Lens

Light

Light is a form of energy which gives sensation of vision. It gives the ability to see the object. It shows unique properties.Being an electromagnetic wave, it undergoes reflection,refraction,absorption,transmission,etc.Here we discuss about refraction of light.

Refraction of light:

The phenomenon of bending of light as it passes from one medium to another medium is called refraction of light. The refraction is caused due to change in velocity of light as it passes from one medium to another medium.

Lens:

a curved piece of glass,or other transparent material, used in cameras, glasses, and scientific equipment, which makes objects seem closer, larger, smaller, etc is called the lens.

Convex lens

A lens, which is thicker at center and thinner at edges, is called convex lens.

When theparallel beam of light incident on convex lens, after refraction, it converges at a single point. So it is called converging lens.

Concave lens:

The lens that is thicker at the edges and thinner at centre is known as concave lens.

When a parallel beam of light incident on concave lens, refracts and appears to diverse from a point. So, the concave lens is called diverging lens.

Some related terms

Center of curvature

Centers of curvature of a lens are the centers of the spheres, from which the lens is formed. In figure c₁ and c₂ are the center of curvature.

Radius of curvature

The radius of thesphere whose parts form a lens is known as radius of curvature.

Optical center

The geometrical center of the lens is known as theoptical center. In the following figure, 'o' is the optical center

Principal focus or focal point

When a parallel beam of light incident on a lens after refraction, it appears to meet from a point or it meet at a point. That point is called principal focus or focal length.

Principal focus is real for theconvex lens and virtual for theconcave lens.

Principal axis:

The line passing through the centre of curvature of both spheres is called principal axis.

Focal length:

The distance between theoptical center and principal focus is called focal length.

Rules of refraction in convex and concave lens:

1. An incident ray parallel to the principal axis of a convex lens passes through the principal focus of the lens after refraction. Incaseofconcave lens, the ray appears to be diverging from the focus.

    

   

2. An incident ray, passing through the principal focus of a convex lens, becomes parallel to the principal axis after refraction.

3. An incident ray passing through the optical center of a convex lens and concave lens goes straight without deviation after refraction.

Power of a lens:

The ability of a lens to converge or to diverge a beam of light is called power of a lens. The unit of power of a lens is Dioptre (D)

Mathematically,
Power of a lens: \(\frac{1}{focal\;length}\)
Note: focal length should be in "meter".

Characteristics of image when placed at different places:

1. Convex lens

   

2. Concave lens

   

When object is placed anywhere between optical center and infinity, the image is formed between optical center and principal focus, which has following characteristics:

• The image is virtual, erect and diminished.

Some important definition:

1. One dioptre of a lens: One diopter power is defined as the power of lens whose focal length is 1 m.
2. Lens formula: A formula which gives the relationship between the object distance (u), image distance (v) and the focal length(f) is called the lens formula.
3. Magnification: The magnification (m) produced by a lens is defined as the ratio of theheight of theimage to the height of theobject.It is also equal to the ratio of image distance to theobject distance.