Why Is Radiant Energy Kinetic?

Radiant energy is the energy of electromagnetic radiation. It refers to the energy emitted from sources in the form of electromagnetic waves or photons. Examples of radiant energy include sunlight, heat from fire, x-rays, and gamma rays. Kinetic energy is the energy possessed by an object in motion. The faster an object moves, the more kinetic energy it has. When objects collide, the kinetic energy transfers to heat, sound, and deformation.

Radiant Energy Defined

Radiant energy is electromagnetic radiation transmitted through space. It refers to energy that travels in the form of electromagnetic waves or particles called photons. Some examples of radiant energy include visible light, ultraviolet light, infrared radiation, gamma rays, and radio waves. Radiant energy does not rely on a material medium like air or water to transmit it. Rather, it can travel through the vacuum of space. The different types of radiant energy have varying wavelengths and frequencies across the electromagnetic spectrum. However, they all move at the speed of light in a vacuum – approximately 186,000 miles per second. Radiant energy derives from the accelerating charge of electrons and is propagated through electromagnetic fields. It exhibits wave-particle duality, having both wave-like and particle-like properties. The term radiant energy encompasses all forms of electromagnetic energy.

Kinetic Energy Defined

Kinetic energy is the energy possessed by an object due to its motion. It depends on the mass and velocity of an object and is directly proportional to both of them. Kinetic energy can be described by the following equation:

Kinetic Energy = 1/2 x Mass x Velocity2

The faster an object moves, the more kinetic energy it possesses. Kinetic energy is a form of mechanical energy and is present in all moving objects, from a rolling ball to a flying airplane. Some examples of kinetic energy in everyday life include a flowing river, wind, and any moving vehicle. The kinetic energy of an object changes with its velocity. If the object speeds up, its kinetic energy increases. If it slows down, the kinetic energy decreases. Kinetic energy is a key concept in physics and understanding motion.

Light as Electromagnetic Radiation

Light is a form of radiant energy that travels in waves. Light is part of the electromagnetic spectrum, which includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. The only difference between these types of radiation is their wavelength and frequency.

Visible light that humans can see is just a small part of the full electromagnetic spectrum. The wavelengths of visible light range from about 380 nanometers (violet) to about 740 nanometers (red). Other forms of electromagnetic radiation outside this range are not visible to the human eye.

All forms of electromagnetic radiation travel at the speed of light in a vacuum, which is approximately 186,000 miles per second or 300,000 kilometers per second. They travel in transverse waves, meaning they oscillate perpendicular to the direction they are traveling. The oscillations are electric and magnetic fields that propagate through space and carry energy.

When light travels from one medium to another, its speed and wavelength change but its frequency remains constant. This means that visible light stays visible even when moving between materials. Overall, light is a propagating electromagnetic wave that transfers energy through space.

Wave-Particle Duality

Light exhibits properties of both waves and particles. This seemingly contradictory concept is known as wave-particle duality. As a wave, light can be described by a wavelength and frequency. However, light can also behave as a stream of particles called photons. The energy of a photon depends on its frequency. So at the quantum level, light has characteristics of discrete particles. Yet at larger scales, light interacts as an electromagnetic wave that can diffract and interfere. Wave-particle duality is a fundamental principle of quantum mechanics. It illustrates that matter and energy can display both wave-like and particle-like properties, depending on the experiment performed.


Light energy is carried in discrete packets called photons. A photon is a quantum, or smallest possible discrete amount, of electromagnetic energy. They are massless particles that exhibit properties of both waves and particles. Photons travel at the speed of light and each photon contains a specific amount of energy determined by its frequency. Higher frequency photons contain more energy than lower frequency photons. This quantized nature of light energy carried in photons is a key reason why radiant energy is kinetic.

Photon Momentum

One of the key properties that demonstrates radiant energy is kinetic is photon momentum. Despite having no mass, photons exhibit momentum according to the formula:

p = h/λ

Where p is the photon’s momentum, h is Planck’s constant, and λ is the photon’s wavelength. Even with no mass, photons have momentum proportional to their frequency or inversely proportional to their wavelength. This momentum allows photons to exert force and transfer energy when absorbed by matter, providing direct evidence that light behaves as kinetic energy.

The fact that photons have momentum despite lacking mass puzzled early quantum physicists. It seemed paradoxical based on Newtonian mechanics. However, numerous experiments have definitively proven that photon momentum is real. When a beam of light strikes an object, it imparts a small force in the direction of the beam proportional to the number of photons and their energy levels. This demonstrates that light transfers kinetic energy and momentum to matter through radiation pressure.

Absorption of Light

When a photon is absorbed by an atom or molecule, all of the photon’s energy and momentum is transferred to that atom or molecule. This absorption causes the electron in the atom or molecule to jump to a higher energy state. As a result, the atom or molecule gains both energy and momentum from the photon.

Specifically, the momentum of the photon is directly transferred to the electron during absorption based on the law of conservation of momentum. Since the photon has momentum according to the formula p=h/λ, where p is momentum, h is Planck’s constant, and λ is the wavelength, absorption of a photon imparts its momentum to the absorbing particle.

The amount of momentum transferred depends on the energy and wavelength of the photon. Higher energy, shorter wavelength photons impart greater momentum when absorbed. This direct transfer of photon momentum to matter demonstrates that radiant energy in the form of light behaves as kinetic energy during absorption.

Radiant Energy is Kinetic

The fact that light, a form of radiant energy, can be described as both a wave and a particle is known as wave-particle duality. When light is described as a particle, these discrete packets are called photons. Photons have momentum, which means they possess kinetic energy.

When a photon is absorbed by an atom or molecule, its momentum is transferred. This transfer of momentum provides evidence that radiant energy in the form of photons has kinetic properties. The absorption and emission of photons is how energy is transmitted by electromagnetic radiation. Thus, radiant energy can be classified as a form of kinetic energy.


In conclusion, radiant energy such as light exhibits kinetic properties through the momentum of photons. Even though photons have no mass, they carry momentum in the form of electromagnetic radiation. When a photon is absorbed by an object, it imparts that momentum to the object, like a billiard ball striking another. This transfer of momentum demonstrates that light behaves as a kinetic form of energy. While radiant energy such as visible light was long thought to be purely electromagnetic waves, quantum mechanics revealed the particle-like behavior of photons. The quantized momentum of these massless particles shows that radiant energy is indeed kinetic at a fundamental level.

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