Light can exert a force on objects through a phenomenon called radiation pressure. When light interacts with matter, it transfers momentum to the object it strikes. This transfer of momentum creates a force that can push or exert pressure on the object.
However, it's important to note that the speed of light itself does not change during this process. According to Einstein's theory of special relativity, the speed of light in a vacuum is constant and does not depend on the motion of the source or the observer. So, even though light can transfer momentum and exert a force on objects, its speed remains the same.
The energy required for light to exert a force comes from the light itself. When light transfers momentum to an object, its energy decreases by a small amount. This decrease in energy corresponds to a decrease in the light's frequency or wavelength. The energy lost by the light is transferred to the object in the form of kinetic energy, causing the object to experience a push or pressure.
In summary, light can push objects through radiation pressure by transferring momentum, but its speed remains constant. The energy required for this transfer of momentum comes from the light itself, resulting in a decrease in the light's energy and a corresponding decrease in frequency or wavelength.