Wave Interactions

Material Interface

When a wave, such as light or sound is travelling through a single consistent material there is no change in the properties of the wave over short distances. However, over larger distances and when a material changes this can affect the properties of the waves.

__Absorption and Transmission __

As sound travels through the air it is being transmitted, that is it able to pass through the material.The energy of the wave is, however, being slowly absorbed by the air. This is why sounds are harder to hear the further away from the source you move.

Most noticeable changes in wave properties occur where the wave moves from one material to another. The different properties of the materials might allow for the wave to move through it (transmission ) or be stopped (absorption).

Example: When light from the sun reaches your house the bricks absorb this wave, but the glass in the windows allows it to pass through, as the glass is transparent to light waves. The ultra-violet waves are mostly absorbed by the glass. What about infrared (heat) can that pass through glass?

Answer: Yes Visible light and infrared from the sun are slowed down a little by the glass but not absorbed by it, the UV is absorbed by the molecules of the glass.

The properties of materials can affect the way different wavelengths of the same wave behave when they interact with the material.

White light is made up of all the colours of the rainbow, but if it has to pass through a coloured filter then all but one part of the spectrum is absorbed, only a limited range of wavelengths is transmitted.

A blue filter will transmit light waves with wavelengths between 490 - 450 nm, other wavelengths are absorbed by the material.

Reflection

Many surfaces reflect waves, some selectively. For sound waves, the reflected wave is called an echo. This can be used to measure the depth of water below a ship.

Sound waves of a high frequency are emitted from the boat and the echo is detected, knowing the speed of sound in water the computer can calculate the depth by taking the time for the signal to return to the boat, dividing that by 2 and multiply by the speed of sound in water.

A similar system is used by bats to echolocate in the dark.

Selective Reflection

Coloured surfaces appear a specific colour because the material absorbs some wavelengths of light, but reflects others.

Mirrors

For a plane mirror (flat mirror) the angle that the light hits the mirror (angle of incidence) will equal the angle of reflection. This is measured from an imaginary line at 90° to the mirror called the normal.

Θ_r =_ θ

Refraction

When a wave passes from one transparent material to another the density differences in the material can affect the velocity of the wave. Glass and water are both denser than air, so light passing from the air into them (or the other way around), slows down. This change in speed at the interface (boundary) between the materials can produce a bending of the ray of light known as refraction.

This also happens to sound, water, radio and seismic waves too.

Lenses make use of this property of waves and materials to bend light and help focus it in glasses, contact lenses and camera etc.

When the velocity changes the wavelength and the frequency may change too, given that v = fλ

In a lens this can cause a change in the colour of the light, know in photography as colour aberration.

At extreme angles of incidence, waves can be made to bend so much that they appear to be ‘reflected’ off the transparent sides of the material, this is known as Total Internal Reflection. Note that this is an example of Refraction, not Reflection. It is used to send laser signals through fibre optics. So if you have fibre broadband at home then your TV and Internet is being delivered by total internal refraction.

A sonar on a ship sends a sound wave into the water and it returns in 1.5s, given that sound in water has a speed of 1498 m/s, what is the depth of water?

Your answer should include: 1123.5 / 1123.5m
Explanation: Time / 2 = 0.75s v=x /t, so x = vt = 1498 x 0.75 = 1123.5m

Explain why a red shirt will look black through a blue filter.

Your answer should include: dye / reflects / reflect / wavelengths / wavelength / filter / absorb / absorbs / blue light
Explanation: The dye in the red shirt reflects all the wavelengths of light except red. The filter absorbs all but blue light, given that there is no blue light present the object appear black.

If a light ray is incident with a plane mirror at 40°, what will be the angle of reflection?

Your answer should include: 40 / 40°
Explanation: 40° as the angle of incidence = the angle of reflection.