While looking at the stars, we see the light emitted by them thousands of years ago. But why photons disappear at the moment when we turn off the switch?
The light travels almost infinite distances through a huge, often empty interstellar and intergalactic space, without colliding with anything.
Cosmos, in turn, is a unique case: in the intervals between massive objects, light travels almost through an absolute vacuum.
It means that the possibility of collision of light with something is extremely small. That is why it is easy for light to overcome large distances without any barrier.
So what happens when you turn off the light in the room? There are two options: either reflection or absorption. We can see a reflection in the case of a mirror which reflects every visible object. This happens every time when light collides with a smooth surface.
How smooth the surface should be for this, depends on the wavelength of the light. Thus, optical light needs a smoother surface for pure reflection than radio waves, which are much longer.
The second option is absorption. Through this process, the stones heat up in the sun. The energy coming out of the sun makes their surface hot. Any light can be absorbed, and not only its infrared (thermal) part.
A bad mirror can absorb enough light to make your reflection look like a subtle image. It is more difficult to feel the heat energy from the electric bulb today because they are produced in such a way that the energy is used to spread the amount of light in the room, which helps us to see in the darkness.
So, when a light is lit, it emits photons that fly in all directions around the room and collides with every object in it. These objects absorb most of the cutting photons, but also reflect some of them, which, in fact, allows us to see things in the room.
When the light is turned off, new photons are no longer emitted, and those that are still inside the room are reflected from objects countless times until they are completely absorbed.
If you are fond of elementary particle physics, then you most likely know that light consists of millions of tiny particles known as photons. These fundamental particles carry all types of electromagnetic radiation, including radio waves, ultraviolet waves, microwaves and, of course, visible light.
When you enter the room and turn on the lamp, it immediately fills with light. And if more precisely, the room is filled with trillions of photons that help us see everything inside it.
But where does the light go when you turn off the lamp? What happens to the billions of photons in a room? Do they disappear somewhere or just cease to exist?
Before answering these questions, let’s refresh the main concepts.
Photons: elementary particles carrying light
Perhaps you already know that visible light is a form of electromagnetic radiation and a small part of the electromagnetic spectrum, which includes radio waves, infrared radiation, ultraviolet radiation, gamma radiation, and so on.
A photon is the most fundamental particle of any type of electromagnetic radiation, whether it be radio waves carrying Wi-Fi signals, microwaves heating food in a microwave oven, or visible light that helps us see the world around us.
With a rest mass equal to zero, a photon moves at a speed of almost 300 thousand kilometers per second in a vacuum (this, in turn, is the speed of light).
How photons light things?
A light source, such as a lamp, being in a room, emits millions of photons that fly in all directions when turned on. Since the lamp is in the room (that is, in an enclosed space), the photons emitted by it collide with everything that is in their path, thereby illuminating everything in the room.
Like this, a photon beam illuminates a small closed space:
Where photons disappear when the light source turns off?
While the lamp is shining, the room receives a constant stream of photons. Of all the countless photons crashing into an object in the room (for example, a table), some will be absorbed, while others will reflect and lose some of the energy in the process.
These reflected photons will collide with something else in the room and lose some more energy. In fact, the photon will continue to bounce off objects until some of them absorb it.
Thus, the room remains lit for as long as the lamp is working. However, as soon as you turn it off, everything changes quickly.
Photons emitted before turning off the lamp will continue to bounce off objects until objects in the room completely absorb them. And it manages to happen in just a fraction of a millisecond.
If the lamp continued to shine, the rapid absorption of these photons would not change anything, since it would constantly emit new portions of photons into the room. But, as we have already found out when the lamp turns off and new photons are no longer emitted, objects in the room absorb previously emitted photons. Their energy is used to heat the objects that have absorbed them, since, according to the first law of thermodynamics, energy cannot be created or destroyed, it only transfers from one process to another.
All of these — the emission of photons by a lamp, their reflection, and absorption by other objects — takes place in about one million seconds.
This is unusually fast so that a person can perceive, or at least makeup, about any phenomenon that can be readily digestible. That is why the room is plunged into darkness when the light goes out.
Returning to what we talked about at the beginning of the article, we note that the situation in outer space would be somewhat different.
Unlike turning off the light in a room on Earth, photons emitted in outer space would continue to move through an endless vacuum for a long time before colliding with anything.
Before Leaving Read It: Atmosphere Facts: Earth Losing 90 Tons Of Oxygen Everyday into The Space.