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# Basic Waves

### foundation GCSE and iGCSE waves

A TRANSVERSE wave has vibrations that are at right angles to the direction of travel of the wave.
You find crests and troughs in a transverse wave
A LONGITUDINAL (or compression) wave has vibrations that are parallel to the direction of travel of the wave.
You find compressions and rarefactions in a longitudinal wave.
One noisy example of a longitudinal wave is sound.
One wet example of a transverse wave is surface water waves.
Frequency is defined as the number of vibrations (or waves) per second.
The unit of frequency is Hertz (Hz)
Speed is defined as distance travelled per second.
Units for speed could be metres per second or cm/s
Wavelength is the distance from crest to crest.
The symbol for wavelength is the Greek letter called lambda (= lower case L)
The wave equation is velocity (or speed) = frequency times wavelength.
We can calculate frequency from velocity (or speed) divided by wavelength.
Take speed of sound = 300 m/s. If it is 5 seconds between seeing lightning and hearing thunder, then the distance away is speed times time = 1500 metres.
If a water wave has a frequency of 6 Hz, and a wavelength of 2 cm, its speed is 12 cm/s
If a sound wave in water travels at 1000 m/s and its frequency is 2000 Hz then its wavelength is 0.5 m
Lowest frequency for normal human hearing = 20 Hz
Highest frequency for normal human hearing = 20 000 Hz = 20 kiloHertz
Sound with frequency greater than 20 kHz is called ultrasound
Sound with frequency lower than 20 Hz is called infrasound
Speeds greater than the speed of sound are called supersonic.
Echo is a word for sound reflection.
Sonar is a system that uses sound reflections to measure distance.
If sound in seawater travels at 1000 m/s and a pulse takes 5 seconds to return from the sea bottom the depth is 2500 m (remember it goes there and back)
Ultrasound scans use ultrasound because it has a small wavelength.
Dish shaped (inside of dish!) reflectors are called concave.
Refraction is change of direction because of change of speed.
Name for wave spreading out as it goes through gap is diffraction.
If the gap is wide compared with wavelength then the amount of diffraction is small.
Sound diffracts through doorways, light doesn't because wavelength of sound is much larger than wavelength of light.
We ignore the time for the light to travel compared with the sound because light travels a million times faster than sound.
You can't hear sound on the Moon because there is no air on the Moon.
Does a water wave travel more slowly or faster in more shallow water? more slowly.
What causes a wave? vibrations
Created by: waslancing

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