Light is massless electromagnetic radiation. It is quite interesting that, according to Albert Einstein’s famous equation, E = mc² then light has no energy. So where does the energy that we use from it every day come from ?

Energy of light

Light actually carries energy without any mass. The above equation is actually a special case of the equation E² =p²c² + m²c⁴ where p is the momentum of the particle and c is the speed of light. It shows that the total energy of a particle is the combination of the mass energy and the motion energy of the particle.

The relationship between energy – mass – momentum according to the Pythagorean theorem.

When a particle is at rest, the equation becomes the familiar E=mc². In contrast, for a massless particle like light the equation would be E=pc and therefore all their energy comes from momentum.

No energy but still momentum

Another interesting question arises. Momentum p=mv right? So light has no energy, where does its momentum come from? We know that light has both wave and particle properties, and its momentum comes from wave properties.

You can imagine it more easily: two people grasping the ends of a rope, if one person pulls the end of the rope hard enough, the other person can be thrown away. The wave on the string has no mass, but it can still transfer momentum to the other person. Light is similar, it has no mass but has momentum, so it has energy.

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