Electromagnetic Spectrum

Water waves, sound waves, light waves, and waves in a string or slinky are examples of waves in everyday life.

Vibrations in matter cause waves. (Students are likely to be familiar with vibrations causing sound waves and with disturbances in water causing water waves. They are not likely to know that the movement of charged particles causes electromagnetic waves.)

Waves can cause matter to move back and forth in place, but waves do not transport matter. Examples include sound causing matter to vibrate as the sound waves travel through a material, a boat rising and falling in the open ocean as waves pass beneath the boat, or a “wave” travelling around a stadium as fans stand up and sit down without moving away from their seats.

Radiation refers to electromagnetic waves, which are one specific type of wave. Light refers to the portion of the electromagnetic spectrum that is in or near the visible light spectrum. So light is both a wave and a type of radiation.

Aside from the type of wave (mechanical vs. electromagnetic), one wave can be distinguished from another by its measurable properties: energy, wavelength, frequency, and amplitude.

Vision (visible light), communications/entertainment (radio waves), medical imaging (X-rays), cooking food (microwaves), heating (infrared), and sunlight (ultraviolet) are examples of waves we encounter in our everyday life.

All electromagnetic waves transfer energy. When this energy is absorbed by matter, it is converted into heat because it causes the particles in the matter to vibrate more rapidly.

The main pattern observed in the electromagnetic spectrum is that waves with shorter wavelengths and higher frequencies have higher energy. For a wave of a given wavelength, amplitude is also related to energy. So a visible light wave with a higher amplitude would have greater energy and be observed to have a higher intensity.

Electromagnetic waves include radio waves, microwaves, infrared waves, visible light, ultraviolet light, X-rays, and gamma rays. All electromagnetic waves transfer energy. They all travel the same speed (i.e., the “speed of light”). The different waves along the spectrum differ in their wavelength, frequency, and energy. As you move from radio to gamma waves, wavelength decreases, frequency increases, and energy increases.

Students might have encountered radio waves (radio, TV, internet, cell phones), microwaves (microwave oven), infrared waves (heat from any object), visible light (sunlight and artificial lights), ultraviolet light (the Sun), possibly X-rays (medical imaging). Some students may have even encountered gamma rays (radiotherapy).

Red light has the longest wavelength and therefore the lowest energy of the visible light spectrum. Note that the prefix infra means below, so infrared radiation is just below red light on the electromagnetic spectrum. Violet has the shortest wavelength and therefore the highest energy of the visible light spectrum. Note that the prefix ultra means beyond, so ultraviolet radiation is just above violet light on the electromagnetic spectrum.

Astronomers use radio waves, microwaves, and infrared waves to view objects in space. Radiologists use X-rays to see inside the body. Thermal imaging cameras show areas of hot and cold. Firefighters can use such cameras to find people inside a smoke-filled room. Bees use ultraviolet waves to help them see patterns on flowers that do not appear to the human eye.