Light and sound compared

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

Reflection

Sound waves and light waves reflect from surfaces. Remember that they behave just like water waves in a ripple tank. The angle of incidence equals the angle of reflectionreflection: There is a reflection when waves bounce off a surface.

Smooth surfaces produce strong echoes when sound waves hit them, and they can act as mirrors when light waves hit them. The waves are reflected uniformly and light can form images.

The waves can:

• Be focused to a point, for example sunlight reflected off a concave telescope mirror.
• Appear to come from a point behind the mirror, for example a looking glass.

Rough surfaces scatter sound and light in all directions. However, each tiny bit of the surface still follows the rule that the angle of incidence equals the angle of reflection.

Refraction

Sound waves and light waves change speed when they pass across the boundary between two substances with different densities, such as air and glass. This causes them to change direction and this effect is called refraction.

There is one special case you need to know. Refraction doesn't happen if they cross the boundary at an angle of 90° - in that case they carry straight on.

Remember that sound and light waves behave just like water waves in a ripple tank. The bending follows a regular pattern. Check your understanding of refraction by having a go at the animation.

Total internal reflection

You should be able to describe a use of total internal reflection with the help of a diagram.

Beyond the critical angle

Waves going from a dense medium to a less dense medium speed up at the boundary. This causes light rays to bend when they pass from glass to air at an angle other than 90º. This is refraction.

Beyond a certain angle, called the critical angle, all the waves reflect back into the glass. We say that they are totally internally reflected. Have a go at the animation to check your understanding of this.

All light waves, which hit the surface beyond this critical angle, are effectively trapped. The critical angle for most glass is about 42 °.

Optical fibres

An optical fibre is a thin rod of high-quality glass. Very little light is absorbed by the glass. Light getting in at one end undergoes repeated total internal reflection, even when the fibre is bent, and emerges at the other end.

Optical fibres

• Optical fibres are used in endoscopes that allow surgeons to see inside their patients.
• Optical fibres can also carry enormous amounts of information as pulses of light.

Diffraction

When waves meet a gap in a barrier, they carry on through the gap. However, the waves spread out to some extent into the area beyond the gap. This is diffraction.

You should know that the amount of diffraction depends on the wavelengthwavelength: The length of a single wave, measured from one wave crest to the next. and the size of the gap.

The extent of the spreading depends on the width of the gap compared with the wavelength of the waves. The smaller the width of the gap compared with the wavelength of the wave, the stronger the diffraction. For example, when waves spread into a harbour, they spread out more if the harbour mouth is narrow.

Sound

Sound can diffract through a doorway or around buildings. Lower pitched sounds travel better than high-pitched sounds. This is because low-pitched sounds have a long wavelength compared with the width of the gap, so they spread out more.

Ultrasound

Ultrasound is sound with a high frequency. It has a very short wavelength compared with most gaps, so there is very little spreading. This makes sharp focusing of ultrasound easier, which is good for medical scanning.

Light

Light has a very short wavelength compared with most everyday gaps such as windows and doors. There is little obvious diffraction, so it produces sharp shadows.

Radio waves

Long wave radio signals are much less affected by buildings and tunnels than short wave radio signals or VHF radio signals. Because of diffraction, radio signals can sometimes be received in the shadow of hills.

Interference

Where two waves meet, their effects are added together. This is called interference.

Constructive interference

When they arrive in step, they reinforce each other to give a wave of greater amplitude [amplitude: The maximum height of a wave, measured from the mid-point of its vibration. ].

This is called constructive interference.

Constructive interference

Destructive interference

When they arrive out of step, they cancel out.

This is called destructive interference.

Destructive interference

Interference of light

If a laser is shone on two slits very close together, the diffracted beam can be seen on a screen to have bright and dark bands on it.

Interference of light

The bright bands show constructive interference of light.

The dark bands show destructive interference of light.

Interference and diffraction of light and sound show that light and sound are both types of wave.

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