|6 factoids about X-rays
|They are part of the electromagnetic spectrum. They cause ionisation. They affect a photographic film in the same way as light. They are absorbed by metal and bone. They are transmitted by soft tissue. Their wavelength is of the same order of magnitude as the diameter of an atom.
|What are X-rays used for? (4 egs)
|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.
|Precautions to be taken when X-ray machines and CT scanners are in use.
|Lead screening for patient and operator Limit to number for patient Limit of exposure for operator
|Range of human hearing
|Candidates should know that the range of human hearing is about 20 Hz to 20 000 Hz.
|What is ultrasound?
|Electronic systems can be used to produce ultrasound waves, which have a frequency higher than the upper limit of hearing for humans.
|How are ultrasonic waves used to find cracks in materials?
|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.
|Calculation of the distance between interfaces in various media.
|Candidates may be required to use data from diagrams of oscilloscope traces. s is distance in metres, m v is speed in metres per second, m/s t is time in seconds, s
|Use of ultrasound in medicine
|Examples include pre-natal scanning and the removal of kidney stones.
|What is refraction?
|Refraction is the change of direction of light as it passes from one medium to another.
|What does a lens do to light to form an image?
|A lens forms an image by refracting light.
|What does a convex lens do to 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.
|What defines the 'nature' of an image?
|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.
|Closer than focal length: enlarged, upright, virtual. At focal length: no image. Beyond focal length: Reduced, inverted, real.
|The nature of the image produced by a concave or diverging lens.
|Diminished, inverted, virtual.