What represents the invisible lines that indicate the direction and strength of a magnetic field?

The representation of invisible lines that indicate the direction and strength of a magnetic field is known as magnetic field lines. These lines are an important conceptual tool in understanding magnetism; they show where the magnetic field is directed and how strong the field is at different points. The density of these lines indicates the strength of the magnetic field—the closer the lines are, the stronger the magnetic field in that area.

Magnetic field lines emerge from the north pole of a magnet and loop around to enter the south pole, illustrating the directional nature of the magnetic forces at play. This visualization helps in predicting how a magnetic field will interact with charged particles or other magnets.

The other options refer to different concepts: electric field lines represent electric fields created by electric charges, induction lines are not standard terminology in physics, and flux lines typically refer to various types of flow, such as electric or magnetic flux, but do not specifically indicate magnetic field direction and strength like magnetic field lines do.