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What is radiant energy?
The warmth we feel when the sun touches our skin is one of many examples of radiant energy.
This type of energy travels in the form of electromagnetic waves, such as visible light, infrared radiation, and ultraviolet rays.
The same thing happens when we stand near a heater or turn on a lamp. Radiant energy is also present in less visible phenomena, like radio waves or Wi-Fi signals.
This means solar energy travels through space without requiring direct contact or a physical medium.
Origin of the term and basic concept
The concept of radiant energy has its roots in 19th-century physics. The term comes from the Latin radius, meaning ‘ray,’ a reference to the way energy spreads in rays or waves.
It was first used in 1890 by American physicist Nikola Tesla to describe energy emitted as electromagnetic radiation. However, its theoretical foundation was established decades earlier thanks to James Clerk Maxwell and his famous electromagnetism equations (1865).
In essence, radiant energy is energy that travels as electromagnetic waves, a phenomenon that doesn’t require air or surfaces to propagate and can travel even through a vacuum.
The sun, for instance, sends us its energy as electromagnetic radiation, which then turns into heat or light when it reaches Earth.
Radiant energy vs. radiation
Although the terms are sometimes used interchangeably, radiant energy and radiation are not the same.
- Radiation: the act of emitting and propagating waves or particles through space. It is the movement or the journey itself.
- Radiant energy: the content being transported during that journey —the ‘cargo’ carried by the waves.
A simple metaphor helps: radiation is like a moving truck, while radiant energy is the merchandise inside.
The same occurs with the sun: it emits radiation at many wavelengths (ultraviolet, visible, and infrared). On the other hand, radiant energy is the portion we harness —in photosynthesis, through solar panels, or as the sensation of heat on our skin.
Understanding this difference is essential since radiation can occur without becoming usable energy (for example, part of the sun’s radiation is reflected by the atmosphere), while radiant energy is what actually impacts our surroundings and can be used or felt.
Everyday examples of radiant energy
- The sun: the largest source of radiant energy, essential for life and Earth’s climate.
- Light bulbs and LED lamps: convert electricity into light energy.
- Microwave ovens: transmit radiant energy in the form of microwaves to heat food.
- Radio and TV antennas: emit electromagnetic waves that allow us to hear music or watch programmes.
- Solar panels: capture the sun’s radiant energy and convert it into usable electricity or heat.
How is radiant energy measured?
Radiant energy is measured in joules (J), the standard unit in the international system. When measuring how much energy reaches a surface over a period of time, units such as watts per square meter (W/m²) are used.
Why radiant energy matters
Radiant energy is essential in three key areas. Biology: without it, photosynthesis wouldn’t exist. Plants convert solar radiation into chemical energy —the foundation of the food chain.
Technology: telecommunications, radio, television, remote controls, and the Wi-Fi we use daily rely on it.
Energy: through photovoltaic and thermal solar panels, we can harness solar radiation to produce clean electricity and sustainable heat.
Radiant energy is invisible, but it’s with us every step of the way —
in light, in heat, in communication, and in energy generation. Understanding it means recognising that it exists in every sunbeam, every bulb, and every wireless connection.
Q&A
No. Solar energy is a type of radiant energy —the kind emitted by the sun. But radiant energy also includes other types, such as X-rays, microwaves, and infrared radiation.
It depends. Visible light and infrared radiation are not harmful under normal conditions. Still, excessive exposure to ultraviolet rays, X-rays, or gamma radiation can be dangerous to your health.
Radiant energy travels in the form of electromagnetic waves, while thermal energy refers to the heat contained within a material. When the sun warms your skin, it first arrives as radiant energy and then becomes thermal energy.
No. A large part is reflected or scattered in the atmosphere. But thanks to technologies such as solar panels, we’re able to capture more of it —and do so increasingly efficiently.