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# Why are there magnets around the DVD laser player? Answered

I just disassembled an old DVD player. The laser is 'suspended' between two fairly powerful small batteries. Any idea WHY the batteries are there?

The link below shows the laser and the magnets.
Video

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They are used to focus the laser

I'm glad Frollard clarified your description, Steve! Otherwise, I would have posted something highly sarcastic about linear Maxwell's equations and the inability of magnetic fields to affect the propagation of light.

It does make a lot of sense to use non-mechanical actuators to adjust the laser position. Much faster response and tighter control.

In the same spirit, I am honour bound to point out that moving a lens around has to count as mechanical actuation.....

:-D Yeah, yeah. Though using a non-contact method (magnets) is generally better than gears and levers.

Ah, stupid me, making the spherical cow approximation again.

Yup, in conjunction with a small coil (electromagnet) the laser can be moved with insanely good precision - limited only by the accuracy of the circuit controlling the coil.

magnets and coils of wire = movement. L

Since we're on a picky kick today courtesy of Dr. K...magnets and coils = force. Movement is optional.

level 3 pickyness:
magnets and energised coils.

L

I'll see you and raise you to level 4. A shorted coil will also exhibit a force if moved.

you excite it with the magnetic-field, so technically it would still be energised/excited?

L

F = dp/dt = m dv/dt + v dm/dt. You could use force to change mass, but that's probably a lot more difficult than changing velocity :-)

Isn't pointless Talmudic debate fun?