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# Photons Save Print Edit
Photons
• Astrophysics • Atoms and Radioactivity • Electricity • Energy Physics • Engineering Physics • Fields in Physics • Force • Further Mechanics and Thermal Physics • Magnetism • Measurements • Mechanics and Materials • Medical Physics • Nuclear Physics • Particle Model of Matter • Physical Quantities and Units • Physics of Motion • Radiation • Space Physics • Turning Points in Physics • Waves Physics Photons are particles that have no weight and are thus massless. They are the particles responsible for carrying energy in the form of electromagnetic waves. When a particle releases energy, photons appear. The process of radioactive disintegration of unstable isotopes is one of the many events that can emit photons.

## Photon properties

Photons, which travel in the vacuum of space at nearly 300,000,000m/s, are the fastest particles in the known universe.

They have a wavelength, which defines their energy. Shorter wavelengths, as found in radiation known as gamma rays, have more energy. Radio waves, on the other hand, have larger wavelengths and low energy.

Photons have no mass but are a form of pure energy; they are not made by other particles. Photons do not react to gravity forces until they are massively large such as in black holes. Figure 1. The sunlight is made of photons. In a rainbow, we see the decomposition of light in photons with different wavelengths. Source: Eric Dillalogue, Flickr (CC BY 2.0).

### The energy of a photon

The energy of a photon depends on its wavelength. Shorter wavelengths have higher energy as they oscillate faster, while larger wavelengths, which oscillate more slowly, have lower energy. To obtain the photon energy, you need to multiply the velocity of the light c in the vacuum by the Planck constant h and divide this by the wavelength of the photon λ. The formula to obtain the photon energy is: The Planck constant is equal to 6.63 ⋅ 10 ^ -34 J s, and the light velocity in the vacuum is close to 3 ⋅ 10 ^ 8 m/s.

### Energy wavelength dependence

The wavelength-energy relationship is true for all forms of electromagnetic energy. Measuring the wavelength of a photon can tell us some of its characteristics. We can calculate the energy of several wavelengths and compare them in order to understand how the wavelength affects a photons energy.

You might hear the radio presenter mention that they transmit on FM at 90 Mhz. If you want to know the energy of the radio waves, you first need to obtain the wavelength. To calculate this, you use the relationship between the wavelength λ and the frequency f. V is the velocity of the wave, which you can take as the light velocity in the vacuum, i.e., 3⋅10^8 m/s. The frequency is in Hertz, which is the inverse of the second, and the prefix is mega, which means 1⋅10^6.  This wavelength has a familiar scale. Now you can calculate the energy by using the photon energy equation:  That is not too much energy, and the wavelength is, of course, very large.

The visible light energy (yellow colour)

If you want to calculate the energy in the visible spectrum for the colour yellow, a quick internet search will tell you that it has a frequency of 5.12 ⋅ 10 ^ 14 Hz. You then divide the velocity of the light by the frequency of the yellow light.  The wavelength value is very small. We are talking micrometres here or 1 ⋅ 10 ^ -6 m scales. To calculate the energy, you need to use the energy-photon relationship:  The amount of energy, in this case, is still small compared to everyday things, but it is much larger than the energy of the radio waves.

The photons from an x-ray machine

The frequency of the x-ray machine used by your doctor has a value of 3 ⋅ 10 ^ 17 Hz. First, you determine the wavelength λ.  Calculating the energy of the x-rays using this value, we get the energy of the x-ray photons. The energy coming from the x-rays is three orders of magnitude larger than the energy of the visible spectrum of light in the colour yellow. Figure 2. Approximate spectrum of energy vs wavelength for any photon. Source: Manuel R. Camacho, StudySmarter.

### Light velocity

It is important to point out that the velocity of light varies depending on the medium in which it moves. Light travels faster in a vacuum, i.e., at a speed of around 300,000,000 m/s. In the atmosphere, light travels a little slower, and in solid materials such as diamonds, it travels significantly slower, so much so that the speed of photons reduces to less than half their speed in a vacuum.

## Photons - key takeaways

• Photons are particles with no mass that are responsible for carrying energy in the form of electromagnetic waves.
• The visible light, such as the sunlight, consists of photons.
• The energy of a photon depends on its wavelength. Larger wavelengths have less energy, while smaller wavelengths are more energetic.
• Photons are the fastest particles in the known universe, travelling at a speed of 300,000,000 m/s in a vacuum, which is known as the speed of light.

A photon is a particle that is responsible for exchanging energy and carry the electromagnetic force.

Photons are basic particles that are not made of any other particle.

Photons have one quantity that can be measured. This is their wavelength, which also defines their energy. To measure the wavelength, you need electronic sensors that can detect the photon’s frequency. The wavelength is equal to the velocity of light divided by the photon’s frequency, i.e.:

f = c/λ

## Final Photons Quiz

Question

What is a photon?

The particle responsible for carrying energy and the electromagnetic force.

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Question

What is the approximate velocity of the photon in a vacuum?

300,000,000 [m/s].

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Do photons have mass?

No, they don’t.

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Are photons pure energy?

Yes, they are.

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Do photons appear in the process of disintegration?

Yes, they are part of the disintegration process.

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Can the disintegration of a radioactive material emit photons?

Yes, it can.

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On which variable does the energy of photons depend?

On the photon’s wavelength.

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Name a photon with a large wavelength.

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Name a photon of a very small wavelength.

X-rays.

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Does the velocity of light depend on the medium in which it travels?

Yes, it does.

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In a diamond, does light travel faster or slower?

It travels slower.

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Is a radio transmission a form of electromagnetic waves?

Yes, it is.

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Is visible light a form of photons?

Yes, it is.

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What is the unit of the Planck constant?

Joules per second.

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What is the unit of the frequency of light?

Hertz.

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Are hertz the inverse of the time unit?

Yes, they are.

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Are photons the fastest objects in the universe?

Yes, they are.

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