Electromagnetic Wave Spectrum

Visible light, radio waves and X-rays may seem very different, but they belong to the same family. All electromagnetic waves are transverse, can travel through a vacuum, and travel at the same velocity through a vacuum or air.

Together they form the electromagnetic spectrum. It is continuous: wavelengths and frequencies vary across it without gaps. The seven named groups describe different regions of this spectrum, rather than seven completely separate kinds of behaviour.

The groups are arranged using two wave properties. Wavelength is the distance between corresponding points on consecutive waves, such as neighbouring crests. Frequency is the number of complete waves passing a point each second.

The wave equation links these properties:

$$v = f\lambda$$

Here, $v$ is wave speed, $f$ is frequency and $\lambda$ is wavelength. Because electromagnetic waves have the same speed in a vacuum or air, a shorter wavelength means a higher frequency. More closely spaced waves pass a point more often.

Reading the spectrum in order

From longest to shortest wavelength, the order is:

Radio → microwave → infrared → visible light → ultraviolet → X-rays → gamma rays.

This is also the order from lowest to highest frequency. Radio waves therefore have the longest wavelengths and lowest frequencies, while gamma rays have the shortest wavelengths and highest frequencies.

Read the wavelength and frequency arrows in opposite directions. Moving towards gamma rays decreases wavelength but increases frequency; it does not increase the speed of the waves.

The small region our eyes detect

Our eyes detect only visible light, a limited region of the electromagnetic spectrum between infrared and ultraviolet. The other regions are still electromagnetic waves even though we cannot see them.

Within visible light, the colours run from red, orange, yellow, green, blue, indigo to violet as wavelength decreases. Red has the longest wavelength and lowest frequency in the visible range; violet has the shortest wavelength and highest frequency.

Being invisible does not mean a wave transfers no energy. Infrared and microwaves, for example, can warm objects without being detected by our eyes.

Following energy from source to absorber

A useful way to explain electromagnetic energy transfer is to follow the chain source → electromagnetic waves → absorber → effect.

In a microwave oven, the microwave source emits microwaves. Water molecules in the food absorb energy carried by these waves. Energy is transferred to the food’s thermal energy store, increasing its temperature.

A hot object emits infrared radiation. When another object absorbs this radiation, energy is transferred to its thermal energy store and it warms up. No contact between the two objects is needed for this transfer.

The Sun emits electromagnetic waves that travel through space to Earth. Infrared radiation absorbed at Earth’s surface warms it. The Sun also emits visible light, which our eyes detect, and ultraviolet radiation, which can cause sunburn when absorbed by skin. These examples show that electromagnetic radiation transfers energy even across a vacuum.

Radio → microwave → infrared → visible light → ultraviolet → X-rays → gamma rays.

$v = f\lambda$: at constant speed, shorter wavelength means higher frequency.

Visible light

• Human eyes detect only the visible region.
• Red → orange → yellow → green → blue → indigo → violet is decreasing wavelength and increasing frequency.
• Red: longest visible wavelength. Violet: shortest visible wavelength.

Energy transfer

Recall the chain: source → waves → absorber → effect.

Microwaves absorbed by food and infrared absorbed by objects transfer energy to their thermal energy stores.