What is the Meissner effect?

Hint: In the question, we are asked to explain what Meissner effect. As a first step, one should be aware under which concept this effect belongs. After recalling this you could think of all the points related to this effect. Therefore, you could get a clear picture about this effect.

Complete step-by-step solution:
In the question, we are asked to explain what the Meissner effect is. The Meissner effect basically is the expulsion of a magnetic field out of a superconductor while its transition to the superconducting state takes place while being cooled below the critical temperature. Now, this expulsion is known to be strong enough so as to repel a nearby magnet.

Additional information:
This phenomenon was discovered by German physicists Walther Meissner and Robert Oshsenfeld. This phenomenon was discovered by measuring the magnetic field distribution that is present outside superconducting tin as well as lead samples. In the presence of an applied field, these samples were cooled way below their superconducting transition temperature. Here, these samples had cancelled nearly all of the interior magnetic fields. Since this magnetic flux is conserved by a superconductor this effect can be detected indirectly only. Also, when the interior field shows a decrease, the exterior field increases.

Note:Another interesting fact is that a superconductor with very little or may be even no magnetic field is said to be in the Meissner state. Also, this state however breakdown when the applied magnetic field is way too strong. We even classify the superconductors into two classes according to how this breakdown is known to occur.