Glossary

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ultrasonic frequencies - frequencies above 20,000 Hz, the highest frequencies detected by most humans.

ultrasound - an imaging technique that uses high frequency sound waves and their echoes. The technique is similar to the echolocation used by bats, whales and dolphins, as well as sonar used by submarines.

ultraviolet light - part of the electromagnetic spectrum found between the violet side of visible light and x-rays.

umbra - a total shadow

vertex - the geometric center of a mirror.

virtual image - an image that cannot be projected on a screen. The image appears behind (mirror or lens).

vitreous humour - a clear jelly-like substance found inside the eye cavity.

wave - the transfer of energy in the form of a disturbance.

wavelength - the distance from crest to crest, trough to trough, or other equivalent distance in a wave.

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