Warning: this is a maths blog!

Some time ago, light was discovered to be a wave/particle duality. Count Louis de Broglie wondered (as you do) if electrons could be part of a wave.

They could.

He derived an incredibly beautiful formula from his research:

[image] http://www.s-cool.co.uk/a-level/assets/learn_its/alevel/physics/wave-particle-duality-and-electron-energy-levels/wave-particle-duality/equation1.gif[/image]


The funny h is wavelength, the p is momentum and the normal h is Planck's constant which is 6.626068 x 10^-37

Light definitely has a wavelength so it must have momentum which means it must also have mass. But what is its mass?

To find momentum, we would have to divide Planck's constant by its wavelength. Light's wavelength is 0.5 x 10^-6 which means that it's momentum is:
1.3252136 x 10^-27 kgm/s.

Now, momentum is mass multiplied by speed. We want mass so we divide momentum by its speed. The speed of light is 300,000,000 m/s. Therefore its mass is
4.417378667 x 10^-36 kg.
Multiply by 1000 to get the mass in grammes.

Don't forget that this mass is still really small.

Now for explanations. The momentum it exerts only effects atoms in our body. It can't effect our entire bodies. So the momentum is transferred in the quantun scale.

As for the mass, it isn't technically mass in a conventional sense. Mass, in physics, is defined as the amount of 'stuff' in a substance which is basically your photons (light particles). A common misconception is that weight is mass. Weight is a force due to gravity. Light doesn't have this weight. But, gravity can still influence it.

Take a black hole for instance. In a mediocre gravitational strength, light is bent inwards slightly. As we increase the strength, light gets bent further inwards until it eventually is pulled into the centre of the black hole. This means light can't escape. Thus, black holes would appear black since light wouldn't reach an observer.