LUMINOUS BODIES: CHARACTERISTICS AND HOW THEY GENERATE THEIR OWN LIGHT - PHYSICAL - 2025

A luminous body is called any natural or unnatural object that emits its own light, this being the part of the electromagnetic spectrum visible by human eyes. The opposite of a luminous object is a non-luminous one.

Non-luminous objects are visible because they are illuminated by the light emitted by luminous objects. Non-luminous bodies are also called illuminated bodies, although they are not always in that state.

The Sun, a luminous body that illuminates the sky and the sea. Source: pixabay

Luminous objects are primary sources of light since they emit it, while non-luminous objects are secondary sources of light because they reflect that produced by the former.

Examples of luminous and non-luminous bodies

Luminous objects

There are objects in nature capable of emitting light. These include:

- Sun.

- The stars.

- Luminescent insects, such as fireflies and others.

- The rays.

- The aurora borealis or northern lights.

The following are man-made luminous objects:

- Incandescent lamps or bulbs.

- The flame of a candle.

- Fluorescent lamps.

- Led lights.

- The screen of a mobile phone.

Non-luminous objects

In nature there are many objects that do not emit light by themselves, but can be illuminated:

- The Moon, which reflects the sunlight.

- The planets and their satellites, which also reflect the sunlight.

- Trees, mountains, animals reflect the light of the sky and the Sun.

- The blue sky and clouds. They are visible due to the scattering of sunlight.

The artificial luminous body bulb that illuminates our nights. Source: pixabay

Characteristics of luminous bodies and their light

The main characteristic of luminous bodies is that the light with which we can see them is produced by the object itself.

We can see people and objects thanks to the light emitted by luminous bodies, whether natural or artificial. And also because nature has endowed us with the organs of vision.

In the absence of luminous bodies it is impossible to see everything that surrounds us. If you have ever experienced total darkness, then you know the importance of luminous bodies.

That is, without light there is no vision. Human and animal vision is the interaction between the light emitted by luminous bodies and that reflected by non-luminous bodies with our light sensors in the eye and with our brain, where the image is finally constructed and interpreted.

Vision is possible because the light emitted or reflected by objects moves through space and reaches our eyes.

Photons

A photon is the smallest amount of light that a luminous body can emit. Photons are emitted by atoms of luminous bodies and reflected or scattered by non-luminous ones.

Vision is only possible when some of these photons, emitted, scattered or reflected, reach our eyes, where they produce an electronic excitation in the endings of the optic nerve that carry an electrical pulse to the brain.

How do luminous bodies generate light?

The photons are emitted by the atoms of the luminous bodies when they have been excited in such a way that the electrons of the atomic orbitals go to states of higher energy, which later decay to states of lower energy with the consequent emission of photons.

Every body, if its temperature is increased, becomes a light emitter. A piece of metal at room temperature is a non-luminous body, but at 1000 degrees Celsius it is a luminous body, because electrons occupy higher levels and when they decay to lower levels they emit photons in the range of the visible spectrum.

This is what happens at the atomic level with all luminous bodies, be it the Sun, the flame of a candle, the filament of an incandescent light bulb, the atoms of the fluorescent dust of the energy saving light bulb or the atoms of the LED diode, which is the latest artificial light body.

What varies from one case to another is the excitation mechanism for electrons to pass to higher energy atomic levels and then decay and emit photons.

All we see is the past

Vision is not instantaneous, since light travels at a finite speed. The speed of light in air and in a vacuum is of the order of 300 thousand kilometers per second.

Photons of light that leave the surface of the Sun take 8 minutes and 19 seconds to reach our eyes. And the photons emitted by Alpha Centauri, our closest star, take 4.37 years to reach our eyes if we are looking at the sky.

The photons that we can observe with the naked eye or through a telescope in the Andromeda galaxy, the closest to our own, will have emerged from there 2.5 million years ago.

Even when we see the Moon, we are seeing an old Moon, because what we are looking at is an image from 1.26 seconds ago. And the image of the players of a soccer game that we see in the stands 300 meters from the players, is an old image one millionth of a second in the past.

Duality of light

According to the most accepted theories, light is an electromagnetic wave, as are radio waves, microwaves with which food is cooked, microwaves from cell phones, X-rays and ultraviolet radiation.

However, light is a wave but it is also made up of particles called photons, as we stated earlier. Light has this dual behavior, which is known in physics as wave-particle duality.

All the variety of electromagnetic waves differ in their wavelength. The part of the electromagnetic spectrum that the human eye is able to perceive is called the visible spectrum.

The visible spectrum corresponds to a narrow range of the electromagnetic spectrum between 0.390 microns and 0.750 microns. This is the characteristic size of a protozoan (amoeba or paramecium).

Below the visible spectrum, in wavelength, we have ultraviolet radiation whose wavelength is comparable to the size of organic molecules.

And above the visible spectrum is infrared radiation, the size of which is comparable to the tip of a needle. At the tip of this needle, 10 to 100 protozoa can fit, that is, 10 to 100 wavelengths of the visible spectrum.

In contrast, microwaves have wavelengths between centimeters and meters. Radio waves have lengths from hundreds of meters to thousands of meters. X-rays have wavelengths comparable to the size of an atom, while gamma rays have wavelengths comparable to the atomic nucleus.

Colors and visible spectrum

The visible spectrum includes the variety of colors that can be distinguished in a rainbow, or in sunlight scattered on a glass prism. Each color has a wavelength that can be expressed in nanometers, which is one millionth of a millimeter.

The light spectrum and its wavelengths in nanometers (nm), from highest to lowest, are as follows:

- Red. Between 618 and 780 nm.

- Orange. Between 581 and 618 nm.

- Yellow. Between 570 and 581 nm.

- Green. Between 497 and 570 nm.

- Cyan. Between 476 and 497 nm.

- Blue. Between 427 and 476 nm.

- Violet. Between 380 and 427 nm.

The luminous black body, energy and momentum

Light has energy and momentum. Each color in the visible spectrum corresponds to photons of different energy and different momentum or momentum. This was known thanks to the pioneers of quantum physics such as Max Planck, Albert Einstein and Louis De Broglie.

Max Planck discovered that light energy comes in packets or quanta, whose energy E is measured in Joules and is equal to the product of a fundamental constant of nature known as Planck's constant, which is denoted by the letter h and the frequency f in Hertz.

E = h ∙ f

This discovery was made by Planck in order to explain the radiation spectrum of a luminous body, which only emits radiation but does not reflect any, known as the "black body" and whose emission spectrum changes according to temperature.

Planck's constant is h = 6.62 × 10 ^ -34 J * s.

But it was Albert Einstein who affirmed, undoubtedly, that light were photons with energy given according to Planck's formula, as the only way to explain a phenomenon known as the photoelectric effect, in which a material illuminated with light emits electrons. It was for this work that Einstein received the Nobel Prize.

But the photon, like every particle and despite not having mass, has an impetus or momentum given by a relationship discovered by Louis De Broglie in the framework of the wave-particle duality of the photon and quantum objects.

The de Broglie relation states that the momentum p of the photon is equal to the quotient of Planck's constant h and the wavelength λ of the photon.

P = h / λ

The color red has a wavelength of 618 × 10 ^ -9 m and a frequency of 4.9 x 10 ^ 14 Hz, so the energy of a photon is 3.2 × 10 ^ -19J and its momentum is 1.0 × 10 ^ -27 kg * m / s.

At the other end of the visible spectrum is violet with a wavelength of 400 × 10 ^ -9 m and a frequency of 7.5 x 10 ^ 14 Hz, so the energy of a photon is 4.9 × 10 ^ -19J and its momentum is 1.7 × 10 ^ -27 kg * m / s. From these calculations we conclude that violet has more energy and more momentum than red.

References

1. Tippens, P. 2011. Physics: Concepts and Applications. 7th Edition. Mac Graw Hill. 262-282.
2. Wikipedia. Visible Spectrum. Recovered from wikipedia.com
3. Wikipedia. Electromagnetic spectrum. Recovered from wikipedia.com
4. Wikipedia. Light source. Recovered from wikipedia.com
5. Wikibooks. Physics, optics, nature of light. Recovered from: es.wikibooks.org