Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.

Materials required

  • a drawing board
  • a white sheet of paper
  • a few common pins and a plane mirror.

Procedure

  • Pin the white sheet of paper firmly on the drawing board.
  • Place the plane mirror on it and trace its outline on the paper.
  • Then remove the mirror and draw the normal.
  • The normal is the 90 degrees vertical line drawn to the outline of the mirror.
  • Now place the mirror again on the outline. The normal will be reflected clearly in the mirror.
  • Next place two pins in a straight line on one side of the normal on the white sheet of paper.
  • Next place two pins on the other side of the normal in such a way that these two pins are in a straight line with the reflection of the two pins on the other side of the normal.
  • Now remove the mirror and the pins and join the pin marks to the normal.

Observation and Conclusion

It will be seen that the angles which the lines make on both sides of the normal will be equal (refer to the attached image). So, if one line represents the incident ray – the ray which is travelling from the source of light – and the other line represents the reflected ray – the ray of light which has been reflected –

  • it is proved that the angle which the incident ray makes to the normal is always equal to the angle which the reflected ray makes to the normal, i.e. <i=<r
  • Since the lines representing the normal and the incident and reflected rays are all represented on the sheet of plain paper, the incident ray, the reflected ray and the normal all lie on the same plane.

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