I understand light transfer through space as an oscillating electromagnetic field just like sound wave travels through air making air molecules compress and expand.

Now as we already observe effects of high intensity sound making things to vibrate with the compression and expansion of surrounding air, why dont we observe same effect with high intensity light on surrounging electric and magnetic field. Just for an example , magnetic compass which aligns itself in the direction of magnetic field of earth, why doesnt light from Sun interferes with the magnetic field produced by earth which may affect the reading of needle in magnetic compass?


1 Answer 1

  • In normal lighting conditions, the light waves reaching the compass are incoherent, meaning the light affecting one part of the needle has a random phase relationship with the light hitting another part of the needle. So the light reaching all the different parts of the needle are working against each other rather than together, and the net effect on the needle is very small (probably much smaller than the effect of brownian motion from the fluid the needle typically floats in)

  • Normal light is also temporally incoherent, meaning the phase of the oscillations of the field do not remain consistent over time. That means that the effects of the magnetic field on the compass needle are highly randomized in time as well as space, so again the effects are not working together to move the needle, but only randomly to cause a random motion of the needle.

  • The electric and magnetic fields in a visible light wave oscillate with a frequency in the hundreds of terahertz, that is, hundreds of trillions of times per second. While the field may exert a force on the needle, the mass of the needle and the viscosity of the fluid around it mean the needle will not move very far in the few femtoseconds the field might be driving it in a particular direction. This damps the response of the needle at these frequencies to essentially zero. So even if you were to illuminate the compass with a laser (providing coherent light) rather than the usual incoherent sources, the effect would be insignificant.


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