Physics CH # 14

Snell’s Law:

      According to Snell’s law

"The ratio of the sine of the angle of incidence to the sine of the angle of refraction is always constant. "

    Mathematically,

    Sine <i/sin <r = constant or
sinα </sin<r = m

    where m = Refractive index of the material of medium.

Total Internal Reflection:

    When light rays enter from one medium to the other, they are refracted. If we increase the angle of     incidence, angle of refraction will also increase. At certain angle of incidence light rays are reflected     back to the first medium instead of refraction. This condition or phenomenon is called Total Internal     Reflection.     For latest information , free computer courses and high impact notes visit : www.citycollegiate.com

Critical Angle:

      The angle of incidence at which the angle of refraction will become 90^o is called Critical Angle. If angle     of incidence further increased then instead of refraction, reflection will take place.

Defects Of Vision:

    There are four common defects of vision:
    1. Short sightedness or myopia
    2. Long sightedness or hyper metropia
    3. Astigmatism

Short Sightedness Or Myopia:

Symptoms:

      In Myopia, a person can not see distant objects clearly, but he can see clearly the objects near to him.

Reason:

      The reason for Myopia is either the focal length of lens of eye is too short or the eyeball is very much     elongated.

What Happens In Myopia:

      In Myopia, light rays from a distant object are focused in front of the Retina.

Correction Of Defect:

      This defect can be corrected by using a concave lens of suitable focal length

Long Sightedness Or Hyper Metropia:

Symptoms:

      In hyper metropia, a person can not see objects clearly which are near to him, but he can see clearly     distant objects

Reason:

      The reason for HYPER METROPIA is that either the focal length of the lens of eye is too long or the     eyeball is too short.

What Happens In Hyper Metropia:

      In hyper metropia, light rays from a near object are focused behind the Retina.

Correction Of Defect:

      This defect can be corrected by using a convex lens of suitable focal length

Power Of Lens:

    Power of lens is defined as the reciprocal of the focal length of the lens in meters.    

 Formula:

Power = 1/f(in meter)

    Unit of power of lens is Dioptre.

Dioptre:

      Dioptre is defined as the power of lens whose focal length is one meter     if f =1 meter then the power of the lens = 1 dioptre.

Image Formation By Convex Lens:

When the object is placed at infinity :

Nature And Position Of Image:
1. The image will form at the principal focus (F).
2. The image will be real and inverted.
3. The image will be very small in size.

When The Object Is Placed Beyond 2F :

Nature And Position Of Image:
1. The image will form between F and 2F.
2. The image will be real and inverted.
3. The image will be smaller in size.

When The Object Is Placed  At  2F :

Nature And Position Of Image:
1. The image will form at 2F.
2. The image will be real and inverted.
3. The image will be equal in the size of object.

When The Object Is Placed Between The Pole (P) And F :

Nature And Position Of Image :
1. The image will form on the same side of object.
2. The image will be virtual and erect.
3. The image will be magnified.

 Compound Microscope:

         Compound microscope is an optical instrument which is used to obtain high magnification.

Construction:

   It consists of two converging lenses:
    Objective
    Eye piece

Objective:

   The lens in front of object is called objective. Its focal length f₁= f_o is taken to be very small .The    objective forms a real, inverted, and magnified image of the object placed just beyond the focus of    objective.

Eye Piece:

   The lens towards the observer's eye is called piece .Focal length of eye piece is greater than the focal    length of objective. Eye piece works as a magnifying glass.

Working:

   The objective is so adjusted that the object is very closed to its focus. The objective forms a real,
   inverted and magnified image of the abject beyond 2fo on the right hand side. The eye piece is so    adjusted that it forms a virtual image at the least distance of distinct vision "d" .The final image is
   highly magnified.

Astronomical Telescope:

Introduction:

   It is an optical instrument used to view heavenly bodies such as moon,stars, planets and distant object.

Construction:

   Astronomical telescope consists of two convex lenses:
Objective
Eye piece

Objective:

   The objective is a convex lens of large focal length and large aperture. It usually made of two convex lenses    in contact with each other to reduce the chromatic and spherical aberrations.

Eye Piece:

   The eye piece is also a convex lens .Its focal length is smaller than that of objective. It is also a    combination of two lenses.    The objective is mounted on a wide metallic tube while the eye piece is mounted on a small tube .The    distance b/w the eye piece and the objective can be changed by moving tubes.

 Working:

     The rays coming from a distant object falls on objective as parallel beam at some angle say "a" and these    rays after refraction and passing through the objective converge at its focus and make an inverted & real    image AB. This image acts as an object for the eye piece. The distance of the eye piece is so adjusted that    the image AB lies within the focal length of the eye piece. The eye piece forms the final image .The final    image is magnified ,virtual and inverted with respect to object. The final image is formed at infinity.