0625 · 3.3Electromagnetic spectrum

Electromagnetic wave properties and spectrum

0:00A television remote, a mobile phone and an X-ray scanner seem to have very little in common. Yet each uses a member of the same wave family. Their waves behave differently when they meet materials, so they have different uses and risks. Understanding those differences turns the electromagnetic spectrum from a list to memorise into a useful explanation of everyday technology. In this video, we will connect each region of the spectrum to its uses and possible harmful effects.

0:25We will explain why electromagnetic waves carry messages through space, walls and glass fibres. We will also distinguish analogue signals from digital signals. By the end, you should understand both the wave family and the communication systems that depend on it. First, recall three ideas about waves. Wavelength is the distance between corresponding points on neighbouring waves, such as two neighbouring crests. Frequency is the number of complete waves passing a point each second.

0:52Waves transfer energy, and their speed equals frequency multiplied by wavelength. We will use this relationship without reteaching the wave properties topic. Electromagnetic waves are transverse waves that transfer energy. Unlike sound, they do not need a material medium, so they can travel through a vacuum. A vacuum is a region without matter. Visible light is one example, but most electromagnetic waves cannot be seen by human eyes. The word spectrum describes the whole continuous range of these waves.

1:19All electromagnetic waves travel at the same high speed in a vacuum. The Supplement requirement for Extended students gives this speed as 3.0 × 10⁸ metres per second. That is three hundred million metres per second, and the speed is approximately the same in air. Higher frequency does not mean higher speed when comparing electromagnetic waves in a vacuum. In order of increasing frequency, the regions are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.

1:48Along that same order, wavelength decreases. Radio waves therefore have the longest wavelengths, while gamma rays have the shortest. To give the order of increasing wavelength, reverse the list, beginning with gamma rays and ending with radio waves. This opposite relationship follows from the wave equation. At a fixed speed, increasing the number of waves passing each second requires each wave to occupy less distance. Imagine the same length of road containing more equally spaced cars.

2:15The spacing must become smaller. Frequency and wavelength are not independent when the wave speed stays constant.

Radio waves and microwaves

2:21Let us work through a calculation using that relationship. A radio wave has a frequency of one hundred million hertz and travels through a vacuum. We need its wavelength, so wavelength equals speed divided by frequency. Three hundred million divided by one hundred million gives a wavelength of three metres. The units are metres because we are calculating a distance. Now consider a microwave with a wavelength of three centimetres.

2:44Convert this to zero point zero three metres before substituting into the equation. Frequency equals speed divided by wavelength. Three hundred million divided by zero point zero three gives ten billion hertz. Its shorter wavelength corresponds to a higher frequency, not a higher speed. The regions have different uses because materials transmit, absorb and detect them differently. When a material absorbs electromagnetic radiation, energy is transferred to that material. This can produce heating or other changes.

3:13When a receiver detects changes in a wave, those changes can carry information. These two ideas explain many applications across the spectrum. Radio waves carry radio and television transmissions from transmitters to receivers. Information is represented by controlled changes in the transmitted waves. Radio astronomy uses radio waves arriving from objects in space, rather than relying only on visible light. This allows astronomers to obtain information that an ordinary optical telescope would not collect.

3:41Radio waves are also used in radio frequency identification. A reader communicates with a tag attached to an object, such as an access card or a labelled product. The tag provides identifying information through radio communication. The reader can therefore recognise the object without needing to see a printed label directly. Microwaves are used for satellite television, mobile phones and microwave ovens. In communication, controlled changes in the microwaves carry information.

4:06In an oven, food absorbs microwave energy, causing heating. These uses depend on different effects of the same wave region. A wave can carry a message, transfer energy for heating, or do both. Communication with artificial satellites is mainly by microwaves. They can travel through the atmosphere and across the vacuum between Earth and the satellite. The satellite receives a signal and transmits a signal onwards. Some satellite phones use low orbit satellites, which are relatively close to Earth.

4:34These satellites move across the sky relative to a person on the ground. Some satellite phones and direct broadcast satellite television use geostationary satellites instead. A geostationary satellite remains above the same position on Earth. A television receiving dish can therefore stay pointed in one direction. For this topic, the important connection is between the satellite system and microwave communication. Detailed orbital calculations belong in the space physics topic. We now add the Supplement explanation of wireless communication for Extended students.

5:04Mobile phones and wireless internet use microwaves because they can penetrate some walls. Their short wavelengths also allow short aerials for transmission and reception. This helps make portable devices practical. However, some walls transmit microwaves poorly, so reception depends on the materials and surroundings. Bluetooth uses radio waves, which can pass through walls but become weaker when doing so. A wall does not simply leave every signal unchanged or stop it completely.

5:31Some energy may be transmitted, while some is absorbed or reflected.

Infrared, light and optical fibres

5:36Infrared is closely associated with heating because absorbed infrared transfers energy to materials. Electric grills use infrared radiation to transfer energy towards food. Television remote controllers use infrared for short range communication. They send coded signals to a receiver. Infrared is also used in intruder alarms, which can detect changes in the infrared radiation reaching a sensor. Thermal imaging detects infrared radiation from objects and uses it to produce an image.

6:03Warmer regions can appear differently from cooler regions, revealing temperature patterns that our eyes cannot directly see. The displayed colours represent information detected by the camera. They do not mean the object itself is giving off those visible colours. Infrared is also used in optical fibres, alongside visible light. Visible light itself allows vision when light enters our eyes from objects. Photography records images using detected visible light, while illumination supplies light so that objects can be seen.

6:31These uses depend on light reaching a detector, whether that detector is an eye or a camera. Here is the Supplement explanation of optical fibre communication. Optical fibres carry visible light or infrared signals for cable television and high speed broadband. Glass is transparent to visible light and some infrared, allowing these waves to pass through it. The fibre guides the signal along its length. The light topic explains how total internal reflection keeps suitable rays within the fibre.

6:58Visible light and short wavelength infrared can carry high rates of data. Information is represented by rapid changes in the light signal, which a receiver detects. The fibre provides a guided route between transmitter and receiver. Notice that high speed broadband refers to the rate of transferring data, not to light travelling faster than other electromagnetic waves in a vacuum.

Ultraviolet, X-rays, gamma rays and risks

7:18Ultraviolet is used for security marking and detecting fake bank notes. Certain substances absorb ultraviolet and then emit visible light, making otherwise inconspicuous marks appear. Human eyes are detecting that emitted visible light, not the ultraviolet itself. Ultraviolet is also used to sterilise water by killing or disabling microorganisms. Its ability to damage cells makes this useful. X-rays are used in medical scanning and security scanners. They pass through some materials more easily than others.

7:46A detector records these differences to form an image of structures inside a body or a bag. In a medical image, bone and surrounding soft tissue can appear differently because they absorb different amounts of the radiation. Gamma rays are used to sterilise food and medical equipment by killing microorganisms. They are also used in the detection and treatment of cancer. Detection can involve observing gamma radiation from a substance introduced into the body.

8:10Treatment uses radiation to damage cancer cells. These are separate applications, although both depend on how radiation interacts with the body. The useful transfer of energy can also cause harm when exposure is excessive. Microwaves can cause internal heating of body cells. Infrared can cause skin burns. These are heating effects, but the affected regions of the body can differ. We should therefore name the specific harmful effect rather than describe every type of radiation as having the same danger.

8:37Excessive ultraviolet exposure can damage surface cells and eyes, leading to skin cancer and eye conditions. X-rays and gamma rays can cause mutation or damage to cells inside the body. A mutation is a change in genetic material. The same ability to damage cells explains both some medical applications and the need to control exposure carefully. A useful distinction is between heating and ionisation. Ionisation means removing electrons from atoms, which can lead to damage in living cells.

9:04X-rays and gamma rays are ionising radiation, whereas microwaves do not cause harm by that mechanism. Non-ionising does not mean harmless, because sufficiently intense radiation can still cause harmful heating. Risk depends on the radiation and the exposure.

Analogue and digital signals

9:19We now reach the Supplement requirements on analogue and digital signals. A signal is a changing quantity used to represent information. An analogue signal varies continuously and can take any value within its range. A digital signal uses discrete values rather than a continuous range. Common digital systems use two levels to represent zero and one. Sound can be transmitted using either kind of signal. For analogue transmission, a microphone produces an electrical signal whose variation represents the sound.

9:47That information is carried to a receiver and used to reproduce sound. For digital transmission, measurements representing the sound are converted into numbers. Those numbers are sent as digital signals and used to reconstruct the sound. Do not confuse the information with the wave carrying it. A sound recording can be carried by electromagnetic waves even though sound itself is not electromagnetic. The transmitter sends information representing the sound, not the original sound wave through space.

10:13This distinction explains how speech can pass through a satellite link, where sound could not cross the vacuum. During transmission, unwanted changes called noise can distort a signal. In an analogue system, noise changes the continuously varying information itself. Simply amplifying the signal also amplifies that noise. In a digital system, a receiver can often still recognise the intended discrete levels. It can then regenerate a clean signal representing the original data.

10:41Accurate signal regeneration increases the range over which digital information can be transmitted reliably. Digital signalling also supports increased rates of data transmission. The key point is not that digital waves travel faster, but that information can be transmitted efficiently and restored accurately. However, sufficiently severe noise can make the original digital values impossible to identify correctly.

Exam practice and recap

11:03Now try an exam style question that asks you to explain why microwaves are suitable for mobile phone communication. The command word explain requires a reason connected to the application. Microwaves can penetrate some walls. Their short wavelength means that only a short aerial is required for transmission and reception. These properties allow communication using compact portable devices. A second question asks you to explain why digital signalling allows communication over an increased range.

11:30Noise can distort a signal during transmission. The discrete levels of a digital signal can still be identified if the distortion is not too large. The signal can therefore be regenerated accurately. Repeated regeneration allows reliable transmission over longer distances. Watch for three common mistakes when answering spectrum questions. Reversing frequency and wavelength gives the wrong order because they vary oppositely at a fixed speed. Saying gamma rays travel faster is wrong because all electromagnetic waves have the same vacuum speed.

11:59Saying digital signals are completely unaffected by noise is also wrong. Their advantage is accurate regeneration when the intended values remain recognisable. Finally, connect each application to the relevant interaction. Communication needs signals that can reach and be detected by a receiver. Heating requires absorption of energy, while scanning uses differences in transmission and absorption. Sterilisation and cancer treatment exploit damage to cells. These connections help you choose a suitable region instead of guessing from a memorised list.

12:29Let us recap the central ideas. From radio waves to gamma rays, frequency increases while wavelength decreases. All electromagnetic waves travel at the same speed in a vacuum, approximately the same as in air. Different interactions with matter explain their uses and harmful effects. Analogue signals vary continuously, while digital signals use discrete values that allow accurate regeneration.