28 August 2015

Light Reflection and Refraction

CBSE NCERT Class X (10th) | Science

Light Reflection and Refraction

Reflection of light


  • Light is a form of energy due to which we are able to see the objects which emits light for example objects like sun, lamp, candle emits light of their own and thus they are known as luminous objects.
  • There are objects like table , chair etc. which are not luminous objects and still we are able to see them and this happens because they reflects lights which falls on them from a luminous object like sun, lamp etc. and when this reflected light reaches our eyes we are able to see such non luminous objects.
  • Light rays basically consist of electromagnetic waves which do not require any material medium (like solid, liquid or gas) for their propagation.
  • The wavelength of visible light waves is very small and is of the order of 4×107m to 8×107m .
  • Speed of light waves depends on the medium through which they pass as speed of light in air is slightly less than the speed of light in vacuum 8×108m/s same way speed of light in water and glass is much less than that in air.
  • When light falls on the surface of an object it can either be
    1. Absorbed:- If an object absorbs all the light falling on it , then it will appear perfectly black for example a blackboard
    2. Transmitted: - An object is said to transmit light if it allows light to pass through itself and such objects are transparent.
    3. Reflected:- If an object sends back light rays falling on its surface then it is said to have reflected the light

Reflection of Light

  • The process of sending back light rays which falls on the surface of an object is called REFLECTION of light
  • Silver metal is one of the best reflectors of light.
  • Mirrors we use on our dressing tables in our home are plane mirrors.
  • ray of light is the straight line along which the light travelled and a bundle of light rays is called a beam of light.
  • Laws of Reflection of light
    1. The angle of incidence is equal to the angle of reflection, and
    2. The incident ray, the reflected ray and the normal to the mirror at the point of incidence all lie in the same plane.
  • These laws of reflection are applicable to all types of reflecting surfaces including spherical surfaces

Real and Virtual images

  • An image is formed when the light rays coming from an object meet at a point after reflection from a mirror (or refraction from lens).
  • The images are of two types
    1. Real Images:- Real images are formed when rays of light that comes from an object (or source) meets at a point after reflection from a mirror (or refraction from a lens). Real images can be formed on a screen and can be seen with the eyes.
    2. Virtual images:- Virtual image is an image in which the outgoing rays from an object do not meet at a point. It will appear to meet at a point in or behind the optical device (i.e., a mirror) but they do not actually meet after reflection from a mirror (or refraction from a lens). A plane mirror always forms virtual images.
  • Characteristics of images formed by mirrors:-

    (a) Images formed by mirrors are always virtual and erect
    (b) Size of image is always equal to the size of the object and the image is laterally inverted.
    (c) The images formed by the plane mirror are as far behind the mirror as the object in front of the mirror.
  • Lateral inversion:- If an object is placed in front of the mirror, then the right side of the object appears to be the left side and left side of the object appears to be the right side of this image. This change of sides of an object and its mirror image is called lateral inversion.

Refraction of Light


  • We know about light and also know that light travels in a straight line path in a medium or two different mediums with same density.
  • Now a question arises what happens when light travels from one medium to another with different densities for example from air to glass.
  • When light ray is made to travel from one medium to another say from air to glass medium then light rays bend at the boundary between the two mediums.
  • So, the bending of light when it passes from one medium to another is called Refraction of light.
  • The refraction of light takes place on going from one medium to another because the speed of light is different in two media.
  • Medium in which speed of light is more is called optically rarer medium and medium in which speed of light is less is known as optically denser medium. For example glass is an optically denser medium than air and water.
  • NOTE:- When light goes from rarer medium to denser medium it bends towards the normal and when it goes from denser medium to rarer medium it bends away from the normal.

Refraction through a rectangular glass slab

  • To understand the refraction of light through a glass slab consider the figure given below which shows the refraction of light through a rectangular glass slab.
    refraction of light through a rectangular glass slab
  • Here in this figure AO is the light ray travelling in air and incident on glass slab at point O.
  • Now on entering the glass medium this ray bends towards the normal NN’ that is light ray AO gets refracted on entering the glass medium.
  • After getting refracted this ray now travels through the glass slab and at point B it comes out of the glass slab as shown in the figure.
  • Since ray OB goes from glass medium to air it again gets refracted and bends away from normal N1N'1 and goes in direction BC.
  • Here AO is the incident ray and BC is the emergent ray and they both are parallel to each other and OB is the refracted ray.
  • Emergent ray is parallel to incident ray because the extent of bending of the ray of light at the opposite parallel faces which are PQ (air-glass interface) and SR (glass-air interface) of the rectangular glass slab is equal and opposite.
  • In the figure i is the angle of incidence, r is the angle of refraction and e is the angle of emergence.
  • Angle of incidence and angle of emergence are equal as emergent ray and incident ray are parallel to each other.
  • When a light ray is incident normally to the interface of two media then there is no bending of light ray and it goes straight through the medium.