Sound and light both travel in waves, which are caused by vibrations. The vibrations begin when an object at rest is set in motion (e.g. a guitar string or electrons). This vibrating motion can begin (initial displacement) in two ways, longitudinal or transverse. Light waves are transverse. This is when the movement of electrons is perpendicular to the motion of the wave. An example is a football stadium wave. Everyone is waiting for their turn, and when it comes, they sit and stand in an up and down motion, and the wave continues forward. Light waves, therefore, move faster than sound waves, which are longitudinal. Light waves can also travel through space.
In the case of sound waves, which are longitudinal, the vibrations begin when a movement is made that is parallel to the wave path (a push instead of a rise). This is sort of like a slinky being pushed, the wave that comes from it moves away from it in a parallel direction, without moving up and down. Sound waves, being longitudinal, are slower than light waves, and do not travel through a vacuum (e.g. space).
Finally, and again, because of their initial displacement and type of motion, sound waves travel faster through denser mediums (solids), while light waves travel faster through less dense mediums (gases).
Light travels as transverse waves and can travel through a vacuum. Sound travels as longitudinal waves and needs to travel through a solid, liquid or gas: it cannot travel through a vacuum.
Light and sound can be reflected and refracted, just like water waves. Light and sound can also be diffracted, just like water waves, but diffraction in light is less obvious than in sound.
Light and sound both travel as waves, but they are not identical. The table summarises the similarities and differences between them.
Table comparing light and sound waves
Type of wave
|Transversetransverse waves: Waves in which the vibrations happen at right-angles to the direction of travel. Light travels as transverse waves.||Longitudinallongitudinal waves: Waves in which the vibrations happen in the same direction as the direction of travel. Sound waves are longitudinal waves.|
Can they travel through a vacuumvacuum: A volume that contains no matter - space is almost a vacuum.?
|Yes||No. Sound waves can only pass through a solid, liquid or gas|
Can they be reflectedreflection: There is a reflection when waves bounce off a surface.?
Can they be refractedrefraction: There is refraction when waves change direction as they move from one transparent substance to another.?
Can they be diffracteddiffraction: The spreading out of waves when they pass through a gap or around an obstacle.?
Can they interfere?