(a) Define optical centre of a lens. What happens when a ray of light passes through the optical centre of lens ? (b) Define principal focus and focal length of a lens. Draw ray diagram to show the position of principal focus of a lens.

(a) Define optical centre of a lens. What happens when a ray of light

1.	(a) Define optical centre of a lens. What happens when a ray of light passes through the optical centre of lens ? (b) Define principal focus and focal length of a lens. Draw ray diagram to show the position of principal focus of a lens.
Answer» Solution :(a) The OPTICAL centre of a thin lens is a point on its principal axis, a ray of light passing through which goes straight without any bending (or DEVIATION). In FIGURE, the point O is the optical centre. (b) Principal focus of a lens is a point where a light beam incident parallel to the principal axis of the lens, after refraction, actually converges to (in case of a convex lens) or diverges from (in case of a concave lens). Since, a lens has two refracting surfaces, a lens has two principal foci. `F_(1) and F_(2)` on either side of lens. The distance of principal focus of a lens from its optical centre is called its focal length f. Thus, `f = OF_(1) = OF_(2)`.