Properties of electromagnetic waves 1

What you'll learn

• How electromagnetic waves can be absorbed, transmitted, reflected or refracted by materials.
• How to draw ray diagrams for refraction at a boundary between two media.
• Why, on Higher Tier, changing wave speed causes refraction.
• How to investigate how different surfaces absorb and emit infrared radiation.

Starting point: electromagnetic waves and materials

Electromagnetic waves are waves that can transfer energy without needing particles to carry them. They include radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.

In a vacuum, all electromagnetic waves travel at the same speed: about 300 000 000 metres per second. In a material, their speed can be different.

When an electromagnetic wave meets a material, what happens depends on both:

• the material it meets
• the wavelength of the wave

This wavelength part is especially important for Higher Tier: the same material might transmit one type of electromagnetic wave but absorb another.

Absorption, transmission, reflection and refraction

When an electromagnetic wave meets a boundary or a material, four main things can happen.

Absorption

Absorption happens when energy from the wave is transferred to the material. This often makes the material warmer. For example, a black surface absorbs lots of infrared radiation.

Transmission

Transmission happens when the wave passes through the material. For example, visible light is transmitted through clear glass.

Reflection

Reflection happens when the wave bounces off a surface. A shiny metal surface reflects a lot of infrared radiation and visible light.

Refraction

Refraction happens when a wave changes direction as it passes from one medium into another, such as from air into glass.

Refraction: bending at a boundary

Refraction happens at a boundary, which is the surface where two media meet, such as air meeting glass.

To draw refraction properly, you need three important parts:

• the incident ray: the incoming ray before it reaches the boundary
• the normal: an imaginary line drawn at 90° to the boundary
• the refracted ray: the ray after it enters the new medium

When light travels from air into glass, it usually slows down and bends towards the normal. When it travels from glass back into air, it speeds up and bends away from the normal.

For a rectangular glass block, the ray leaving the block is usually parallel to the ray entering the block, but shifted sideways. This shift is called lateral displacement.

Why refraction happens

This explanation is Higher Tier content, but it is useful for everyone.

Refraction happens because the wave changes speed when it enters a different medium. If the wave enters the boundary at an angle, one side of the wave reaches the new medium first.

Imagine a wavefront reaching glass from air:

• the part of the wavefront that enters glass first slows down first
• the other part is still moving faster in air
• this makes the wavefront rotate
• the direction of travel changes

The wave equation is:

$$v=f\lambda$$

where $v$ is wave speed in metres per second (m/s), $f$ is frequency in hertz (Hz), and $\lambda$ is wavelength in metres (m).

When a wave crosses a boundary, its frequency stays the same because the source has not changed. So if the wave speed decreases, the wavelength must also decrease.

No bending when the ray meets the boundary normally

If a ray travels along the normal, it still changes speed when it enters the new medium, but it does not change direction.

This is because the whole wavefront enters the new medium at the same time, so one side does not slow down before the other.

Required practical: infrared radiation and surfaces

Infrared radiation is an electromagnetic wave that transfers energy by heating. The required practical investigates how the amount of infrared radiation absorbed or radiated depends on the nature of the surface.

The key comparison is usually:

• matt black surfaces absorb and emit infrared radiation well
• shiny silver surfaces absorb and emit infrared radiation poorly, but reflect infrared well

Investigating absorption

To investigate absorption, you can shine an infrared lamp at different surfaces and measure their temperature rise.

A simple method:

1. Set up two identical metal containers or plates, one matt black and one shiny silver.
2. Place them the same distance from an infrared lamp.
3. Put a thermometer or temperature probe in contact with each container.
4. Switch on the lamp for the same length of time.
5. Record the temperature increase for each surface.
6. Compare the results.

The independent variable is the surface type. The dependent variable is the temperature rise. Control variables include distance from the lamp, starting temperature, surface area, material, time exposed and lamp power.

Investigating radiation emission

To investigate how well surfaces radiate infrared, you can fill identical containers with hot water and record how quickly they cool.

A simple method:

1. Fill identical matt black and shiny silver containers with the same volume of hot water.
2. Make sure they start at the same temperature.
3. Record the temperature every minute using thermometers or temperature probes.
4. Compare how quickly the temperatures fall.
5. The container that cools faster is radiating infrared energy faster.

Making the practical fair and reliable

A fair test changes only one variable: the nature of the surface.

To improve reliability, repeat the experiment and calculate a mean temperature change. You should also try to reduce heat loss to the surroundings in ways that are not being tested, for example by using lids or insulation where appropriate.

Be careful with infrared lamps and hot water. Lamps can get very hot, and hot water can scald, so handle equipment sensibly.