X-Rays are electromagnetic waves with a short wavelenth.
They are ionising, so potentially dangerous.
Ultrasound waves are sound waves with a frequency above 20,000 Hz.
Ultrasound reflects off boundaries between materials.
Refraction is the bending of light when it moves from one medium to another.
Lenses use refraction to form an image.
Refractive index is a measure of the speed of light through a material compared with the speed of light in a vacuum.
The nature of an image is determined by its distance from the lens, its size, its orientation, and wheter it is real or virtual.
The image produced by converging lenses is real and inverted, or virtual, upright and magnified.
The image produced by diverging lenses is virtual, upright, and diminished.
Magnification = image height/object height
The human eye has a range of vision from 25cm to infinity.
Features of the eye: retina; cornea; iris; pupil; ciliary muscles; suspensory ligaments.
Short-sight is inability to focus on far away objects.
Corrected by diverging lenses.
Long-sight is inability to focus on near objects.
Corrected by converging lenses.
In a camera a converging lens focuses light onto photographic film or a charge-coupled device (CCD) to produce a real image.
More powerful lenses have shorter focal lengths.
Lens power is measures in dioptres.
Power = 1/focal lenght.
If the angle of light travelling from a denser medium towards a less dense medium exceeds the critical angle, total internal reflection (TIR) occurs.
Optical fibres channel light through total internal reflection, and are used for communication and in endoscopes.
Laser beams remain very narrow even over long distances.
Their intensity means they can be used to cut, etch, or cauterise objects - including in laser eye surgery.