detected How it forms an image Why is it good? Where it is used
Visible light Light is reflected along It can be used to see Endoscopes
optical fibres from the inside the patient to
endoscopes. investigate problems
without the need for
invasive surgery.
X-rays X-rays are absorbed by It can be used to X-ray photography and
dense materials in the body diagnose bone CAT scanners
such as bone and metal but fractures.
not others. A negative
image is produced.
X-rays (CAT scanners) Intense beams of X-rays are Unlike standard X-ray CAT scanners
used. photographs, they can
image soft tissue as well
as bones.
Gamma rays The movement of Can be used to find PET scanners
substances emitting gamma cancer tumours in the
rays inside the body are body.
monitored and the positions
shown on a screen.
Ultrasound High-frequency sound Can be used to diagnose Ultrasound scanners
waves reflect off features soft tissue problems and
inside the body to form an to perform pre-natal
image. scans of a foetus in the
Ultrasound (breaking down An ultrasound beam The patient doesn't Breaking down kidney
kidney stones) concentrations high energy need surgery and it is stones
waves at the kidney stone relatively painless.
which turns it into sand-like
Ultrasound (measuring the As ultrasound works in real It can be used to Speed of blood flow
speed of blood flow) time it can be used to show measure the speed of
how things are changing. blood flow. Problems
can be investigated in
the heart and liver.
Intensity of Radiation
Radiation: This is energy that originates from a source.
Radiation covers all types of radiation in physics, and not just ionising nuclear radiation. Light emitted from a
star is radiation for example. Radiation can be in the form of a wave or a particle.
The intensity of radiation from a source decreases as distance increases. This can be checked with a heat
source; the further away you get the less heat you feel.
The intensity of radiation also depends on what it's passing through. In theory, unless the radiation is in a
vacuum some of the radiation will always be absorbed in the medium it's passing through.

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Generally, the denser the medium the more radiation will be absorbed thus decreasing the intensity of the
Intensity = Power of incident radiation per unit area
The more intense the radiation, the more energy it carries per second or the higher the power. Similarly, the
higher the power the more energy that gets transferred per second when it hits an object.
Surface area of an object is also a factor; the bigger the object the more radiation that hits it than a smaller
object.…read more

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The near point is the closest distance that the eye can focus on. For the average adult that is 25cm.
Short-sighted people
Short-sighted people cannot focus on objects that are far away ­ this happens if their far point is closer than
Images of distance objects are brought into focus in front of the retina.
Short sight can happen for many reasons. The cornea may be curved too sharply or the eyeball is too long.…read more

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Contact lenses or Laser eye surgery
Contact lenses: The cornea does most of the focussing for the eye, and when it's the wrong shape it can be
too weak or too powerful making it potentially responsible for both long and short sight. Contact lenses sit
on top of the cornea and are shaped to compensate for the fault. Like glasses, contact lenses can be
converging or diverging.
Laser eye surgery: A laser can be used to vaporise tissue, changing the shape of the cornea.…read more

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Different lenses
There are two main types of lens ­ converging and diverging.
Converging lens are convex and they cause parallel rays of light to converge to a focus at the lens focal point
Diverging lens are concave and cause parallel rays of light to diverge. The focal point of a diverging lens is the
point where rays hitting the lens parallel to the axis appear to have come from.…read more

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The position of the object affects the image
Position of object At 2F Between F and 2F Nearer than F
Real or virtual Real Real Virtual
Image orientation Inverted Inverted Right way up
Image size Same as object Bigger than object Bigger than object
Image position At 2F Beyond 2F Same side of the
lens as the object
Three rules for refraction in a diverging lens
1.…read more

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Diverging lenses always produce a virtual image.
The image is right way up, smaller than the object and on the same side of the lens as the object no matter
where it is.
Power and the Lens Equation
The focal length is related to power. The more powerful the lens, the more strongly it converges rays of
light, so the shorter the focal length.
Power of lens = 1
focal length
The unit for lens power is dioptres, or D.…read more

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If the angle of incidence is equal to the critical angle the emerging ray comes out along the surface. There's
quite a bit of internal reflection.
If the angle of incidence is greater than the critical angle no light comes out. It's all internally reflected i.e.
total internal reflection.
You can investigate the critical angle for a water and air boundary by shining a beam of light up through a
rectangular tank.…read more


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