Sound and light
Sound waves are longitudinal waves that must pass through a medium. Echoes are reflections of sounds. Light and other forms of electromagnetic radiation travel as transverse waves. These waves can travel through a vacuum, and they all travel at the same speed in a vacuum.
Sound waves are longitudinal waves. Their vibrations occur in the same direction as the direction of travel. Sound waves can only travel through a solid, liquid or gas.
When an object or substance vibrates, it produces sound:
These diagrams show snapshots from oscilloscope traces of three sounds.
Sounds 1 and 2:
Sounds 2 and 3:
For the exam, you only need to know about sounds in the normal range of human hearing. This is between about 20 Hz and 20 kHz, but the range becomes less as we get older. Sounds with frequencies above about 20 kHz are called ultrasound.
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