Radiation Pressure is exerted by Electromagnetic Radiation. Electromagnetic Radiation has momentum associated with its energy it carries, and its momentum is the energy divided by its speed, i.e., of light (c). Such pressure results when the radiation is absorbed or reflected, or when something emits radiation, and the total momentum is conserved, transferring the force, thus resulting in the pressure.
The pressure is small enough to be negligible in everyday life, but can be significant when it is steady over long intervals, e.g., for interplanetary navigation, which requires it as a factor. It is a factor in Planetary Migration, which are subject to pressures continuously for millions of years.
When it is extreme, such as inside or at the surface of a star, it is also potentially a significant factor.
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