Optics – Mirrors & Lenses

  • Light follows a rectilinear propagation (3 * 108 m/s)
  • Umbra Point source of light Shadow (Total dark)
  • Penumbra →Extended source of light Shadow (Partial dark)


Plane Mirror

Reflection on mirrors


Angle of incidence (i) = Angle of reflection (r)





Image on plane mirror





Image formed on Plane mirrors

  • Virtual & Erect
  • Equal distance & size
  • laterally inverted





Concave & Convex Mirrors


Concave mirrors


Image formed by Concave Mirror

Image formed by Concave Mirror


Image formed by Convex Mirror

Image formed by Convex Mirror


Distinction between a Plane, Concave & Convex Mirror

Distinction between a Plane, Concave & Convex Mirror


Use of Concave & Convex Mirrors

Concave Mirrors Torch,Vehicle headlightsShaving mirrors,Dentist MirrorConc. sunlight to produce heat  Convex mirrors Rear view mirrorsAlways gives erect though diminished imageProvides wider view 



  • Velocity is higher in less denser medium
  • n21 = v1/v2 (n21 refractive index of medium 2 wrt medium 1)
  • A ray of light will bend towards the normal if it is travelling from rarer to denser medium & away from the normal if it is travelling from denser to rarer medium


Few examples of refraction

  • Twinkling of stars
  • Sunrise & sunset (we can see the sun 2 min before the sunrise & 2 min after the sunset)
  • Bottom of water filled body appears to be raised

Refraction on water


Concave & Convex Lenses

Concave & Convex Lenses

  • Convex Lens – Converging Lenses
  • Power –  Positive
  • Concave Lens – Diverging Lenses
  • Power –  Negative

Power of a lenses Diopter


Dispersion of light

Dispersion of light


Total Internal Reflection

  • Light can not always pass through denser to rarer medium
  • if the angle of incidence > critical angle, it leads to total reflection of light
  • Examples : Optical fiber, Mirage effect (Usually associated with hot deserts)


Mirage effect


Mirage Effect

  • Air near desert surface is hot & which cools rapidly with height (2 different mediums density wise)
  • Rays from top of trees exceeds critical angle & TIR occurs
  • Observer sees tree upside down giving illusion of water
  • Vice a versa if air near the ground is cold & up above hot



mirage effect in desert



  • Formed opposite of sun
  • Process Refraction & Dispersion + TIR + Refraction & Dispersion

How rainbow is formed


Scattering of Light

  • A molecule of a medium emits incoming light in all possible directions known as scattering (Tyndall Effect)

Scaterring of light

Blue Color Sky

  • When sunlight reaches earth’s atmosphere, Blue color scatters more strongly than the red
  • When this scattered color enters our eye, sky looks blue to us

Blue sky & red sunset

Reddish Sunrise / Sunset

  • Light from sun near horizon passes through thicker layers of air & travel larger distances in earth’s atmosphere before reaching our eyes.
  • Blue light & shorter wavelengths scatters away soon letting longer wavelengths of red light reach our eyes


Interference of waves

Superimposition of 2 waves of same kind which passes through same point at the same time

Wave Interference

  • Constructive interference –   Same phase interference
  • Destructive interference –   Opposite phase interference

Constructive interference

Example Colors in soap bubble & oil on water in presence of white light (Alternate black & white spots in case of monochromatic light)


  • Spreading of light through a narrow slit or aperture
  • Failure of light to travel in a straight line (due to the wave nature of light)


Diffraction Grating A device used to cause diffraction for ex. Parallel equidistant lines on glass or metals (as in CD)

Holograms → Result of Laser + Interference + Diffraction

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