The Electromagnetic Spectrum
Describe the usual seven-region electromagnetic spectrum in order of increasing frequency and photon energy; explain the shared vacuum speed and the frequency-wavelength relationship; and describe uses and hazards, including the role of exposure.
What a learner can do afterwards
- Lists the usual seven regions in order of increasing frequency and relates higher frequency to shorter wavelength in a vacuum
- States the shared vacuum speed as approximately 3 × 10⁸ m/s and uses c = fλ to calculate frequency or wavelength
- Gives at least one use and one hazard for each region of the spectrum
- Links higher frequency to higher photon energy and explains that tissue damage also depends on absorption and exposure
1 · Read
You have met frequency and wavelength before. Electromagnetic (EM) waves have electric and magnetic fields oscillating perpendicular to their travel. They need no material medium. All EM waves share one speed in a vacuum, or empty space: approximately 3 × 10⁸ m/s, meaning 300,000,000 metres per second. Frequency counts cycles per second. This speed rule is for a vacuum; light travels more slowly in water or glass.
EM radiation transfers energy in packets called photons. Higher frequency means more energy per photon. It does not tell you the total energy delivered each second. An oven can heat food quickly by delivering many low-energy microwave photons. Each UV photon has more energy than a microwave photon because UV has a higher frequency.
Radio waves carry radio and TV broadcasts. Very intense radio exposure can heat tissue and cause burns. Microwaves belong to the broader radio range, but this chart shows them separately. They carry WiFi and many mobile phone signals. In an oven, water in food absorbs microwave energy and the food warms. Strong microwave exposure can also heat and burn body tissue.
Infrared lamps provide heating; intense exposure can burn skin. Warm objects emit infrared. A thermal camera detects it and turns its measurements into a visible image. Human eyes detect visible light directly, not infrared. Visible light lets us see, but intense visible light, such as a laser beam, can damage eyes.
Ultraviolet (UV) can damage DNA in cells. UV lamps can disinfect water by damaging microbes. UV from sunlight also helps skin make vitamin D. Exposure can cause sunburn, eye damage and a higher risk of skin cancer. A useful effect does not make direct exposure safe. Follow protective measures for sunlight and UV lamps.
X-rays form images of bones and teeth. Bone absorbs more of the beam than soft tissue, so fewer X-rays reach the detector behind bone. Bone does not stop every X-ray. X-ray photons can remove electrons from atoms, called ionization, and damage cells. Exposure can increase cancer risk. Medical staff weigh the benefit against the risk and control the dose.
Gamma rays can kill cancer cells in radiotherapy. They are ionizing radiation and can also damage healthy tissue, so treatment needs a planned exposure. Gamma rays occupy the high-frequency end of the usual chart. X-ray and gamma-ray ranges overlap; their names also depend on how the radiation is produced.
Radio waves, microwaves, infrared and visible light are non-ionizing: their photons cannot directly ionize atoms. Strong exposure can still cause heating or eye injury. Risk depends on the radiation, what absorbs it and the exposure. If tissue absorbs the same energy each second, twice the time gives twice the absorbed energy. Workers use exposure limits, distance and suitable shielding.
For a vacuum, use c = fλ: speed c equals frequency f times wavelength λ. Use frequency in hertz (Hz) and wavelength in metres. Find wavelength with λ = c ÷ f, or frequency with f = c ÷ λ. Use c = 300,000,000 m/s for calculations here. One megahertz (MHz) means 1,000,000 Hz. A calculator can handle the large numbers and small decimals.
At 150,000,000 Hz, wavelength = 300,000,000 ÷ 150,000,000 = 2 m. To find frequency from a wavelength of 0.000001 m, calculate 300,000,000 ÷ 0.000001 = 300,000,000,000,000 Hz. Enter the full numbers into a calculator. Dividing by a small decimal can give a very large result.
At the fixed vacuum speed, fλ stays constant. Doubling frequency halves wavelength. Tripling frequency divides wavelength by three: a wavelength of 0.24 m would become 0.08 m. A higher frequency does not make an EM wave travel faster in a vacuum.
All EM waves share a vacuum speed. Higher frequency means shorter wavelength and more energy per photon. Each region has uses and hazards; risk also depends on absorption and exposure.
Lesson, questions and study cards written for this learning dataset.
Questions, readings and study cards written for this dataset. Linked video: "Light waves, visible and invisible", TED-Ed.
More to read
What the boxes on a spectrum chart mean
Check the direction of a spectrum chart before using it. From radio toward gamma, the usual school chart runs toward higher frequency and shorter wavelength. Energy per photon increases in that direction too.
Seven equal boxes do not mean seven equal frequency ranges. The boxes help you follow the names in order. Microwaves are part of the broader radio range, even when they have their own box. X-ray and gamma-ray ranges overlap, and their names also depend on how the radiation is produced.
The visible region is the part human eyes detect directly. A screen can show measurements taken in another region without making our eyes sensitive to that radiation. For example, a thermal camera detects infrared and creates a visible display from its measurements.
Changing position on the chart does not change the vacuum speed. Every region shares that speed.
Three questions about an exposure
To understand an exposure, ask what radiation is present, how much the material absorbs and how long the exposure lasts. A region's name alone does not answer all three questions.
A microwave oven and a WiFi transmitter both use microwaves. That does not mean they deliver the same energy to nearby objects. Low energy per photon does not prevent a beam from transferring enough total energy to cause heating.
The non-ionizing regions include radio, microwave, infrared and visible light. Strong exposure can still cause burns or eye injury. X-rays and gamma rays can ionize atoms and damage cells. UV can damage DNA and can harm skin and eyes.
A useful application still needs appropriate precautions. UV can disinfect water, and gamma rays can treat cancer, but both can also harm healthy tissue. Medical exposures are planned by trained staff. Never use a radiation source as a home experiment.
Two ways to collect an invisible signal
An X-ray image and a thermal image collect different information.
For a bone image, an X-ray beam is directed through the body toward a detector. More of the beam is absorbed along a path through bone than along a path through soft tissue. The detector records different amounts of radiation from those paths. Those differences give the image contrast. Bone does not absorb every X-ray.
A thermal camera detects infrared arriving from warm objects. It uses the measurements to produce a visible display. It does not use an X-ray beam to show bones inside the object.
In each case, the displayed picture represents measurements. It is not the same as an eye directly seeing the invisible radiation. The devices also have different safety considerations. X-rays can ionize atoms, while intense infrared can injure tissue through heating.
Check a wave calculation
The equation c = fλ links vacuum speed, frequency and wavelength. Use hertz for frequency and metres for wavelength. For the calculations here, use the rounded speed 300,000,000 m/s.
Suppose the frequency is 75,000,000 Hz. To find wavelength, divide speed by frequency: 300,000,000 ÷ 75,000,000 = 4 m. Check by multiplying back: 75,000,000 × 4 = 300,000,000.
Now suppose the wavelength is 0.000002 m. Frequency is speed divided by wavelength: 300,000,000 ÷ 0.000002 = 150,000,000,000,000 Hz. A calculator can work with the full numbers. Keep the units beside the result.
A direction check can catch a mistake. At fixed speed, a larger frequency requires a smaller wavelength. If you double frequency and also double wavelength, their product becomes four times as large, so it no longer equals the unchanged speed.
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