Mass form of energy - The PHOTONS

 

Image: pixabay 

It took our scientists more than 100 years to figure out whether light is a particle or a wave. But after gaining a clear understanding of light, man was able to answer many of the mysteries of the universe. In this collection, we will discuss in detail why light is called a wave and a particle, the effects of gravity on light, and whether the energy of light has mass. 

Every object we can see has an energy. That energy can be potential energy or kinetic energy. The energy of an object at rest is called potential energy and the energy of an object in motion is called kinetic energy . 

In that sense, light has no potential energy. Because light does not stand still. It has only kinetic energy. Here we will see in detail whether this kinetic energy goes in wave form or in particle form. 

Wave form of light:

Newton, who lived in the 16th century, said that light propagates in the form of particles. He predicted that if the particle is very small, it will be blue and if it is large, it will be red. Physicist Christiaan Huygenes , who lived in the same period , said that light propagates in the form of waves. He believed that light oscillates up and down in directions perpendicular to the direction of propagation. 

Image: wikimedia commons 

Image: wikimedia commons

In the 1800s, Thomas Young , a university researcher, conducted an experiment to prove his claim . In this experiment a light beam passes through some slits as shown in the figure above. Here the slits must be smaller than the wavelength of light. At the end of this experiment he noticed that the light was bright in some places and absent in others. This is not possible with a particle.

For example, when we throw a ball towards a hole of smaller diameter, the ball cannot penetrate through the hole. But when the water from a water pipe is pushed rapidly through the small hole, the water, although it cannot follow a straight path, will penetrate through the edges of the hole and go to the next area. 

The same has happened in this experiment. Perhaps if light had been a particle, it certainly could not have traveled across the gap. Thus Thomas Young proved that light does not propagate in the form of particles, but in the form of waves . The image below explains why the light appears brighter in just a few places.

When two waves collide, they propagate as above. Therefore, at the end of the test, the light appeared bright only in certain areas.

Particle form of light:

After Thomas Young's experiments, everyone began to believe that light travels in the form of waves. After him, the famous physicist Albert Einstein ran his thinking about light at the quantum level. In 1905 he experimentally demonstrated that light also has a particle form. The name of the experiment is the photoelectric effect . 

Image: wikimedia commons

At the end of this experiment, he found that when light is incident on a metal, electrons are ejected according to its frequency. The test results are as follows.

• There is no time lag between the incient of light rays and the emission of electrons.
• There is no change in the kinetic energy of the emitted electrons when increasing  the intensity  of the light beam  .
• As the frequency  of the light beam increased, the kinetic energy of the electrons increased.

Maybe if light were in wave form...

• It takes a certain amount of time for the electrons to pick up energy from the light waves and leave.  
• And when increasing the intensity of light, the kinetic energy of electrons and their number should increase. 

When none of this happens properly, Einstein proved that light propagates in the form of energy pockets . These packets of energy were later called photons .

According to Thomas Young's experiment, light propagates in wave form. But according to Albert Einstein's experiments, light propagates in the form of particles. Both of these tests are correct! Hence, Einstein concluded that light has both a wave form and a particle form. 

He won the Nobel Prize in 1921 for his photoelectric effect.

Mass Form of Energy:

Whether photons, packets of energy, have mass is a slightly different question. Actually these photons have no mass. They are, however, subject to gravity! An example of this is that light is attracted through black holes. 

How can a massless object be subject to gravity?

Light has no rest mass. Rest mass is the measured mass of an object when it is at rest . Light has no rest mass. Because light does not stand still. But it has inertial mass. Ordinarily, the momentum of an object is the product of the object's mass and velocity. 

P=MV

Here inertial mass is momentum divided by velocity. 

M=P/V

Light, though massless, has a constant momentum and velocity. So with these, we can find the inertial mass of light. But the mass of light has nothing to do with its momentum. The momentum of light can be written as below.

P= hν/c 

Here, h represents Planck's constant, ν represents the frequency of light, and c represents the directional speed of light. So, here it is clear that light has no rest mass. But for this, it must behave like a particle with mass, since it has specific momentum and velocity. Those particles are photons . Or, they can also be called energy packs . 

Effect of Gravitation on Light:

Image: picryl 

When light has no rest mass, it must not be bound by gravity. But when light approaches a black hole, it is pulled in by the hole's gravitational pull. The reason for this is as follows:

 

For example, imagine dropping a stone of a certain mass downwards from a certain height on Earth. First, when we drop the stone, its kinetic energy would be zero. As the stone approaches the ground, its kinetic energy has reached its maximum. 

 

So, here the velocity of the stone when it reaches the ground is greater than the directional velocity it had when it was released. This is because of the Earth's gravity. Since photons have inertial mass, they must be bound by gravity. Because according to physics, inertial mass and gravitational mass are equal. 

 

Do not think of photons as particles. It just goes in the form of energy packs. However, as we have said before, it has a certain momentum and velocity, so it is considered a particle in relation to a gravitating object. But the photons, from the surface of the Earth, do not increase in speed when they reach the ground. Because the speed of light in the universe is constant.

 

Instead, the frequency of the light is increased. This increases its momentum and also increases its inertial mass. But on Earth, the change in frequency of photons is very small. Only in highly gravitating objects, such as black holes, do the frequencies of photons increase greatly. 

 

As the frequency increases, the inertial mass of the photons also increases. As this inertial mass increases, the photons become more susceptible to the influence of gravity. This is how light is bound by gravity.