CHAPTER - 6

Chandra Sekhar Venkata Raman was born on 7th November, 1888 at Tiruchirapalli in Tamilnadu with equipment worth hardly Rs.200 and meagre facilities, Raman was able to make a discovery which won him the 1930 Nobel Prize in Physics.
  When he was only 19 years, he became a member of the Indian Association for cultivation of Scinece. In 1921 on his return journey abroad the blueness of the sky and sea intrigned him. He declared the Raman effect on February 28. Now this day is being observed as the 'National Science day'.

KEY WORDS

KEY WORDS - EXPLANATION

Lens: A lens is formed when one medium is separated from another medium by two surfaces, one of which is curved.
Focal length: The distance between optic centre (P) and focus (F) of the lens is known as focal length of the lens.
Focus: A parallel beam of light incident on the lens converges to a point or seems to emanate from point on the principal axis. This point is called focus of the lens.
Optic centre: The mid point of a thin lens is called optic centre (P) of the lens.
Principal axis: The line joining the two centres of curvature and the optic centre of the lens is called principal axis.
Radius of curvature: The distance between the centre of curvature and the curved surface is called radius of curvature.
Centre of curvature: The centre of the sphere, of which curved surface is a part, is called centre of curvature (C).
Convex lens: A lens may have two spherical surfaces bulging outwards. Such a lens is called double convex lens. It is thick at the middle as compared to edges.
Concave lens: A double concave lens is bounded by two spherical surfaces curved inwards (Bio concave lens). It is thin at the middle and thicker at the edges.
Refraction of light at a curved surface: As in the case of plane surfaces, a ray will bend towards the normal if it travels from a rarer to a denser medium and it bends away from the normal if it travels from a denser to a rarer medium.
 

Lens formula: 

SYNOPSIS

* The centre of the sphere, of which curved surface is a part is called as centre of curvature. It is denoted by letter 'C'.
* Any line drawn from the centre of curvature to a point on the curved surface becomes normal to curved surface at that point.
* Any line drawn from the centre of curvature to a point on the curved surface becomes normal to the curved surface at that point.
* The direction of the normal changes from one point to another point on the curved surface.
* The centre of the curved surface is called the pole (P) of the curved surface.
* The line that joins the centre of curvature and the pole is called 'Principal axis'.
* According to Snell's law the ray which travels along the normal drawn to the surface does not deviate from its path.
* A ray will bend towards the normal if it travels from a rarer medium to a denser medium and it bends away from the normal if it travels from a denser to a rarer medium.
* If the rays move very close to the principal axis, the rays can be treated as parallel and are called paraxial rays.
* When a ray enters a medium with refractive index n₂ from a medium with refractive index n1, at a curved surface of radius of curvature R, then the formula used is
                     
* Where u and v are the object distance and image distance from the pole of the interface (P).
* A lens is formed when a transparent material is bounded by two surfaces of which one (or) both surfaces are spherical.
* Lenses can be of various types. A lens may have two spherical surfaces bulging outwards. Such a lens is called double convex lens. It is thick at the middle as compared to edges.
* Biconvex, Biconcave, Plano-convex, Plano-concave, Concavo-convex are the different types of lenses.
* We are concern only with thin lenses i.e. the thickness of the lens is negligible.
* Each curved surface of a lens is a part of a sphere. The centre of the sphere which contains the part of the curved surface is called centre of curvature. It is denoted by the letter 'C'.
* If a lens contains two curved surfaces then the centres of curvatures are dentoed as C₁ and C₂.
* The distance between the centre of curvature and curved surface is called radius of curvature (R).
* Radii of curvature are represented as R₁ and R₂ respectively.
* The line joining the points C₁ and C₂ is called principal axis. The mid point of a thin lens is called the optic centre (P) of lens.
* A ray passing along the principal axis is undeviated.
* A ray passing through the optic centre is also undeviated.
* A parallel beam of light incident on a convergent lens converge at a point on the principal axis. This point is called the focus (F) of the convergent lens.
* A parallel beam of light incident on a divergent lens seems to emanate from a point on the principal axis. This point is called the focus (F) of the divergent lens.
*   is the lens formua, here f is the focal length of the lens, u is the distance of the object from the lens and v is the distance of the image from the lens.
* The focal length of lens depends upon the surrounding medium in which it is kept.
*  is the lens maker's formula. Here n is the absolute refractive index of the lens and R₁ and R₂ are the radii of curvature of the two curved surfaces respectively.
 

Image formation of an object by a convex lens

Writer: C.V. Sarveswara Sarma