Tracing the path of the rays of light through a glass prism


To trace the path of the rays of light through glass prism.

Materials Required

  • A drawing board
  • Few drawing pins
  • A glass prism
  • Few sharp pointed alpins
  • Three sheets of white paper
  • A measuring scale
  • A pencil
  • A protractor


A prism is a portion of refracting medium bounded by the planes inclined to each other at an angle.

When a ray of light travels from a rarer medium towards denser medium, it bends towards the normal. if the ray of light goes from denser to rarer, it bends away from the normal.


  1. Fix a sheet of white paper on a drawing board.
  2. Place the prism on the paper and mark its boundary as shown in figure.
  3. Remove the prism and draw a normal at E on side AB.
  4. Draw a line PE making an angle 30° with the normal.
  5. Fix two pins at P and Q on the line PE about 6 cm apart from each other.
  6. Place the prism back to its position.
  7. Look through face AC for images of pins P and Q. Fix points R and S so that they appear in line with the images of pins P and Q.
  8. Remove the pins and join RS to give emergent ray.
  9. Measure angle of emergence and angle of deviation after removing prism and the pins.
  10. Repeat the above procedure for two more angles of incidence i.e., 45° and 60°.


Angle of prism = 60°

S.No. Angle of incidence (∠i) Angle of emergence (∠e) Angle of deviation (∠δ)
2.45°54° 39°
3.60°40° 40°

*These values may be different for different prism.


i + e = A + δ

Error Reduction and Precautions

  • The angle of incidence should be in between 30° and 60°.
  • Use a sharp pencil to properly draw boundary of prism and rays of light.
  • The alpins should have sharp tip and should be fixed vertically.
  • The distance between the alpins should be atleast 6 cm for greater accuracy.
  • The prism should be smooth, transparent and clean.

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