Light and Sound

Physics:  Back ] Next ]  Home

Light    Sound

Light and sound are made of WAVES. They are both forms of ENERGY.

Light travels much faster than sound. This is why the flash of lightening is seen long before the sound of thunder is heard, even though they are formed at the same instant.

Speed of light 3x1010 m/s (300,000 km/s)
Speed of sound 330m/s

Light is given out from a luminous source eg the sun or a projector lamp.

Light travels in straight lines.

In order to see anything light has to be reflected off of it and enter our eye.

Shadows are formed when RAYS of light are stopped by an object that does not TRANSMIT light.

When light hits a rough surface the light gets scattered.

When light hits some coloured paper (eg a red book) some colours are absorbed but the red light is scattered which is why the book looks red.

When a light ray hits a smooth surface it is reflected in a regular way. A mirror reflects the ray at the same angle that it hits it.

When a light ray hits a mirror it is always reflected by the same angle as it hits the mirror
ie Angle of incidence=angle of reflection 


When light travels from air into water or air into glass it gets bent. This bending is called refraction. Refraction is responsible for making swimming pools look shallower than they really are, and for making a pencil resting half in water appear bent.

Refraction happens because the light rays travel slower through the water or glass than they do through air.

Words you need to know from this topic
REFLECTION When light bounces off a smooth surface (eg a mirror) and forms an image behind it.

REFRACTION When light gets bent by passing from air into water or glass (or passing back again).

ABSORPTION When light hits an object and does not get reflected back (eg when light hits a piece of black paper it is absorbed, this is why the paper looks black)

TRANSMISSION When light passes straight through something like a piece of transparent paper.

DISPERSION The splitting of white light into a SPECTRUM. This is often done by a using a PRISM

Optical fibres are strands of thin glass. Light can bounce from one end of the strand and come out of the other.

They are used in communications where they are now used to carry telephone or computer messages instead of wires.

Some ray diagrams

A periscope, using two plane mirrors 
A coin appears to be at a lower depth (ie the water looks more shallow) due to refraction


Sound       (light)

In order to produce a sound something has to vibrate.

The vibrating object causes compressions in the air which in turn cause the ear drum in our ear to vibrate.

The frequency of the vibrations determine the pitch of the note: Faster vibrations produce a note with a higher pitch.

The size (amplitude) of the vibrations determine the volume of the sound: If the amplitude increases then the sound will get louder.

The diagram shows a vibrating string. It has an amplitude of 2cm.
The string can be made to vibrate with a higher note 3 ways:

1.  Make the string tighter
2.  Make the string shorter
3.  Make the string lighter (a heavy object vibrates slower)

Sound travels faster in solids and liquids than it does in air.

Sound will NOT travel through a vacuum.

How sound is produced in different musical instruments

instrument part which vibrates instrument part which vibrates

Trumpet: Lips Organ: Air

Clarinet: Reed Guitar: Strings

Piano: Strings Drum: drum skin


An echo is heard when sound is reflected off a distant object.

Sonar make use of echoes to measure the distance (or shape) of an object (eg the sea floor).

It does this by measuring the length of time it takes to hear the echo.

Ultra sound is too high for us to hear (maybe about 40kHz). It is used to produce pictures of unborn babies,

in burglar alarms and also in some cleaning devices


Words to know:

Frequency: the number of vibrations per second.

Pitch: how high or low a note sounds.

Amplitude: the height of a wave

Volume: How loud a note is.

If the frequency increases then the pitch will increase. 
If the amplitude increases then the volume will increase.

Some waveforms

This page is part of a site that uses frames.
If you cannot see navigation links to the left or top then
click here to return to the start.