Let r be the distance of a point on the axis of a bar magnet from its centre. The magnetic field at such a point is proportional to
For a bar magnet with length 2l, and magnetic moment M, the magnetic field on any point in the axis at a distance r from the centre can be expressed as,
Hence, we can understand that, magnetic field is inversely proportional to (r2-l2)2, that is,
Hence option D is the correct option.
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