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  4. In case of reflection over spherical surface, which of these are correct ? I. Normal is taken as perpendicular of tangent at point of incidence. II. Perpendicular to incident ray which is perpendicular to plane of incident ray. III. Line joining centre of curvature of mirror with point of incidence. IV. Line joining centre of curvature and pole of curved surface.

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Q. In case of reflection over spherical surface, which of these are correct ?
I. Normal is taken as perpendicular of tangent at point of incidence.
II. Perpendicular to incident ray which is perpendicular to plane of incident ray.
III. Line joining centre of curvature of mirror with point of incidence.
IV. Line joining centre of curvature and pole of curved surface.

Ray Optics and Optical Instruments

Solution:

Normal is perpendicular to the tangent to surface at the point of incidence i.e., the normal is along the radius, the line joining the centre of curvature of the mirror to the point of incidence.
So, geometric centre of a spherical mirror is called its pole while that of a spherical lens is called its optical centre. The line joining the pole and the centre of curvature of the spherical mirror is known as the principal axis. In the case of spherical lenses, the pricipal axis is the line joining the optical centre with its principal focus.