Shulou(Shulou.com)11/24 Report--

When you are excited because of the sunny weather, you pick up the camera and take a few pictures, only to find it difficult to get an image of the simplest three-dimensional sphere of the sun in the two-dimensional plane. what appears on the paper is the sun that cannot see the boundary clearly and the "dazzling light" that keeps emitting outward.

I believe that many friends have found this phenomenon, this time Xiangning will lead us to seriously think about what this is all about. Interpret "dazzling Light" in physical language.

Sunlight in the camera | Source: author's mobile phone shoot

The light under the lens | Source: the author's cell phone photography is a clich é: the wave-particle duality of the sun or the light around the lamp has a proper term-starlight 🌟; the physical mechanism of starlight is familiar to everyone-the diffraction of light.

Before we understand the diffraction phenomenon of light, we must first mention the wave-particle duality, which is familiar to all of us. Light has the properties of both waves and particles, which is the basic property of light.

The fluctuation of light was first seen in the theory of the Dutch physicist Huygens. However, on the one hand, this theory has been shelved because it is unable to explain the diffraction of light, and on the other hand, because of Newton's authority. Until the phenomenon of double-slit interference experiment and the success of Maxwell equations in theoretical derivation, the wave theory of light is gradually stabilized.

Interference of light | Source: 51sole.com the particle property of light can be proved by photoelectric effect. The phenomenon described in this experiment is the phenomenon that the metal emits electrons when a beam of light is irradiated on the metal surface. It was for photoelectric effect (attention, not relativity) that Don Einstein won the Nobel Prize in physics.

It is a clich é: the diffraction of light can be described clearly through the classical single-slit diffraction experiment of light. In this experiment, after a beam of light passes through a slit, a series of diffraction stripes are displayed on the screen. It is like when light encounters an obstacle in the process of propagation and continues to travel around the obstacle.

Single slit diffraction of light | Source: blog.sciencenet.cn this clearly reflects the fluctuation of light. Moreover, this diffraction stripe is most obvious when the width of the slit is about the same as the wavelength of the light.

The diffraction of light is usually divided into two types: Fresnel diffraction and Fraunhofer diffraction. Fresnel diffraction occurs when the distance between the obstacle and the light source and the diffraction pattern is relatively close. Fraunhofer diffraction can be said to be a special case of Fresnel diffraction, which occurs when the distance between the obstacle and the light source and the diffraction pattern is infinitely far. It is conceivable that the incident light in this case is almost parallel.

Diffraction of light | Source: 51wendang.com light is so wonderful that many friends may not remember the relationship between the interference of light and the diffraction of light. Then use the screenshot of 👇 below to help you remember. You are welcome to take it.

The difference between diffraction and interference | Source: MOOC at this time, what will happen if a single seam is replaced with a small hole? It is not difficult to imagine that at this time the diffraction images become larger and larger radius, sharing a center and interlocking diffraction rings.

In fact, when the shapes of slits, polygonal holes and round holes are different, the diffraction patterns are also various. and the research shows that this diffraction phenomenon also follows the law that "the even-numbered holes produce corresponding to the even number of thorns, and the odd-numbered holes produce 2 * corresponding to the corresponding edges." It's wonderful.

Diffraction pattern of regular polygonal holes (simulation) | Source: [1]

Diffraction pattern of round hole and round bottle | Source: physical explanation of 51wendang.com light. Now that we understand the diffraction of light, we can talk about why "dazzling light" can be seen behind the lens, the sun on the screen and around the lamp.

We think of the lens as a round hole and the sun as a point light source. It's just that the light emitted by the sun 149.6 million kilometers away, for anything on earth, can be seen as a parallel incident beam in most cases. In this case, the Fraunhofer diffraction of sunlight appears under the lens.

As the saying goes, the end of physics is mathematics, and the end of mathematics is philosophy. Of course, the wonderful mathematical language also describes the properties of diffraction stripes, which is reflected incisively and vividly even in the simplest wave optics theory.

Suppose an is the width of a single slit, D is the distance from the slit to the light screen or the focal length of the lens, and K is the number of diffraction stripes, the width of the diffraction fringes, the wavelength, and the angle between the diffraction stripes and the normal of the slit plane. The above parameters satisfy the following formula. Through these formulas, we can know the diffraction pattern under certain conditions like a prophet. Of course, everyone was already a prophet when they were in high school.

Through this calculation, we can find that the shorter the light wavelength is, the smaller the width of the diffraction fringe is, the closer the diffraction fringe is to the middle region, and the longer the light wavelength is, the larger the width of the diffraction fringe is, the wider the diffraction pattern is.

Spectrum | Source: cwwz.net based on the above calculation, let's take a look at Xing Mang's "true face". As we all know, the visible band is from 380nm to 750nm. Assuming that a light source can emit the red light of 750nm, the green light of 530nm and the purple light of 400nm at the same time, then each order of the diffraction pattern around the light source in this lens will be a mixed region from the middle to the middle, gradually changing to purple-green-red color stripes, that is, colorful stars.

The aperture in the camera lens is usually polygonal. When we shoot with the camera, when the incident light passes through the aperture, it diffracts unilaterally along the edge of the aperture, and then forms a diffraction pattern with a lot of "dazzling light" on the receiving screen, which is the star.

You may have to ask, if the aperture is round, will there be stars? Of course not, just like the diffraction of a circular hole, if the aperture is a perfect circle, the stars will disappear and be replaced by concentric diffraction rings. This is also determined by the diffraction characteristics of light.

Of course, due to the construction of the aperture: the aperture is generally made up of multiple blades. The amount of light received by the camera sensor (CCD or CMOS) through the aperture is determined by changing the composition of the blade. Therefore, when shooting distant light sources such as the sun with a camera or shooting with a small aperture, there is a high probability that stars will be produced; moreover, there are even numbers of stars.

Squinting to open a new world has a similar phenomenon. When we raise our heads affectionately, our eyes are stung by the light that is not soft; but when we squint our eyes quickly, we pull out long rays around our eyes. What's wrong with this?

Lens and human eye | Source: [1] it is not difficult to guess, which is also due to the diffraction of light. Assuming that the human eye is the same as the aperture of the lens, if it is compared to a small hole in the diffraction experiment of light, the retina of that person's eye system, like the screen behind the lens in the camera system, can be regarded as a light screen.

So, when the eyes squint into a line, the upper and lower eyelids form a slit. In our sophisticated visual system, the diffraction of light also occurs. As a result, long lines of light will be pulled out around the eyes, especially in the upper and lower directions.

Finally, Xiangning reminds you that the light and sunlight will be relatively strong, and the direct exposure of the sun and strong light for a long time may not only bring damage to the camera's photosensitive elements, but also cause more irritation and damage to the eyes. So adjust the aperture, wear sunglasses firmly, and it's better not to look directly at them!

References:

Chen Gengjian, Zhou Xinyu, he Chunqing, et al. The star of the light is explained by the far-field diffraction of the polygonal hole [J]. Physics experiment, 2019. 39 (03): 27-31.

Xin Xiu, Xian Fuzheng, Sun Shangqian, Han Guangbing, Xu Jianqiang. Research on Star Mang phenomenon based on Fraunhofer diffraction [J]. College Physics experiment, 2021 Journal 34 (01): 39-43.

Wang Ying, Zhou Yuhan, Xia Xiangsuo. Theoretical and experimental study of star awn phenomenon [J]. College Physics, 2020. 39 (09): 41-46. 71.

Han Peishan, Wu Hongjin, Huang Bowen, Yang Haolin, Jia Xinyan, Liu Qijun, Chang Xianghui, Wei Yun, Fan Daihe. Study on the phenomenon of stars in camera negatives [J]. Physics and Engineering, 2019 and 29 (04): 82-87.

Chen Xianqin, Zhang Zhihua, Zhang Rui. The realization of matlab simulation of "Star Mang" phenomenon [J]. Physics and Engineering, 2019 and 29 (S1): 101-105.

Chen Jiafu, Wang Zhiyong, Yu Bin, Lin Danying. Design and analysis of single slit diffraction simulation experiment [J]. College Physics, 2020. 39 (12): 67-74.

This article comes from the official account of Wechat: Institute of Semiconductors, Chinese Academy of Sciences (ID:bdtdsj), author: Yan Xiangning, Editor: southern Cat

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