If light has energy but no mass, how can it have momentum?
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Light has the ability to exert a force on things – it can “push” objects. That’s how a solar sail works.
Looking at light as a wave, it has an electric field and a magnetic field. When the electric field hits an object, it causes charged particles (typically electrons) to move from side to side. Then the magnetic field exerts a force on those moving particles, pushing the object in the direction of the light.
And, of course, momentum is conserved. So when a light source emits light, the source gets pushed backwards (we just don’t notice this because things like light bulbs are so heavy compared to the momentum of the light they emit). This momentum is eventually transferred to whatever the light hits. But between the light being emitted and absorbed, that momentum must be somewhere, right? And it is – it’s in the light.
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