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Light is a kind of radiant energy, which propagates straight in a vacuum at a speed of 3.0x 10 ^ m / s. It is believed that light is both a kind of electromagnetic wave and a kind of particle flow. This understanding of the wave-particle duality of light is the basis of light quantum theory. The theory of light quantum theory believes that different light rays can cause photochemical reactions due to the different energy of the light quantum. Photoelectric effects. Fluorescent grazing. Puncture effects and thermal effects. These effects have become the theoretical basis of phototherapy medicine.