USES OF LENSES

(A) CONCAVE LENS : 

1-) Concave lens are used in a variety of instruments e.g., microscope,telescope,photographic camera, film projector etc. 

2-) To view the smallest parts of watch by watch maker. 

3-) To detect eye and ear defects by a doctor. 

4-) To remove the defect of hypermetropia.

(B) CONVEX LENS :

1-) In the construction of Gallileo's telescope.

2-) To remove the defect of myopia. 

IDENTIFICATION OF CONVEX AND CONCAVE LENSES

1-)  By Touching : The convex lens is thicker at the centre relative to its edges, while the concave lens is thinner at the centre than its edges. 

2-) By Image Formation : On viewing the distant objects by convex lenses the image is seen as inverted and diminished. In case of concave lenses, the images is erect  and diminished. 

On viewing the near object, e.g., printed matter in a book, it looks lager by convex lenses whereas the same appears smaller by concave lenses.

In case of a rectangular glass plate, unlike lenses, the near and far objects appear the same as they are.

LENSES

Any transparent refracting medium bounded by two surfaces ( in which at least one surface should be curved surface)is called a "lens". The lenses used in laboratory are generally made up off glass. There are two type of lenses :  

1-) Convex lenses : These lenses are thicker at the centre and thinner at their edges. These are also called converging lenses. There are three type of convex lens :

a-) Double convex or biconvex 

b-) Plano-convex 

c-) Concavo-convex 

2-) Concave lenses : These lenses are thinner at the centre than at edges. A concave lens is also called as diverging lens. There are three type of concave lenses : 

a-) Double concave or biconcave 

b-) Plano-concave

c-) convexo-concave 

 

DOUBLE REFRACTION

When a beam of light of unpolarized light is allowed to pass through a calcite crystal, it is split up into two refracted beams in place of the usual one as in glass. This phenomenon is known as double refraction and was first discovered by a Dutch philosopher Erarmus Bartholinus in the year 1669. 

PLANE POLARIZED LIGHT BY REFLECTION

The simplest way to produce plane polarized light is by reflection. It was discovered by Malus in 1808 that partially polarized light is obtained when ordinary light is reflected from the surface of a transparent medium like glass or water and that the degree of polarization varies with the angle of incidence. At a certain angle of incidence, depending upon the nature of the reflecting surface, the reflecting light has the greatest percentage of the polarized light. This angle of incidence is called the angle of polarization for that medium and for ordinary glass its value is 57.5*. As the refractive index depends upon the wavelength, the angle of maximum polarization varies with the wavelength and hence the complete polarization is possible with monochromatic light and not with white light. 

METHODS TO PRODUCE PLANE- POLARIZED LIGHT

Various method to produce plane polarized light may be broadly summarized as below: 

1-) Polarization by reflection 

2-)Polarization by refraction

3-)Polarization by double refraction

4-) Polarization by selective absorption by crystals 

POLARISATION OF LIGHT

The phenomenon of interference and diffraction have proved beyond any doubt that light is some from of wave motion. But these properties of do not tell us whether the waves so essential for explaining these phenomena are longitudinal or transverse. Also regarding the nature of vibrations(whether linear, circular, elliptical or torsional) no clue is provided by these phenomena. It is the phenomenon of polarisation which could decide this question and show that waves are definitely transverse in nature.

 

plane polarized light:

When ordinary light is allowed to pass through a tourmaline crystal, the vibrations are confined only to one direction of propagation of light. This light which has acquired the property of one sidedness and the vibrations are linear is called "plane polarized or linearly polarized light".