Parents and Teachers: Support Ducksters by following us on or .

Science >> Physics for Kids

# Physics for Kids

## Photons and Light

We learned on the light page that light has the unique characteristic of behaving like both a wave and a particle at the same time. We then learned some of the ways that light behaves like a wave. Now we want to explore how light also behaves like a particle. Scientists have named the particles that make up light photons.

What is a photon?

In physics, a photon is a bundle of electromagnetic energy. It is the basic unit that makes up all light. The photon is sometimes referred to as a "quantum" of electromagnetic energy.

Photons are not thought to be made up of smaller particles. They are a basic unit of nature called an elementary particle.

Properties of a Photon

Photons have some basic properties that help define what they are and how they behave. These properties include:
• They have zero mass.
• They have no electric charge.
• They are stable.
• They carry energy and momentum which are dependent on the frequency.
• They can have interactions with other particles such as electrons.
• They can be destroyed or created by many natural processes.
• When in empty space, they travel at the speed of light.
Photons Interact with Matter

Photons behave like particles in that they can interact with matter. In some cases the energy of the photon is absorbed by the matter. In this case the extra energy may be emitted as heat. One example of this is the blacktop of the road getting hot in the sun.

Our eyes also interact with photons. When a photon strikes the eye it is turned into electrical energy that is then transmitted to brain to form an image.

When the energy from photons is absorbed by matter, the matter can emit electrons. This process is called the photoelectric effect. The photoelectric effect is a property of light that is not explained by the theory that light is a wave. This is one of the main reasons that scientists chose to treat light as both a wave and a stream of particles.

Planck's Constant

The relationship between the energy of a photon and its frequency can be described by the following equation:

E = hv

where E is energy, v is frequency, and h is Planck's constant. Planck's constant is always the same (i.e. the "constant") and equals 6.62606957 * 10-34 m2 kg / s.

• Not only is light made up of photons, but all electromagnetic energy (i.e. microwaves, radio waves, X-rays) is made up of photons.
• The original concept of the photon was developed by Albert Einstein. However, it was scientist Gilbert N. Lewis who first used the word "photon" to describe it.
• The theory that states that light behaves both like a wave and a particle is called the wave-particle duality theory.
• Photons are always electrically neutral. They have no electrical charge.
• Photons do not decay on their own.
Activities