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Light has accompanied humanity from bonfires, torches, and candles to lamps, screens, lasers, microwaves, and space telescopes. But behind all these forms of illumination and radiation, there is the same physical basis: photons, particles associated with electromagnetic radiation.
Simply put, a photon can be understood as a “packet” of light energy. NASA explains that electromagnetic radiation can be described as a flow of photons, each carrying energy and moving, in a vacuum, at the speed of light. The difference between radio waves, visible light, X-rays, and gamma rays is not in the “type” of particle, but in the energy of each photon.
Photons have no rest mass, but they have energy
A common mistake is to say that the photon “has no mass and no energy.” The first part needs to be specified: the photon has no rest mass. The second is wrong: photons have energy. This energy depends on the frequency or wavelength of the radiation.
This is why blue light photons have more energy than red light photons, and ultraviolet photons have more energy than infrared photons. NASA describes photons as light particles with a specific amount of energy. Electromagnetic radiation forms a broad spectrum: radio, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.
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Why does the light go out so quickly when we turn off the lamp?
When a lamp is on, it continuously emits photons in various directions. These photons hit walls, furniture, clothes, skin, and other objects. Some of them are reflected, allowing us to see the environment. Another part is absorbed by the materials, transforming light energy into other forms of energy, mainly heat on a microscopic scale.
When the switch is turned off, the lamp stops producing new visible photons. The photons that were already in the room continue traveling for an extremely short time until they are absorbed by the surfaces. Since the speed of light is very high and the distances within a room are small, this process happens too quickly for human perception. Therefore, to us, the room seems to go instantly from lit to dark.
What happens to the light from the stars?
The light from stars is also made up of photons. In the case of the Sun, the radiation originates in the interior of the star, in an extremely dense environment. There, the photons do not travel in a straight line to the surface. They interact many times with particles in the solar plasma, being absorbed and re-emitted in different directions. NASA describes this path as a kind of “drunken walk,” in a zigzag, which can take from tens of thousands of years to much more, depending on the region considered.
After the light escapes the solar surface, the situation changes. In the space between the Sun and the Earth, it travels about 150 million kilometers in approximately eight minutes. This happens because space is much emptier than the solar interior, drastically reducing interactions along the way.
Does light lose energy simply by traveling?
Not in the way many popular videos suggest. A photon traveling through a vacuum does not “spend” energy like a car consuming fuel. It can cross enormous distances without colliding with anything. That’s why we can observe very distant stars and galaxies.
However, the light from very distant objects can undergo redshift, known as cosmological redshift. According to NASA, this happens because, as light travels over great distances, space itself expands, stretching the wavelength of the radiation. When the wavelength increases, the observed energy of the photon decreases.
This is different from the old idea called “tired light,” which suggested that photons would simply lose energy during the journey. This hypothesis is not the explanation accepted by modern cosmology. The dominant interpretation is that the redshift of distant galaxies is linked to the expansion of the Universe.
Why isn’t the night sky completely bright?
If there are so many stars, it seems reasonable to ask: why isn’t the night as bright as the day? The answer involves several factors: the Universe has a finite age, light takes time to travel, galaxies are distributed over great distances, and the Universe is expanding. Furthermore, part of the very old radiation has been stretched to longer wavelengths.
ESA explains that the cosmic microwave background radiation is a kind of “fossil light” from the young Universe. It was once much more energetic, but today it is mainly detected in the microwave range, precisely because its light has been cooled and stretched over cosmic history.
Light connects our daily life to the Universe
The same physics that explains a lit lamp also helps to understand stars, galaxies, telescopes, Wi-Fi, radio, microwaves, and medical exams. All these phenomena involve electromagnetic radiation and photons with different energies.
Therefore, the central idea of the original text is correct: photons are fundamental for us to see the world and study the cosmos. However, the explanation needed important adjustments. Photons have energy, they do not disappear by magic, they do not simply wear out by traveling, and the redshift of galaxies is better explained by the expansion of the Universe, not by a common loss of energy along the way.
Sources consulted: information about photons and light energy was verified at NASA and in the agency’s material on the electromagnetic spectrum. The explanation about sunlight reaching Earth is based on NASA Goddard. The cosmological redshift was checked at NASA/Hubble, and the cosmic background radiation was verified at the ESA.
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