Radiant energy, also known as electromagnetic radiation (EMR), is energy transmitted without the movement of mass. Practically speaking, this is the energy found in electromagnetic waves, also known as light. Light is made of individual particles called photons, each carrying a small "packet" of energy. Because photons are so small, light energy is often measured in electron volts. Examples of radiant energy include the warmth that radiates from a hot stove and the warmth from direct sunlight.
Not all radiant energy is visible (see figure 1). Only photons within a very small range of energies can be seen by people, commonly known as visible light. Photons with lower energies are found in microwaves, radio waves, and as infrared radiation (which is felt as heat). Photons with higher energies are found in ultraviolet rays, in x-rays, and in gamma rays.
Figure 1. The electromagnetic spectrum:
more energy corresponds to shorter wavelength
and higher frequency
since all three quantities are closely related. The radiant energy of a photon can span many orders of magnitude (a factor of 1024
!), but there's only a very small portion that is visible, as shown here.
Ways to use collect radiant energy include:
- Solar power harvests radiant energy carried by the light from our sun by converting it into electricity.
- Biomass from plants. Plants are able to harness and use light energy in a process called photosynthesis. They absorb radiant energy from sunlight and transform it into useful chemical energy contained in molecules within their cells. Animals can obtain some of this chemical energy as food by eating plants, or by eating animals which eat other animals or plants. The biomass created by plants can also be used in biofuels.
- Radioisotope thermal generators use radiant energy from a radioactive source to collect heat or even make electricity.
Ways to use radiant energy include:
- Human vision requires radiant energy (light) to see
- The burner on a stove.
- Sitting around a campfire.
- Getting a sun tan.