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