P3A

X-rays are part of the electromagnetic spectrum. They have a very short wavelength and causenionisation. X-rays can be used to diagnose and treat some medical conditions. Precautions to be taken when X-ray machinesand CT scanners are in use. Examples include CT scans, bone fractures, dental problems and killing cancer cells. The use of charge-coupled devices (CCDs) allows images to be formed electronically.

Properties of X-rays include:

■ they affect a photographic film in the same way as

light

■ they are absorbed by metal and bone

■ they are transmitted by healthy tissue

■ their wavelength is of the same order of magnitude as the diameter of an atom.

Electronic systems can be used to produce ultrasound waves, which have a frequency higher than the upper limit of hearing for humans. The range of human hearing is about 20 Hz to 20 000 Hz.Ultrasound waves are partially reflected when they meet a boundary between two different media. The time taken for the reflections to reach a detector can be used to determine how far away such a boundary is. Ultrasound waves can be used in medicine. Examples include pre-natal scanning and the removalof kidney stones.

s = v x t

Refraction is the change of direction of light as it passes from one medium to another. A lens forms an image by refracting light. In a convex or converging lens, parallel rays of light are brought to a focus at the principal focus.The distance from the lens to the principal focus is called the focal length.The nature of an image is defined by its size relative to the object, whether it is upright or inverted relative to the object and whether it is real or virtual. The nature of the image produced by a converging lens for an object placed at different distances from the lens. The use of a converging lens as a magnifying glass.The nature of the image produced by a concave or diverging lens. The construction of ray diagrams to show the formation of images by converging and diverging lenses.

refractive index  = sin i/sin r

magnification = image height/object height

The structure of the eye is limited to: retina, lens, cornea, pupil, iris, ciliary muscle, suspensory ligaments.

Correction of vision using convex and concave lenses to produce an image on the retina:long sight, caused by the eyeball being too short,or the eye lens being unable to focus OR short sight, caused by the eyeball being too long,or the eye lens being unable to focus.

The eye can focus on objects between the near point and the far point. Comparison between the structure of the eye and the camera. Examples of use include the endoscope for internal imaging. For a given focal length, the greater the refractive index, the flatter the lens. This means that the lens can be manufactured thinner.

The focal length of a lens is determined by: the refractive index of the material from which the lens is made, and the curvature of the two surfaces of the lens.

P=1/f

Visible light can be sent along optical fibres. The laser as an energy source for cutting, cauterising and burning. Examples of use should include the endoscope for internal imaging.

refractive index = 1/sin c