5.2 Interactions of electromagnetic radiation with matter — Learn

Prerequisites: Section 5.1, especially the wave equation and the order of the electromagnetic spectrum; the idea that energy can be transferred to a thermal store.

By the end of this section you can: describe how electromagnetic radiation is absorbed, reflected, transmitted and refracted; and describe a use and a danger of each group of electromagnetic waves.

1. What can happen to a wave

When electromagnetic radiation reaches a material, one or more of four things happens.

Absorption. The radiation is taken in by the material. Its energy is transferred to the thermal store of the material, so the material warms up.
Reflection. The radiation bounces off the surface.
Transmission. The radiation passes through the material.
Refraction. The radiation changes direction as it passes from one material into another.

Which of these happens depends on the material and on the wavelength. A material that is transparent to visible light may absorb infrared radiation. Glass is one example.

2. Absorption and warming

When radiation is absorbed, energy is transferred to the material. The temperature rise depends on the energy absorbed and on the material itself.

A dark, matt surface absorbs infrared radiation well. A shiny surface reflects most of it. This is why a black car warms up faster than a white car in the Sun. It is also why a shiny emergency blanket helps to keep a casualty warm: the shiny surface reflects infrared radiation back to the body.

Worked example 1. A microwave oven uses radiation with a frequency of 2.5 GHz. Calculate its wavelength.

\begin{aligned} \lambda &= \frac{c}{f} \\ &= \frac{3.0 \times 10^8}{2.5 \times 10^9} \\ &= 0.12\,\text{m} \end{aligned}

Worked example 2. A television remote control sends infrared radiation with a wavelength of 1.0 μm. Calculate its frequency.

\begin{aligned} f &= \frac{c}{\lambda} \\ &= \frac{3.0 \times 10^8}{1.0 \times 10^{-6}} \\ &= 3.0 \times 10^{14}\,\text{Hz} \end{aligned}

3. Reflection and refraction

Reflection happens at a surface. The ray that arrives is the incident ray, and the ray that bounces off is the reflected ray. A line drawn at right angles to the surface is the normal.

The angle of incidence is the angle between the incident ray and the normal. The angle of reflection is the angle between the reflected ray and the normal. The angle of reflection always equals the angle of incidence.

Refraction happens when a wave passes from one material into another and its speed changes.

Going from air into glass or water, the wave slows down and bends towards the normal.
Going from glass or water into air, the wave speeds up and bends away from the normal.

Light travels more slowly in glass and in water than in air. The change of speed at the boundary is what makes the ray change direction.

Worked example 3. A ray of light hits a mirror at an angle of incidence of 40°. State the angle of reflection.

The angle of reflection equals the angle of incidence, so the angle of reflection is 40°.

4. Changing the angle

The figure shows a ray of light arriving at the surface of a glass block. You can change the angle of incidence.

Predict first. Do not move the slider yet. Write down your answers.

When the ray enters the glass, will it bend towards the normal or away from it?
Will the angle of reflection be bigger than, smaller than, or equal to the angle of incidence?

Test. Move the slider and watch the three rays. Check the angle of reflection against the angle of incidence each time.

Explain. Light travels more slowly in glass than in air. Why does a change of speed make the ray bend? Write your answer in two or three sentences.

Watch out. Always measure the angles from the normal, not from the surface. A ray that hits a mirror at a small angle to the surface has a large angle of incidence. Also keep reflection and refraction apart: reflection bounces off the surface, while refraction passes into the new material and bends.

5. Uses and dangers

Each group of the spectrum has uses that follow from how it interacts with matter. Some groups are also dangerous if a person receives too much.

Group	A use	A danger
Radio waves	Broadcasting and communication.	None at normal power.
Microwaves	Cooking, and satellite and mobile links.	Heating of body tissue at high power.
Infrared	Thermal imaging, remote controls and heaters.	Burns to the skin at high power.
Visible light	Seeing, photography and fibre-optic communication.	Bright light can damage the eye.
Ultraviolet	Sunbeds, and vitamin D production in the skin.	Skin damage, and damage to the eyes.
X-rays	Imaging bones and luggage.	Ionising, so it can damage cells.
Gamma rays	Sterilising equipment, and treating cancer.	Ionising, so it can damage cells.

Ultraviolet, X-rays and gamma rays are ionising. Ionising radiation has enough energy to remove electrons from atoms. This is why it can damage living cells.

6. Progress check

Return to the chapter problem. The observatory walls are painted white. Decide whether a white wall absorbs more or less infrared radiation than a black wall. Explain how this affects the temperature inside the building.

You should now be able to

describe the four ways electromagnetic radiation interacts with matter;
explain how absorption transfers energy to a thermal store;
state the law of reflection, and measure angles from the normal;
describe how a ray refracts when it enters a denser material;
give a use and a danger of each group of electromagnetic waves;
explain what makes ultraviolet, X-rays and gamma rays ionising.