Refraction by spherical lenses

 

Refraction by spherical lenses

Refraction means to state that the change of the direction of the parallel beam of ray of lights when it passes through one medium to another medium.

The image formed by the convex lens is enlarged. And the image formed by the concave lens is diminished.

 

Lens :--

              A lens is a transparent material bounded by two surfaces of which one or both surfaces are spherical. It means to say that a lens is bounded by at least one spherical surface. And other surface is plane surface.

Or we can say that lens is a transparent material or  glass bounded by the two spherical surfaces and is used for magnifying the objects.

The working of lens is mainly based on the refraction of ray of lights when they passes through it.

There are two types of lens. A concave lens and convex lens.

concave surface 

convex surface

 

Convex lens :--

                      A convex lens is the spherical lens having two spherical surfaces bulging outwards. It is also called as double convex lens.

 It is thicker at the middle as compared to the edges of the lens.

It is also called as converging lens. It converges the light rays when ray of light  passes through the convex lens.  

Take a paper and hold the convex lens in the position that beam of light falls on the paper through the lens. Observe the paper. The paper starts burning and it produces smoke. And after some time it may catches a fire. The light from the sun constitutes parallel ray of lights. These parallel light converged by the convex  lens at the sharp bright spot formed on the paper. and the sharp bright spot is the real image of the sun. the concentration of the parallel sharp bright spot light rays generates the heat. And these heat energy focused sunlight has burned a hole in the paper where the image of the sun is made.

 

Concave lens:--

                          A concave lens is the spherical lens that have two spherical surfaces  which curved inwards. It is also called as double concave lens.

It is thicker  at the edges  as compared to the middle  of the lens.

It is also called as diverging lenses because the refracted ray diverge from the focus of the lens. Concave lens is opposite to the convex lens. And the parallel ray of light is diverged after passing through the concave lens. Concave lens has two principal foci. And they are represented by the letter F1 and F2. And in the concave lens , a parallel ray of light is passing through the concave lens from the left side then they diverge towards left side only. And if the parallel ray of light is passing through the concave lens from the right side then they get diverge towards the right side only.

concave lens

 

Centre of curvature of lens:--

                                                 A lens (concave lens or convex lens) has two spherical surfaces. And each part of these surfaces forms a sphere. The centre of these sphere is called as centre of curvature of the lens.

The centre of curvature of lens is denoted by the letter C. since there are two centre of curvature of lens and they represents as C1 and C2.

 

Optical centre:--

                                  The centre point of the lens is called  as optical centre. It is denoted by the letter O. a beam of light ray is passing through optical centre goes straight. And it does not get deviated.

 

Principal axis:--

                          An  imaginary straight line passing through the two centres of the curvature of a lens is called as principal axis.

Aperture of lens :--

                                 Aperture of lens is the effective diameter of the circular outline of the spherical lens is called as aperture of lens.

 

Thin lens:--

                    Such type of lens whose aperture is much less than the its radius  of curvature. And the two centre of curvature is equidistance from the optical centre O. such type of lens is called as thin lenses with small aperture.

 

Principal focus of concave lens :--

                              Several light rays which are parallel to the principal axis falling on the concave lens. After refraction , All these ray of light after passing through the concave lens  are appearing diverge from a point on the principal axis. This point is called as principal focus of the concave lens. It is represented by the letter F.

 

Principal focus of a convex lens:--

                                

                             Several light rays which are parallel to the principal axis falling on the convex lens. After refraction , All these ray of light after passing through the concave lens  are appear converging  to a point on the principal axis. This point is called as principal focus of the convex lens. It is represented by the letter F.

 

Focal length of a convex lens:--

                                                      The distance of the principal focus from the optical centre of a lens is called as focal length of a lens.

A lens has two principal foci. They are represented by the letter F1 and F2.

Focal length of a lens depends on the:--

Refractive index of the glass

Its curvature

If the refractive index is higher then the focal length is shorter

If the curvature of the lens is more then the focal length is short.

 

                                                      

            

                         

 

 

 

 

What happens when parallel beam of light are incident on a lens? And why?

 

Take a paper and hold the lens in the position that beam of light falls on the paper through the lens. Observe the paper. The paper starts burning and it produces smoke. And after some time it may catches a fire. The light from the sun constitutes parallel ray of lights. These parallel light converged by the lens at the sharp bright spot formed on the paper. and these  sharp bright spot is the real image of the sun. the concentration of the parallel sharp bright spot light rays generates the heat. And this may causes the paper to burn.