Optical analogue of an equipotential surface is the wavefront of light. This concept is related to the propagation of light in space and the behavior of light waves. The following points explain this concept in detail:

Wavefront of Light:
The wavefront of light is the locus of all the points in space that are in the same phase of the light wave. In other words, it is the imaginary surface that connects all the points of the wave that have the same phase. The wavefront can be visualized as a series of imaginary lines or surfaces that are perpendicular to the direction of propagation of the light wave.

Equipotential Surfaces:
An equipotential surface is a surface in a field where the potential is the same at all points on that surface. In the case of electric fields, an equipotential surface is a surface where the electric potential is constant. Similarly, in the case of gravitational fields, an equipotential surface is a surface where the gravitational potential is constant.

Optical Analogue of Equipotential Surface:
The concept of equipotential surfaces can be applied to the propagation of light waves. The wavefront of light can be thought of as an equipotential surface where the optical path length is constant. In other words, the wavefront of light is the surface where the optical path length is the same at all points on that surface.

Optical Path Length:
The optical path length is the distance that light travels through a medium or a combination of media. It is defined as the product of the refractive index of the medium and the physical distance traveled by the light wave. The optical path length is a quantity that is used to calculate the phase difference between two points on a wavefront.

In conclusion, the optical analogue of an equipotential surface is the wavefront of light. The wavefront is the surface where the optical path length is constant, and all the points on that surface are in the same phase of the light wave. This concept is useful in understanding the behavior of light waves and their propagation in space.