# Thread: Why do light pressure exist?

1. Light have no mass , then how do they puss object and how strong is the light pressure?

2. Originally Posted by johnzxcv
Light have no mass , then how do they puss object and how strong is the light pressure?
When it is said that photons have zero mass, it is meant that (the general equation is ). Therefore, photons have non-zero momentum. By reflecting or absorbing photons, the change in momentum of the photon is imparted to the object.

3. Originally Posted by johnzxcv
Light have no mass , then how do they puss object and how strong is the light pressure?
Light has no rest mass. It does have inertial mass (sometimes referred to as relativistic mass). It's defined as the m in p = mv. Therefore light transfers momentum to whatever it impacts and that change in momentum is seen as a force. Force per unit area is pressure.

4. This guy explains it good.

5. Radiation pressure exists, infact there are propulsion devices that uses this concept - Solar Sails, Beam Propulsion etc.So it is very real.
Photons transfers energy from one point to another point in space.
Imagine our sun, it has a lot of thermal energy (which is basically kinetic energy of the molecules in it).
It is always at higher temperature than the surrounding space so it can emit radiations of varying energy.
Radiations contain photon which has a certain energy, which is actually the energy lost by the sun.
When photon hits another body, some of this energy is transferred to the body.
So the body will be subjected to some thrust in the direction in which photon strike the body surface.
The velocity change of the body due to a single photon is microscopic.
So while designing solar sails we use thin ultra light mirrors with huge surface area as sails.
The photon reflected by the object will have a lower energy due to change in its frequency.

Photons have momentum P= planks constant/wavelength & Energy =(momentum*c)

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