why the sun emits most of its energy in the form of visible light?

Human eye could perceive this visible light and used it to look at the objects.

Explanation:

There are 3 forms of radiation emitted by the sun. They are Infrared radiation, Ultra Violet radiation as well as visible radiation.

Visible light is the type of light can perceive by the human eye and utilizes to see the objects.  

Since the sun is hotter (5800 K) than the earth, it emits mostly the radiation energy in the form of this visible light.

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