Lens is a transparent medium surface bounded by two refracting surfaces. Out of these two reflecting surfaces, at least one surface is spherical.
It is of two types: Convex lens and Concave lens.
The lens, which is thick in the middle and thin at the edges, is called convex lens. It is also called converging lens.
The convex lens are of three types:
i) Biconvex lens or double convex lens
ii) Plano convex lens
ii) Concavo convex lens
The lens, which is thin in the middle and thick at the edges, is called concave lens. It is also called diverging lens.
The concave lens are of three types:
i) Biconcave lens or double concave lens
ii) Plano coneave lens
iii) Convexo concave lens
Vocabulary about lens:
i) Centre of curvature (C):
Centre of curvature is the centre of the sphere of which a lens forms its part. A lens has two centre of curvature.
ii) Principal axis:
The imaginary line joining the two centres of curvature of a lens is called principal axis of the lens.
iii) Optical centre:
The point on the principal axis, which is equidistant from the two centres of curvature of the lens, is called optical centre. It is denoted by O.
iv) Principal Focus:
The point on the principal axis where the parallel rays of light converse in convex lens or appears to diverse in concave lens is called principal focus. It is denoted by letter ‘F’.
v) Focal length:
The distance between the principal focus and the optical centre of a lens is called focal length of the lens. It is denoted by the symbol ‘f’.
The process of adjusting distance between the lens and the screen in order to produce a clear and distinct image is called focusing.
Rules of refraction of light through lens:
i) A ray of light travelling parallel to the principal axis passes or appears to pass through the focus of a lens after refraction.
ii) A ray of light passing through the optical centre of the lens travels undeviated after refraction, i.e. passing through the lens.
iii) A ray of light passing through the focus after refraction becomes parallel to the principal axis.
Power of a lens:
Capacity of a lens to converse or diverse the parallel rays of light is called power of the lens.
The power of lens is measured in dioptre (D) unit. Dioptre is also called radian/m.
The power of concave lens is negative (-ve) and that of convex lens is positive (+ve).
Formula to calculate power of the lens:
P = 1/f
P = power of the lens,
f= focal length in meter.
One dioptre power:
One dioptre power is defined as the power of the lens whose focal length is one meter.
Applications of Lenses:
i) Lenses are used in cameras, telescopes, cinema projectors, search lights and microscopes.
ii) A convex lens is used to form the image of an object on the film of a camera.
iii) A convex lens is used is an astronomical telescope to see the heavenly objects like moon, stars, galaxies, etc.
iv) A convex lens is used in terrestrial telescope to see the objects like trees, houses, persons, vehicles, etc on earth.
v) A convex lens of small focal length is used in microscopes to study biological specimens.
vi) A convex lens of small focal length is used as a reading glass.
vii) A convex lens is used to correct hypermetropia or long-sightedness defect of human eye.
viii) A convex lens is used to correct myopia or short- sightedness defect of human eye.
The ratio of height of the image to the height of the object is called magnification. In other words the ratio between the image distance to object distance is called magnification. It is denoted by letter ‘M’. Magnification does not have unit.
Thus, Magnification= Image distance (I) / Object distance(u)
Instruments which form image of an object are called optical instruments.
Optical instruments are of two types:
i) Real image forming optical instruments, for eg. Camera, Eye, Projector, etc.
ii) Virtual image forming optical instruments, for eg. Microscope, Telescope, Binocular, etc.
The farthest point which can be focused distinctly by the normal eye is called the far point of the eye. For the normal eye the far point is at infinity.
The nearest point that can be focused distinctly by a normal eye is called the near point. The near point for a normal eye is at 25 cm from the eye.
Short sightedness (Myopia):
The defect of vision, in which a person can see the near objects but he/she feels difficulty to see the distant object is called short sightedness. In such eye defect, image of the distant object is formed infront of retina.
Long sightedness (Hypermetropia):
The defect of vision, in which a person can see the distant objects but he/she feels difficulty to see the near object is called long sightedness. In such eye defect, the image of near object is formed beyond (behind) the retina.