Colored spectrum

In the previous article “How is the energy of the sun transmitted“, it is explained that the sun transmits energy to the earth’s surface in the form of light radiation. When we talk about the use of solar energy, we use the energy of the sun’s rays. So, what is the nature of sunlight and what are its characteristics?

Modern physics believes that all kinds of light, including sunlight, are a form of existence of matter. Light is both wave-like and particle-like, which is called the wave-particle duality of light. On the one hand, any kind of light is an electromagnetic wave of a certain frequency or frequency range, which is essentially no different from ordinary radio waves, except that its frequency is relatively high and its wavelength is relatively short. For example, the frequency of white light in sunlight is at least 10,000 times higher than that of radio waves in the centimeter band. Therefore, no matter what kind of light, it can produce the phenomena of reflection, refraction, diffraction, and coherence. Therefore, we usually call light “light wave”. On the other hand, the light emitted by any matter is a stream of particles composed of discrete, moving particles with mass and energy. These particles are so tiny that they cannot be seen even with the most powerful electron microscopes in modern times. These microscopic particles are called photons or photons, and they have a specific frequency or wavelength. The energy of a single photon is extremely small, they are the smallest unit of energy. But even in the faintest light, the number of photons exceeds tens of millions. In this way, the concentration can generate energy that people can feel. Scientific research has shown that photons or light rays of different frequencies or wavelengths have different energies, and the higher the frequency, the greater the energy.

The sunlight that people can see is called visible light, which is white. However, scientific practice has proved that it is not a monochromatic light, but is composed of 7 colors of light: red, orange, yellow, green, cyan, blue, and purple, which is a kind of polychromatic light.

Each color of light has a corresponding wavelength range. The wavelength of red light is 700nm, and the spectral range is 640~750nm; the wavelength of orange light is 620nm, and the spectral range is 600~640nm; the wavelength of yellow light is 580nm, and the spectral range is 550~600nm; the wavelength of green light is 510nm and the spectral range is 480~550nm; the wavelength of blue light is 470nm and the spectral range is 450~480nm; the wavelength of purple light is 420nm and the spectral range is 400~450nm. Usually people arrange the light band diagram of the various colors of sunlight in the order of frequency or wavelength, which is called the solar spectrum.

The sun not only emits visible light, but also emits a lot of light invisible to the human eye, and the wavelength range of visible light only occupies a small part of the entire solar spectrum. The entire solar spectrum includes three parts: ultraviolet region, visible region and infrared region. But its main part, that is, the backbone with strong energy, is composed of wavelengths of 0.3 to 3.0 μm. Among them, the ultraviolet region with a wavelength of less than 0.4μm and the infrared region with a wavelength greater than 0.76μm are the ultraviolet and infrared rays invisible to the human eye; the visible region with a wavelength of 0.4~0.76μm is what we see as white light. In the solar radiation reaching the ground, the light in the ultraviolet region accounts for a very small proportion, about 8.03%; mainly the light in the visible region and the infrared region, accounting for 46.43% and 45.54% respectively.

Different wavelengths of light in sunlight have different energies. Light with maximum energy at the outer surface of the Earth’s atmosphere has a wavelength of about 0.48 μm. However, on the ground, due to the existence of the atmosphere, when the solar radiation passes through the atmosphere, ultraviolet and infrared rays are more absorbed by the atmosphere, and the ultraviolet and visible areas are scattered more by atmospheric molecules and particles such as clouds, so the distribution of solar radiation energy with wavelength is more complicated. The general situation is: on a clear day, the sun is 4 to 5 hours before and after noon, and the light with the most energy is the green light and the yellow light part; in the morning and evening, the most energetic light is the red part. It can be seen that the light with the greatest energy on the ground has a longer wavelength than the outer surface of the atmosphere.

In the solar spectrum, different wavelengths of light have different effects on matter and the ability to penetrate objects. Ultraviolet rays are very active, which can produce strong chemical effects, biological effects and excitation fluorescence, etc.; while infrared rays are not very active, and mainly cause thermal effects after being absorbed by objects; as for visible light, because of its wide frequency range, it can not only play a biological role in sterilization, but also can be converted into heat after being absorbed by objects. The growth of plants mainly depends on the absorption of the visible spectrum, and a large amount of ultraviolet rays with wavelengths shorter than 0.3 μm are harmful to plants. Infrared rays with wavelengths exceeding 0.8 μm can only increase the temperature of plants and accelerate the evaporation of water, but cannot cause photochemical reactions (photosynthesis). The main effects of sunlight on human skin are: the formation of erythema and burns, which are mainly caused by ultraviolet rays with a wavelength shorter than 0.38 μm; the photosynthesis of fat on the skin surface into vitamin D3, which can prevent rickets; the skin develops tan, which is mainly caused by light with a wavelength of 0.3~0.45μm.

Light travels very fast. The solar radiation far away 150 million kilometers away only takes 8min19s to propagate to the ground. The most accurate speed of light obtained so far is 299 792.456 2km per second, usually 300,000 kilometers per second.