Light can be divided into two categories, what will cause light pollution and what will make people see the same thing from different directions.

Classification of light: artificial light and natural light.

Light pollution: There are many reasons for light pollution. In the world, the causes of light pollution are generally divided into three categories, namely white light pollution, artificial sunlight pollution and colored light pollution. In daily life, people's common light pollution is mostly caused by the reflection of mirror buildings and unreasonable lighting at night, which will cause discomfort to the human body. In photography, light pollution will affect the quality of photos taken. We usually use ND filter or CPL filter to filter light pollution. If you are an astronomy enthusiast shooting the starry sky, it is recommended to use UHC filter to filter out the urban light damage and the sky glow.

People can see non-luminous objects from different directions because they reflect light. Light on a rough surface will reflect light in all directions, not in a specific direction, so we can see objects from all directions. This phenomenon is called diffuse reflection. Diffuse reflection is the most common reflection phenomenon in our life. Only diffuse reflection can see everything in the world clearly.

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