English: The momentum of an archer's arrow can be measured by topping it with a target and measuring the impulse, ${\displaystyle {\vec {p}}=\int {\vec {F}}dt}$. For light the impulse can be calculated using a thought experiment in which it passes it through a medium of vanishingly small conductivity. Instead of stopping, the light travels at a speed near ${\displaystyle c}$, but decays exponentially with amplitude. The conductivity causes current to flow, and the light's magnetic field exerts a known force. The impulse associated with that force gives light "momentum". In this way, the momentum of something travelling at the speed of light was understood before the theory of special relativity emerged. This classical understanding of light's momentum facilitated the development of w:Modern physics.