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Principle and application of scattered light in photography

The direct sunlight on the photographer is covered by clouds, but we can still see the "light" around us. In fact, what we see is the scattered light generated by the light around us. This scattered light produces a beautiful "radiation" visual effect.

The photographer cleverly blocked a considerable part of the direct sunlight on the camera with leaves, and the surrounding sunlight created a special atmosphere under the scattering effect. Is the author interested in expressing the artistic conception of Buddha's shining light? The smoke produced by the incense sticks in the temple provides the best scattering source, which makes the scattered light stand out in the picture.

Please note that the smoke formed by incense on the screen is light blue. This is because the linearity of carbon particles that make up smoke is very small, and the scattered light generated by smoke conforms to Rayleigh scattering conditions, so the blue light component in the scattered light is much stronger than the red light component. What we usually call "curling smoke" is said to be caused by Rayleigh scattering.

The reason why these photos have a strong "atmosphere" is to skillfully use the scattering phenomenon of light, that is, to use the "scattered light". So, what is scattered light and how is it generated?

Scattering is a ubiquitous optical phenomenon. When light passes through various turbid media, some light will change its original propagation direction and diverge in all directions, and the original incident or refracted beam will be weakened. Even without facing the direction of the incident beam, people can clearly see the light scattered by these media. This phenomenon is the scattering of light.

In the definition of optics, scattering refers to the phenomenon that the light beam deviates from the original propagation direction and propagates around due to the action of tiny particles (heterostructures) or molecules in the medium on light.

The above picture is a photo of laser performance debugging in Tiananmen Square, Beijing. The reason why you can see the laser beam that cuts through the night sky and shoots into the sky is because the laser beam scatters in the process of shooting into the sky. At first, the laser shot into the sky. If there is no scattering phenomenon, we can't see the light beam standing on the ground.

Please take a close look at the work of our high-power laser tube:

The test uses a one-meter-long high-power He-Ne laser. It should be said that the intensity of laser is very strong, but because the laboratory is a clean environment and the scattered light is very weak, we can't see the emitted laser beam at all. If you want to see the laser beam, you have to spray smoke where it passes, but smoking is strictly prohibited in the laboratory.

The previous example shows that scattering only occurs when there is a scattering source-turbid medium.

Please take a close look at the work of our high-power laser tube:

The test uses a one-meter-long high-power He-Ne laser. It should be said that the intensity of laser is very strong, but because the laboratory is a clean environment and the scattered light is very weak, we can't see the emitted laser beam at all. If you want to see the laser beam, you have to spray smoke where it passes, but smoking is strictly prohibited in the laboratory.

The previous example shows that scattering only occurs when there is a scattering source-turbid medium.

In addition, even if all impurities are carefully removed, that is, in very pure gas or liquid, the refractive index is uneven and scattering will occur due to the fluctuation of medium density caused by molecular thermal motion. Although their scattering intensity is much less than that of Tindal scattering, this phenomenon is still widespread. We call the scattering of light in this pure substance molecular scattering.

Experiments show that the scattering laws of tiny foreign bodies (whose linearity is much less than the wavelength of incident light) and molecular scattering are different from those of large foreign bodies (Dindar scattering), and their scattering intensity is related to the wavelength of incident light, that is, the scattering intensity is inversely proportional to the fourth power of light wavelength, which is the Rayleigh scattering law. This kind of scattering is also called Rayleigh scattering. In Rayleigh scattering, due to the short wavelength of blue light, its scattering intensity is stronger than that of red light with long wavelength, so there are more components of blue light in scattered light.

In addition, even if all impurities are carefully removed, that is, in very pure gas or liquid, the refractive index is uneven and scattering will occur due to the fluctuation of medium density caused by molecular thermal motion. Although their scattering intensity is much less than that of Tindal scattering, this phenomenon is still widespread. We call the scattering of light in this pure substance molecular scattering.

The laws of Dindar scattering and Rayleigh scattering are different, which is the fundamental reason why we can see blue sky and white clouds.

We know that the wavelength range of visible light is 400 nm (blue-purple) to 700 nm (red). The wavelength of the red end is 1.75 times that of the blue-violet light. Its fourth power is about 9.38 times. That is to say, in the visible range, the scattering intensity of short-wavelength blue-violet light is nearly ten times that of long-wavelength red light. If you take photos, you should exceed the third exposure (8 times the third exposure).

In the case of good air conditions, that is, the air is relatively clean and there is less suspended dust, the main scattering is Rayleigh scattering, and there are more blue components in the scattered light. This is the blue sky and white clouds we are looking forward to seeing. However, in some cities, especially big cities with serious air pollution, because the air is full of linear suspended dust particles, a large part of the scattered light is produced by Tindal scattering, which is white. Therefore, the sky is white. When I took the picture, the weather was fine and the sky was blue, but there was still too much dust near the ground, which was far less clear than that at high altitude.

White light contains all kinds of light from red to blue and purple. When sunlight passes through the atmosphere, it will scatter, mainly Rayleigh scattering related to the wavelength of light. Under the action of this scattering, the shorter wavelength component (blue light) in the transmitted light is scattered, and the longer wavelength component (red light) is more. The proportion of red light in transmitted light is related to the distance that light travels in the atmosphere. As can be seen from the figure below, the distance of sunlight passing through the atmosphere in the morning and evening is much longer than that at noon (generally 6- 10 times longer), and the scattered blue light is much more. Therefore, the sun looks reddish in the morning and evening.

In addition, we live on the earth and look at the sky. The reason why we see different scenes during the day and at night is also because of the scattering of the atmosphere. If there is no scattering, the sky we see during the day will be the same as at night, with stars twinkling on a black background. The only difference is that there is a very bright sun shining brightly on the black background. This is not a fantasy. In fact, astronauts have seen this phenomenon from space. And because the earth is surrounded by the atmosphere, astronauts see the earth from space and see a beautiful "blue planet".

In the case of small linearity of scattered particles (mainly Rayleigh scattering), scattered light is polarized light. Therefore, polarized filters can be used to enhance the effect of blue sky and white clouds when shooting.

Please look at the effect comparison after using polarizer. The left side of the above picture shows the effect without polarizer, and the right side shows the effect with polarizer.

Because the polarizing filter filters out a part of the white light and lets all the blue polarized scattered light pass through, the sky is bluer and the white clouds are darker, which has a very good effect.

This is a photo of a famous building in Shanghai. The roof of the building is like a lotus pedestal. Every night, several bright lights on the roof pierce the night sky, which is also a scene of the Bund. We can see these beams, which is the function of scattering. If the air over the city is not clean, the more suspended dust there is, the stronger the scattering will be and the light beam will appear very bright. On the contrary, the light beam will appear very weak. If we basically can't see these beams at night, maybe our city will be a blue sky during the day.

Scattered light is often very important when taking such photos. The fog in the mountains is actually small water droplets suspended in the air, which is an ideal scattering source. Because the droplet size is much larger than the wavelength of light wave, it is mainly Dindar scattering, and the scattered light is white.

The mist in the morning and the scattered light generated by the water vapor on the river make the picture feel like a fairy tale scene. Fog is formed by many tiny water spots, which can produce a lot of scattered light. Therefore, the scenery under the mist can clearly distinguish the foreground, the middle scene and the distant view, thus showing the sense of depth of space. In addition, mist can also cover the chaotic background, which is conducive to highlighting the main image in the picture and improving the expressive force of the work.