# . Explain the following terms related to spherical lenses

Explain the following terms related to spherical lenses: (i) optical center

(ii)  centres of curvature

(iii) principal axis

(iv)  aperture

(v)  principal focus

(vi)  focal length

(b) A converging lens has focal length of 12 cm. Calculate at what distance should the object be placed from  the lens so that it forms an image  at 48 cm on the other side of the lens

(i) Optical center. It is a point within the lens that lies on the principal axis through which a ray of light passes undeflected .

(ii) Centre of curvature. The centre of curvature of the surface of a lens is the centre of the sphere of which it forms a part.  A lens has two centres of curvature because it has two surfaces.

(iii) Principal axis. It is a line through the centres of curvature of the lens.

(iv) Aperture. The diameter of the circular boundary of the lens is called the aperture of the lens.

(v) Principal focus. A beam of light parallel to the  principal axis either  converges to a point  or appears to diverge 'from a point on the principal axis after refraction through the lens, is called  the principal focus.  All lenses have two principal focuses.

(vi) FocaL length. The distance between the optical centre and the principal focus of the lens is called its focal length.

(b) A converging lens is a convex lens.

f= +12 cm

u=?

v = +48 cm (+ve as it is formed on other side of the object)

According to lens formula

The object should be placed .16 cm away from the lens