How much does light weigh?

Answer 1

A quantum of light is also called a photon. Photons have energy and so must also have mass. This is from the famous equations of Einstein.

We know the energy of a photon ... (Planck's constant) times (c)/(wavelength).

We know the speed of the photon ... it's the speed of light.

We know the energy is [E= m c2 ].

What if we use these to come up with a mass for the photon ?

Planck = h

Lightspeed = c

Wavelength = L

Photon energy = hc/L

h c / L = m c2

h / L = m c

==> m = h / (c L) That's it !

For L, let's use the officially accepted peak sensitivity of human vision. That's in the

middle of the spectrum, somewhere around green, at 555 nm = 5.55 x 10-7 meter.

h = 6.626 x 10-34

c = 3 x 108

m = h / (c L) = 6.626 x 10-34 / (3 x 108 x 5.55 x 10-7) = 0.4 x 10-35

The green photon's mass is 0.00000000000000000000000000000004 gram.

Near the surface of the earth, it weighs 4 x 10-35 newtons, or about 1 x 10-35pounds.

Answer 2

Well light can weigh something, but I know this for sure: gravity can pull light in so therefor there has to be a number of its weight (even if it is very small).