I do know that magnetic fields are real and do exist. But when can we see magnetic field lines using Magnetic Field Viewing Film? Why are they called imaginary?
Most of us will have experimented with placing iron filings around a magnet to get this sort of thing:
This particular example is taken from Why iron filings sprinkled near a bar magnet aggregate into separated chunks? The iron filings line up in the direction of the magnetic field and this nicely shows us what the field looks like.
Your magnetic field viewing film works in a similar way. It contains flakes of nickel that line up with the field in the same way as the iron filings, and this produces a pattern that shows us what the field looks like.
The magnetic field is certainly real, and it has a direction at every point in space, but the field lines are just lines that trace out the direction of the field. They are no more real than contour lines on a map are real.
This dates back to Faraday. He was a brilliant experimenter, and he discovered a whole lot about electromagnetic stuff. But he didn't do math, and his explanations for what he found were not in mathematical form.
Some other physicists of the time didn't like him, and they didn't like his explanations, and they tried to say he was wrong. Except his explanations were easy to understand, and they did seem to fit the reality.
When you spread iron filings they tend to line up in long lines, for complicated reasons. Each filing turns into a little magnet which affects the field, so instead of just sitting in random positions and turning with the field, they move into line. You get little stringy strong fields and weaker fields in between. When the filings aren't there, the magnetic field is smooth and continual, and it doesn't have individual strong lines in it.
So they wanted to say he was wrong. People who imagine individual lines instead of imagining a smooth field are doing it wrong.
That argument kind of got fossilized. Educators still point out that the lines of force aren't real, because they know that's what they're supposed to say even though nobody really cares about discrediting Faraday any more.
Incidentally, if you draw a picture of a magnetic field with vectors, the vectors are also imaginary. There are no actual arrows, there's only a continuous bunch of points and at each of them the field has a strength and a direction.
Do you care?
Any field is defined as the normalized force acting in a place on an object. Normalized means: if the force is proportional to some kind of "size" of this object, then the field strength is the force acting on that object of size one.
So the question: is there a "field" when there is no object can not be finally answered. No object can be in a place if not moved to there and no object can be changed in size without removing a part of it, as moving an object in a field means transfer of energy, the field may be produced just by bringing objects to the place.
But the lines created by the filings obviously represent reality, so you can call them field lines as they are strongly depending on the existence of a magnet.
But you have to realize: iron concentrates the magnetic field by about 10,000, so every line of iron gives a path to the magnetic flux leaving the direct neighborhood free of the field. Therefore the actual lining of the filings is a question of probability and the picture will be different whenever you repeatedly drop filings.
The lines that result from magnetisable filings are real. The representation of magnetic lines presents excellently how the magnetic field runs. What is still missing is the description of the discontinuum of the field, i.e. its description by means of particles. Physics still lacks an understanding of this.
It should be possible to describe the electric field, the magnetic field and the electro-magnetic radiation on the basis of particles. As long as this quantisation (discretisation) is not carried out, the question of the reality of field lines remains undecided. In a model of discrete particles, with which an electric field, a magnetic field and also EM radiation can then be described, field lines must then be a component of the description.