Lens

**Category : **JEE Main & Advanced

(1) Lens is a transparent medium bounded by two refracting surfaces, such that at least one surface is curved. Curved surface can be spherical, cylindrical etc.

(2) Lenses are of two basic types convex which are thicker in the middle than at the edges and concave for which the reverse holds.

(3) As there are two spherical surfaces, there are two centres of curvature \[{{C}_{1}}\] and \[{{C}_{2}}\] and correspondingly two radii of curvature \[{{R}_{1}}\] and \[{{R}_{2}}\]

(4) The line joining \[{{C}_{1}}\] and \[{{C}_{2}}\] is called the principal axis of the lens. The centre of the thin lens which is on the principal axis, is called the optical centre.

(5) A ray passing through optical centre proceeds undeviated through the lens.

** (6) Principal focus :** We define two principal focus for the lens. We are mainly concerned with the second principal focus (F). Thus wherever we write the focus, it means the second principal focus.

**First principal focus :** An object point for which image is formed at infinity.

** Second principal focus :** An image point for an object at infinity.

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