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Lens is an object, usually made of glass, bounded by one or two spherical surfaces.

Types of lens: (i) convex or converging lens (ii) concave or diverging lens

Lens formula : The relation between object distance u, image distance v, and focal length f of a lens is known as lens formula .


Len's maker's formula : The relation between focal length of a lens, radii of curvature of two surfaces and refractive index of the material is called lens maker's formula.


Combination of thin lens


Power of a lens is a measure of its ability to produce deviation of light.

Power of a lens, P=

Magnification of a lens is defined as the ratio of the size of image produced by it to the size of the object.

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Optical centre is the geometrical centre of the lens through which light never bends after passing through.

Displacement formula,

  • Focal length of a lens depends on radius of curvature, refractive index of the lens and that of the surrounding medium.
  • An air bubble inside the water behave as diverging lens.
  • If one of the surface of a lens is painted to make a mirror, then it behave as a combination of two lens and one mirror.
  • Intensity of the images proportional to the square of the aperture of the lens.
  • Minimum distance between the real object and real image by a convex lens is four times the focal length.
  • If the object is kept between optical centre and the focus of a biconvex lens, then it behaves as a magnifying lens.
  • The plane perpendicular to the principle axis at the focal point is called focal plane.
  • A sun glasses has no power.
  • Depending on the refractive index of the surrounding medium, the nature of the lens can be changed.
  • If a portion of a lens is broken still, then it will form the complete images.