need to give an at most care while solving numerical problems related to lenses in taking proper signs of different quantities. . SIGN CONVENTION Cartesian sign conventions are used for measuring the various distances in the ray diagrams of spherical lenses. According to cartesian sign convention, .
The object is always placed on the left side of the lens. . All the distances are measured from the optical centre of the lens. .
The distances measured in the same direction as that of incident light are taken as positive. . The distances measured against the direction of incident light are taken as negative. .
The distances measured upward and perpendicular to the principal axis is taken as positive. . The distances measured downward and perpendicular to the principal axis is taken as negative. .
MAGNIFICATION OF A LENS Like spherical mirrors, we have magnification for spherical lenses. Spherical lenses produce magnification and it is defined as the ratio of the height of the image to the Optics height of an object. Magnification is denoted by the letter ‘m’. If height of the object is h and height of the image is h ´ , the magnification produced by lens is, m = height of the image height of the object = h' h …… ( .
) Also it is related to the distance of the object ( u ) and the distance of the image ( v ) as follows: m = Distance of the image Distance of the object = v u …… ( . ) If the magnification is greater than , then we get an enlarged image. On the other hand, if the magnification is less than , then we get a diminished image. .
LENS MAKER’S FORMULA All lenses are made up of transparent materials. Any optically transparent material will have a refractive index. The lens formula relates the focal length of a lens with the distance of object and image. For a maker of any lens, knowledge of radii of curvature of the lens is required.
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