Edexcel P3 Notes

P3.1 -- Radiation in medicine:

• Intensity (W/m2) = Power of incident radiation (W) / Area (meters squared)
• The inverse square law states that the intensity of radiation is reduced by the square of the distance each time. That is to say, if the distance is doubled the intensity is reduced by four times, and so on and so forth.

P3.2 -- How eyes work:

• Light enters the eye through the pupil. The cornea of the eye refracts light coming in to the eye, and this light is then focused by the lens. When the light reaches the retina of the eye, the rods and cones within the retina convert the light to signals which can be interpreted by the brain. In addition to this, cillary muscles alter the shape of the lens to keep the image in focus.
• The closest distance at which one can see un-blurred images is called the near point, which in adults is, on average, around 25cm.

P3.3 -- Sight problems:

• Short-sightedness, or myopia, is the result of the cornea refracting light too much, so that at image forms "in front of" the retina. It is hence altered via the use of a diverging lens.
• Long-sightedness, or hypermetropia, is, by contrast, the result of the cornea insufficently refracting light, meaning that an image forms "beyond" the retina. It is hence altered via the use of a converging lens.
• Other treatments include the use of contact lens and laser eye surgery, both of which alter the curvature of the cornea itself.

P3.4 -- Investigating effect of object distance on image characteristics (N/A: Practical)

P3.5 -- Different lenses:

• For a converging lens, light rays are refracted and meet at the focal point, the distance from the lens to this point being the focal length. For a diverging lens, conversely, the focal point is that point from which the rays appear to originate after passing through the lens.
• Power of a lens (dioptre) = 1/ focal length (metres)
• 1/f = 1/u + 1/v where f is the focal length, u the object distance and v the image distance.

P3.6 -- Reflection and refraction:

• For reflection, angle of incidence = angle of reflection.
• Snell's law can be used to describe refraction: it states that sin i / sin r = refractive index of r / refractive index of i.

P3.7 -- Total Internal Reflection in Perspex (N/A: Practical)

P3.8 -- Investigating the critical angle (N/A: Practical)

P3.9 -- Critical angle:

• The critical angle is that angle of incidence at which the angle of refraction = 90 degrees. At angles greater than this, total internation reflection occurs.
• This has several importance applications. It is the principle on which fibre optic cables and endoscopes function, and in addition it is how automated rain sensors on cars work: an infrared source inside the car directs a bean onto the windscreen, and reaches the boundary between the glass and the outside air at an angle greater than the critical angle, meaning that toal internal…