It is true that he has difficulty understanding that something that has no mass can have energy. But, for example, if you imagine an expansive wave you will understand it. A person who is close to an explosion feels that shock wave, feels that a huge force pushes back. That wave or disturbance has no mass, it has the means by which it spreads, but it does have energy, you do not see it but you feel that energy that has pushed you backwards. That's the first thing we have to be clear about, waves have energy. Light is also a wave, so it has energy.
Mass is a quantity that we use in physics to express the amount of matter a body has at rest. And it is true that the light at rest has no mass. But light can not be at rest, it is always moving at the same speed, a speed that when light spreads in a vacuum we call c, those 300,000 kilometers per second. But it is a mistake to always associate energy with mass. Although assimilating it is difficult and even more so in the case of light.
To understand it, you must know that, in addition to a wave as we have already seen, light is also a corpuscle. Before modern physics, physicists and physicists had seen with experiments that light behaved like a wave but Albert Einstein, Based on the work of Planck, he realized that he also acted as a particle that was called "how much light" and what is the same as what we now call photon. Precisely Einstein's most famous equation, that of E = mc2, is where your question comes from because that equation says that the energy of a body at rest (E) equals its mass (m) multiplied by the velocity of the body. light squared (c2). Since this equation needs the mass to determine the energy, it can lead us to think that a massless system can not have energy but it is not. The above equation is only applicable for particles at rest and light is not. And here we come to the second characteristic of photons that allows us to understand their energy. As I explained to you at the beginning, they are always in motion, they do not stop. And in them, the energy comes precisely from there, from the movement itself
In physics, we call the momentum the linear moment and from that linear moment it is from where the energy to photons. To find it we substitute the mass (m) of the Einstein equation for the linear moment (which in the equations we call p) and thus we obtain the amount of energy of the photon. And that means that there are photons like those of visible light, which are able to capture our eyes, which have an energy 10,000 times smaller than, for example, those in X-rays, or millions of times less than the gamma rays that produce the radioactive elements.
Pascuala García Martínez PhD in Physics, Professor of Optics at the University of Valencia.
Question done via email by Bernardo Collado López
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