# Does light contain kinetic energy?

• Ingrid Eldevj
In summary: They are made of many individual waves but as they are interacting they form a single beam with the same momentum and direction. Would you like me to elaborate on that?Yes, please elaborate on this.
Ingrid Eldevj
Does light contain kinetic energy as it moves, or does it require mass.

It has kinetic energy, equal to its total energy because it has no mass.

Yes, light has kinetic energy and the photon is a massless particle so the light has no mass.

mfb said:
It has kinetic energy, equal to its total energy because it has no mass.

This is interesting.. If I make a Fourier series or integral to represent an electromagnetic wave traveling in vacuum, the individual plane wave components act similarly to harmonic oscillators, don't they? Then the individual modes should have Hamiltonian functions that consist of terms that are equivalent to the kinetic and potential energies in ##H = \frac{p^2}{2m}+\frac{1}{2}kx^2##... Does the total amount of "kinetic energy", as defined by this equivalence, have some actual name and physical significance?

hilbert2 said:
the individual plane wave components act similarly to harmonic oscillators, don't they
The concepts are not completely unrelated, but I'm not sure if we can call that "similarly".

The amount of kinetic energy of a beam of light is just its energy. It doesn't need an additional special name because "energy" is a good name already.

mfb said:
It has kinetic energy, equal to its total energy because it has no mass.

It has kinetic energy, equal to its total energy if it has no mass. Light can have mass and that's not limited to exotic objects like a geon. Almost all light you see in everyday life (e.g. sunlight) has mass. Even single photons can have mass (e.g. a single photon bessel beam).

DrStupid said:
It has kinetic energy, equal to its total energy if it has no mass. Light can have mass and that's not limited to exotic objects like a geon. Almost all light you see in everyday life (e.g. sunlight) has mass. Even single photons can have mass (e.g. a single photon bessel beam).

Mind elaborating on this?

Drakkith said:
Mind elaborating on this?

I'm afraid my English is not good enough for this phrase.

DrStupid said:
I'm afraid my English is not good enough for this phrase.

Would you mind elaborating about your previous post? Would you explain it in more detail?

In case of the geon it is quite easy to understand (even though it is was the most complicate example). It consists of light only but has enough mass to contain itself by its own gravity. The mass of normal light is not as easy to explain. Let's try it with the very simple case of two plane light waves in vacuum, superimposing each other with the angle ##\alpha##. That means for the momentums

$\frac{{p_1 \cdot p_2 }}{{\left| {p_1 } \right| \cdot \left| {p_2 } \right|}} = \cos \left( \alpha \right)$

If every wave has the energy E/2 then we also have

$\left| {p_1 } \right| = \left| {p_2 } \right| = \frac{E}{{2 \cdot c}}$

That results in the total momentum

$p^2 = \left( {p_1 + p_2 } \right)^2 = \left[ {1 + \cos \left( \alpha \right)} \right] \cdot \frac{{E^2 }}{{2 \cdot c^2 }}$

and therefore in the mass

$m = \sqrt {\frac{{E^2 }}{{c^4 }} - \frac{{p^2 }}{{c^2 }}} = \frac{E}{{c^2 }} \cdot \sin \left( {\frac{\alpha }{2}} \right)$

of the resulting light wave which travels with the speed

$\left| v \right| = \frac{{\left| p \right| \cdot c^2 }}{E} = c \cdot \cos \left( {\frac{\alpha }{2}} \right)$

along the bisecting line between the original waves. Of course normal light and Bessel beams are much more complex but the principle is the same. Plane parallel light waves (e.g. lasers) are exceptional cases in everyday life.

## What is kinetic energy?

Kinetic energy is the energy an object possesses due to its motion. It is calculated by the mass of the object multiplied by the square of its velocity.

## Does light have mass?

No, light does not have mass. It is composed of particles called photons, which have no mass but still carry energy.

## How does light travel?

Light travels in a straight line and at a constant speed of approximately 299,792,458 meters per second in a vacuum. It can also travel through certain materials such as air, water, and glass.

## Can light transfer kinetic energy?

Yes, light can transfer kinetic energy. When light interacts with matter, it can transfer its energy to the particles it strikes, causing them to move and gain kinetic energy.

## What factors affect the kinetic energy of light?

The kinetic energy of light is affected by its frequency and wavelength. Higher frequency light has more energy and can transfer more kinetic energy, while longer wavelengths have less energy and transfer less kinetic energy.

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