Class 12 Physics Wave Optics Reflection of plane waves

Reflection of plane waves

  • When all the points on an incident wave which are in same phase are considered together, incident wave front will be obtained.
  • After reflection incident waves will bounce back.
  • Waves which are lighter will touch the surface before the waves which are denser.
  • Let the time taken by some portion of wavefront to interact with the boundary is τ.
  • As soon as incident wavefront touches the boundary, every point on the incident wavefront will give rise to secondary wavelets.
  • The wavefront for the secondary wavelets will be a sphere.
  • After some time t1 incident ray is at some other point, again it will give rise to wavefront.
  • Radius of the wavefront = velocity (speed) of the wave x time taken=vxτ.
  • When other points interact with the boundary gradually, both τand radius will keep decreasing.
  • As a result the tangent of all these points will give rise to reflected wavefront.
  • Mathematically:-
    • Speed of the wave =v
    • Time taken by B to reach point C i.e. BC= vx τ
    • At point A it will give rise to secondary waves, therefore radius=vτ.
    • After some time τ will keep on decreasing.
    • Therefore as it moves towards the point C the radius of the sphere (wavefront) will keep on decreasing.
    • If a tangent is drawn then a reflected wavefront is obtained.


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