|MadSci Network: Physics|
the sources for electric fields are charges; i.e. electric field lines begin and end at charges. Similar to that the sources for gravity are masses, more generally speaking energy densities. Magnetic fields are somewhat different. Magnetic field lines are always closed, no magnetic charges have been observed. The "sources" for a magnetic field are electric currents.
If you want to bend the field lines of gravity, you need some masses or energy densities around. They will influence the present gravitational field. In the same way, electric and magnetic fields can be influenced by the presence of other charged particles. But there is nothing like a lense for electric or magnetic fields only. You can only influence these fields by an appropriate adjustment of charged particles and/or electric currents; similarly for gravity.
I hope this helps,
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