Photography inverse square law, what do you mean? In order to help photography, I had to bite the bullet and say it. Inverse square law is one of the most important knowledge in flash photography, which can be used to describe the decrease of light intensity at different distances. I will introduce it in detail below. Let's have a look.

Photography inverse square law 1 Illuminance inverse square law:

The illuminance and zhi of the object surface are not only related to the luminous intensity of the light source, but also related to the distance from the light source to the object surface. When the distance between the object surface and the light source is constant, the illuminance of the object surface is directly proportional to the luminous intensity of the light source. On the contrary, when the luminous intensity of the light source is constant, but the distance from the object surface changes, the illuminance of the object surface will be inversely proportional to the square of the distance.

If the illumination with an illumination distance of 1m is x units, the illumination with an illumination distance of 2m is 1/4x units.

This law only applies to general point light sources, such as electric lights and flashlights. It is not completely suitable for long tubular light sources and convergent light beams.

The importance of photography:

Illuminance inverse square law is often used in artificial light photography, which has important practical value in indoor lighting and indoor natural light exposure estimation. When the intensity of the light source is constant and the distance between the object and the light source changes, the multiple of the surface illumination change of the object is calculated. This multiple is also the multiple that the exposure combination needs to be changed.

Criminal photographic photos are used to record the scene and identify physical evidence.

That photo should have certain clear standards. The shooting environment of criminal photography is often indoor. In the case of poor lighting conditions, it is necessary to increase the light source, and calculate the correct exposure through the illumination inverse square law, so as to ensure that the photos can clearly meet the work needs.

The Historical Background of the Discovery of inverse square law 2 1 and Two Laws in Photography

The law of universal gravitation is the result of more than 20 years' research by the great scientist Newton. He began to think from the apple landing to the interstellar movement and summed up the law of action between objects. Finally published in 1687, its mathematical expression is:

. On the basis of Kepler's and Tycho's research on the laws of planetary motion, he summed up and popularized the gravity of the interaction between any objects, claiming that everything in heaven and earth follows the same law, completely denying the different ideas of religious forces in heaven and earth.

This is a leap in the history of human cognition. Newton successfully explained the tidal phenomenon by using gravity, and then the discovery of Neptune and Pluto further confirmed the correctness and establishment of the law of gravity, unified the movement in the sky and the movement on the ground, unveiled the first veil of the mystery of the universe, and took the first step for human beings to understand the universe and nature.

Coulomb's law was established by French scientist Coulomb in 1785. Coulomb noticed many similarities between electrostatic force between charges and gravity, and boldly assumed that the law of electrostatic action and gravity had similar forms. He expressed the force of charge as:

It is called "Coulomb's Law" by later generations.

Both force and distance obey the inverse square relationship. The electric quantity Q in Coulomb's law is equivalent to M in gravity. The difference is that gravity is always gravity, and coulomb force can be either gravity or repulsion.

2. On the electrostatic constant k and the gravitational constant g.

Many years after Newton discovered gravity108, Cavendish, a British scientist, measured the gravity constant g in 1798 more accurately with a sophisticated torsion balance device. At that time, no one exceeded his measurement accuracy. Before that, people only knew that there was such a constant, but they didn't know how big it was, which hindered people from studying a series of problems related to planets such as mass, density and radius. The gravitational constant is the most inaccurate measurement.

Because to measure G, it is necessary to measure the gravity F first. The gravity is too weak to shield the interference of other objects, so it is difficult to do experiments. The data recommended by the Science and Technology Data Committee of the International Council for Science and Technology 1986 is g = 6.6725985×10-1m 3/kg *.

For the electrostatic constant K, Coulomb also uses torsion to measure the value of K in the process of deducing the law of electrostatic action. I don't know whether Coulomb used a torsion balance inspired by Cavendish's experiment, or whether they thought the same thing. It's still unclear.

The measurement of electrostatic constant is much more accurate than that of gravitational constant, because the force between two charged bodies is obvious and it is easy to shield the external interference. In the international system of units, k = 8.9875×10.9 nm 2/c 2.

3. Whether the square of the distance in the two laws is reliable.

Will inverse square law be biased? That is, whether the exponent of R must be equal to 2 is a problem that scientists care about. If the inverse square ratio in Coulomb's law deviates, the rest mass of photons will theoretically be non-zero, which will lead to the faster speed of light in vacuum (vacuum scattering), the blackbody radiation formula will be modified and the charge will not be conserved, which will shake the foundation of the whole building of electromagnetism and physics.

For hundreds of years, with the appearance of precision instruments and the improvement of experimental technology, the distance index has reached the accuracy of 2+3× 10 with the efforts of many scientists. Although the accuracy is very high, whether it is strictly equal to 2 is still widely concerned by physicists and needs further testing.

If the exponent of gravity deviates, it will cause a series of problems such as the invalidation of Gauss theorem in force field, which is contrary to what we have learned. Of course, it is impossible to imagine that these basic physical laws have been destroyed. For example, it is announced that the exponent of R is slightly larger or smaller than 2, even if it is only a little, physics may be revised again.

Also, whether the similarity in form between the law of universal gravitation and Coulomb's law means some internal qualitative unity of these two functions is still a mystery, which needs us to reveal.

4. Scope of application of these two laws

In middle school, the charge in Coulomb's law should be regarded as a static point charge (two charges are relatively static, both in inertial system), and the object in the law of universal gravitation should be regarded as a particle, but if it can't be regarded as a point charge and a particle, its value can be obtained according to the vector superposition principle of force and calculus theory.

Coulomb's law is the basic law in electromagnetism. A large number of experiments, including the famous A-particle scattering and geophysical exploration, show that Coulomb's law is reliable in the range of 10 ~ 10m, but whether Coulomb's law can be accurately established in the range of 10m and astronomical distance has not been confirmed by experiments.

Of course, there is no reason to expect that Coulomb's law will be broken at a long distance. The law of universal gravitation has achieved great success in discussing the motion of celestial bodies in the solar system. How far can its power extend? Now, the new version of Newton's general theory of relativity has proved that "the universality of the theory of universal gravitation transcends the edge of the universe" (TongFu.kabu Mandarin). Just like this, from the apple landing to the moon, from the sun to the universe, above, he looks for the green void, below, the yellow spring, all physical phenomena involving gravity come down to a simple law. In the vast universe, the seemingly chaotic motion of stars is constrained by a simplified mathematical language-inverse square law. Isn't this the crystallization of the power of knowledge and human wisdom?

There are four basic interactions in nature: gravitational interaction, electromagnetic interaction, strong interaction and weak interaction.

Strong-weak interaction is a short-range force, and its range of action does not exceed nuclear linearity. In the microscopic world, gravity is insignificant compared with electromagnetic force. For example, the coulomb force between electrons and protons is 10 times that of gravity. So in the micro field, coulomb force plays a role.

But in the whole electrically neutral universe, gravity makes celestial bodies move in orbit regularly, and it is like a baton, which regulates the movement of the whole macro world.

The two inverse square law, the foundation of the existence and development of physics, support the giant pillars of physics, but they are presented to us with strikingly similar expressions. We believe that the world is unified and natural science is a science that pursues truth, goodness and beauty.