Example 1: frequency to wavelength
A radio station broadcasts at 90 MHz. Calculate the wavelength.
- Convert: 90 MHz = 90 000 000 Hz.
- wavelength = speed ÷ frequency = 300 000 000 ÷ 90 000 000.
- wavelength = 3.3 m.
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IB DemystifiedMYP Sciences
The electromagnetic spectrum
Radio, microwaves, light and X-rays are all the same kind of wave, travelling at the same speed. Only the wavelength changes, and that decides what each can do, and how safe it is.
Recommended for MYP 4 · eAssessment priority · About 4 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the spectrum
- Uses and hazards
- The spectrum in the real world
- Worked examples
- In the eAssessment
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- state the properties shared by all electromagnetic waves
- order the regions of the spectrum by wavelength and frequency
- describe and explain uses of each region
- explain which radiations are ionising, why they are hazardous, and how risks are reduced
- use wave speed = frequency × wavelength with electromagnetic waves
- explain absorption and emission, including infrared and the greenhouse effect
Before you start
You will use these skills. If any feel shaky, review them first.
- wave properties and wave speed = frequency × wavelength (see Wave properties)
- prefixes: kilo, mega, giga, micro, nano
- the structure of cells and DNA
Key vocabulary
- Electromagnetic wave
- A transverse wave of oscillating electric and magnetic fields that can travel through a vacuum at 3.0 × 10⁸ m/s.
- Spectrum
- The continuous range of electromagnetic waves, from radio waves to gamma rays.
- Ionising radiation
- Radiation with enough energy to remove electrons from atoms: higher-frequency ultraviolet, X-rays and gamma rays.
- Absorb / emit
- To take in radiation (gaining energy) / to give out radiation.
- Infrared
- Radiation emitted by all warm objects; the hotter the object, the more it emits.
- Exposure limit
- A level of radiation set well below that known to cause harm.
Understanding the spectrum
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What is it?
The electromagnetic spectrum is a family of transverse waves that all travel at 3.0 × 10⁸ m/s in a vacuum. In order of decreasing wavelength (increasing frequency): radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays.
speed of light = frequency × wavelength = 3.0 × 10⁸ m/s
-
Why does it happen?
Electromagnetic waves are produced when charges vibrate or change energy: electrons moving in an aerial make radio waves, warm objects give out infrared, and changes in atomic nuclei release gamma rays. Higher frequencies carry more energy in each packet, which is why high-frequency radiation can ionise atoms.
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How do we know?
In 1800 William Herschel placed a thermometer beyond the red end of a spectrum and found it warmed up, discovering infrared. James Clerk Maxwell predicted that light is an electromagnetic wave, and Heinrich Hertz then produced radio waves in his laboratory, confirming that they travel at the speed of light.
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Why does it matter?
Almost all communication, medical imaging, remote sensing and astronomy depends on electromagnetic waves. Knowing which parts are ionising lets us use X-rays and gamma rays safely and reject unfounded fears about radio waves.
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What does it connect to?
The spectrum links to wave properties, energy transfer, radioactivity and astronomy in physics; to DNA and cancer in biology; to the ozone layer and greenhouse effect in environmental science; and to standard form in mathematics.
Uses and hazards
| Region | Uses | Hazards |
|---|---|---|
| Radio waves | Broadcasting, communications | Non-ionising; no established harm below exposure limits |
| Microwaves | Mobile phones, Wi-Fi, satellites, cooking, radar | Heating of body tissue at high intensity |
| Infrared | Remote controls, thermal imaging, optical fibres, heaters | Burns from intense sources |
| Visible | Seeing, photography, optical fibres | Eye damage from very bright sources and lasers |
| Ultraviolet | Security marks, sterilising water, tanning | Sunburn, skin cancer, cataracts |
| X-rays | Medical imaging, airport security | Ionising: damages DNA, increases cancer risk |
| Gamma rays | Sterilising equipment and food, cancer treatment | Ionising: damages DNA, kills cells |
Risk from ionising radiation depends on the dose. It is reduced by shielding (such as lead screens), increasing distance, and limiting exposure time.
The spectrum in the real world
Weather satellites image clouds in visible light by day and infrared by night. Hospitals treat some cancers by aiming beams of gamma rays or X-rays at a tumour from several directions, so the tumour receives a high dose and healthy tissue less. Water-treatment plants in many countries use UV lamps to kill microorganisms without adding chemicals.
Worked examples
Example 2: choosing radiation
Which radiation would you use to check whether a sealed box contains a metal object, and why?
- The radiation must pass through cardboard but be absorbed by metal.
- X-rays pass through low-density materials but are absorbed by dense metals.
- So use X-rays, as in airport baggage scanners.
In the eAssessment
Questions on the spectrum often combine recall of order and uses with calculations and evaluation of claims about safety. Expect:
- Order and identify: regions by wavelength or frequency, and missing regions on a diagram.
- Calculate: wavelength or frequency using 3.0 × 10⁸ m/s, with prefixes (MHz, GHz, nm), and echo or travel times.
- Explain: uses in terms of absorption, transmission and reflection, and hazards in terms of ionisation and DNA.
- Evaluate and discuss: claims about phone masts, sunscreen or ozone, using evidence rather than opinion.
Common ways to lose marks: saying higher-frequency waves travel faster; treating all radiation as ionising; confusing the ozone hole with the greenhouse effect; and forgetting to halve the distance in echo calculations.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: which surfaces emit the most infrared?
Partially guided investigation · about 40 minutes · pairs
- Research question
- How does the surface finish (matt black, matt white, dull metal, shiny metal) affect the infrared radiation emitted from a cube of hot water?
- Scientific background
- All objects emit infrared, and hotter objects emit more. Different surfaces at the same temperature can emit different amounts.
- Hypothesis
- Write your own hypothesis, with a justification. Think carefully about whether visible colour or surface finish matters more.
- Variables
- Identify the independent and dependent variables, and at least three control variables, explaining how and why you will control each.
- Apparatus
- Leslie cube (a metal cube with four different faces), kettle, thermometer, infrared sensor or thermopile with meter, clamp stand, ruler.
- Method
- Fill the cube with hot water and check its temperature with the thermometer.
- Clamp the sensor 5 cm from the centre of one face, pointing straight at it.
- Record the reading, then turn the cube to the next face, keeping the distance the same.
- Measure all four faces quickly, then repeat in reverse order.
- Calculate a mean for each face.
Safety. Take care with hot water: fill the cube on a stable surface, do not touch the hot metal, and wipe up spills. Keep the kettle's cable away from water.
Results from one pair are used in the Criterion C questions below. Then evaluate: why repeat the readings in reverse order?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
|---|