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REFRACTION OF LIGHT

 Refraction of Light

Refraction of Light : Happens in Transparent medium when a light travels from one medium to another, refraction takes place.
A ray of light bends as it moves from one medium to another Refraction is due to change in the speed of light as it enters from one transparent medium to another.

Speed of light decreases as the beam of light travel from rarer medium to the denser medium.



Some Commonly observed phenomenon due to Refraction

  • Your eyes.
  • Rainbows.
  • Light bending in a glass of water.
  • Glasses.
  • Camera lenses.
  • Object dislocation in water.
  • Binoculars..

  • Refraction through a Rectangular Glass Slab

    When a incident ray of light AO passes from a rarer medium (air) to adenser medium (glass) at point. O on interface KL, it will bends towards the normal. At ptO1, on interface NM the light ray entered from denser medium(glass) to rarer medium (air) here the light ray will bend away from normal OO1 is a refracted ray OB is an emergent ray. If the incident ray is extended to C, we will observe that emergent ray O1B I parallel to incident ray. The ray will slightly displaced laterally after refraction.

    Note : When a ray of light is incident normally to the interface of two media it will go straight, without any deviation.

    Laws of Refraction of Light

    The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.

    • The ratio of sine of angle of incidence to the sine of angle of refraction is a constant i.e.

    For given colour and pair of media, this law is also known as Snell’s Law

    Constant n is the refractive index for a given pair of medium. It is the refractive index of the second medium with repect to first medium.

    Refractive Index

    The refractive index of glass with respect to air is given by ratio of speed of light in air to the speed of light in glass.


    Speed of light in vacuum = speed of light in air is marginally less, compared to that in vacuum.

    Refractive index of air with respect to glass is given by

    3/2

    Refractive index of water 1.33

    Spherical Lens

    A transparent material bound by two surfaces, of which one or both surfaces are spherical, forms a lens.

    Convex lens Concave lens
    1. Bulging outwards 1. Bulging inwards.
    2. Converging lens. 2. Diverging lens.
     

    Concave Lens
    A lens bounded by two spherical surfaces, curved inwards is known asdouble concave lens (or simply concave lens)
    It is also known as diverging lens because it diverges the light.

    Few Basic Terms Related to Spherical Lens

    1. Centre of curvature : A lens, either a convex lens or a concave lens is combination of two spherical surfaces. Each of these surfaces forma part of sphere. The centre of these two spheres are called centre of curvature represented by C1 and C2.
    2. Principal axis :Imaginary straight line passing through the two centres of curvature
    3. Optical Centre : The central point of lens is its optical centre (O). A ray of light, when passes through ‘O’ it remains undeviated i.e. it goes straight.
    4. Aperture : The effective diameter of the circular outline of a spherical lens.
    5. Focus of lens : Beam of light parallel to principal axis, after refraction from

    1. Convex lens, converge to the point on principal axis, denoted by F, known as Principal focus

    2. Concave lens, appear to diverge from a point on the principal axis known as principal focus.

    The distance OF2 and OF1 is called as focal length

    Tips for Drawaomg Ray Diagram

    1. After refraction, a ray parallel to principal axis will pass through F.
    2. A ray passes through F, after refraction will emerge parallel to principal axis

    Image formation by a convex lens for various position of object

      

     

    Image Formation by Concave Lens

    Sign Convention for Refraction by Spherical Lens

    Similar to that of spherical mirror, only the difference is that all themeasurement are made from optical centre ‘O’

    Lens formula

    Magnification

    It is defined as the ratio of the height of image to the height of object.


    It is also related to ‘u’ & ‘v’ also written as d⁰ and di respectively.

    From equation (1) & (2)
    If magnification
    m > 1,then image is magnified
    m = 1 ,image is of same size
    m < 1, image is diminished

    Few Tips to Remember Sign Convention for Spherical Lens

    fuv
    CONCAVE-ve-ve -ve(virtual image always)
    CONVEX+ve-ve+ve(real)
    -ve(virtual)

    h is always +ve
    h´  –ve for Real and +ve for Virtual &Errect.

    Power of Lens

    The degree of convergence or divergence of light ray achieved by a lensis known as power of a lens.
    It is defined as the reciprocal of its focal length Represented by P.
    SI unit of power of a lens is “diopter” denoted by ‘D’

    I diopter. It is the power of lens whose focal length is I m

    Power of convave lens or diverging lens is always negative

    If any optical instrument has many lens, then net power will be

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