Tuesday, 31 January 2017

Physics notes on Optics for competitive exams




  • Light is a form of energy, which is propagated as electromagnetic wave.
  • It is the radiation which make our eyes able to 'see the object. Its speed is 3 x 108 m/s. It is the form of energy. It is a transverse wave.
  • It takes 8 min 19s to reach on the earth from the sun and the light reflected from moon takes 28s to reach earth.
  • 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

  • When a ray of light falls on a boundary separating two media comes back into the same media, then this phenomenon is called reflection 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.

Reflection from Plane Mirror

  • If an object moves towards a plane mirror with speed v, relative to the object the moves towards it with a speed 2v.
  • To see his full image in a plane mirror, a person required a mirror of at least half of his height.

Refraction of Light

  • The phenomenon of deviation of light rays from its path when it travels from one transparent medium to another medium is called refraction of light.
  • The cause of refraction is due to the different speed of light in different medium.
  • When a ray of light enters from one medium to other medium, its frequency and phase do not change, but wavelength and velocity change.
  • Due to refraction form Earth's atmosphere, the stars appear to twinkle.

Laws  of Refraction:

  • The incident ray, the refracted ray and the normal at the point of incidence all three lie in the same plane.
  • The ratio of sine of angle of incidence to the sine of angle of refraction remains constant for a pair  of media i.e.
Sin i/Sin r  = constant=μ2/μ1,  this law is known as snell's law

Application of Refraction:

  • When light travels from a denser medium towards  a rarer medium it deviates away from the normal , therefore a pond appear shallower.
  • A coin appears at lesser depth in water.
  • Writing on a paper appears lifted when a glass slab is placed over paper.

Critical Angle:

  • The angle of incidence in a denser medium for which the angle of refraction in rarer medium becomes 90°, is called the critical angle.

Total Internal Reflection:

When a light ray travelling from a denser medium to the rarer medium is incident at the interface at an angle of incidence greater than critical angel, then light rays reflected back into the denser medium , this phenomenon is known as total internal reflection
Sparkling of diamond, mirage and looming, shinning of air bubble in water and optical Fibre are examples of total internal reflection.

Spherical Mirror:

Spherical mirror are of two types
  1. Concave mirror
  2. Convex mirror
  • Image formed by a convex mirror is always virtual, erect and diminished.
  • Image formed by a concave mirror is generally real and inverted.
Uses of Concave Mirror
  • As a shaving mirror
  • As a reflector for the head lights of a vehicle, search light
  • In ophthalmoscope to examine eye, ear, nose by doctors.
  • In solar cookers.
Uses of Convex Mirror
  • As a rear view mirror in vehicle because it provides the maximum rear field of view and image formed is always erect.
  • In sodium reflector lamp.
Important points related to spherical Mirrors:
Centre of Curvature (c): The centre of the hollow glass sphere of which the mirror is a part.
Radius of Curvature(R): The radius hollow sphere of which the mirror is a part.
Pole(P): The mid-point of a spherical mirror is called pole.
Focus (F): when a parallel beam of light rays is incident on a spherical mirror then after reflection it meets or appears to meet at a point on principal axis, called focus of the spherical mirror.
Focal length(f): 
Focal length d= R/2
concave mirror

convex mirroe

Lenses :

A lens is a uniform refracting medium bounded by two spherical surface or one plane surface.

Lenses are of two types:

  • Convex lens
  • Concave lens

Prism :

Prism is a uniform transparent refracting medium bounded by plane surfaces inclined at some angles forming a triangular shape.
Dispersion of light:
When a light is incident on a glass prism, it splits into its seven colour comonents in the following sequence VIBGYOR,  and this is known as dispersion of white light.
The refractive index of glass is maximum for violet colour and minimum for red colour of light, therefore violet colour of light deviated maximum and red colour of light deviated least.