Activity & Practical on Glass Prism | Class 10th level | edugrown

In a rectangular glass slab, the emergent light rays are parallel to the incident light rays because the opposite faces of a rectangular glass slab (where refraction takes place) are parallel to one another.

The triangular glass prism is a transparent object made of glass having two triangular ends and three rectangular sides (or rectangular faces).The opposite faces of a triangular glass prism are not parallel to one another.

They are inclined at an angle to one another.The angle between its opposite faces is called the angle of the prism.

Aim

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

MATERIALS REQUIRED

A drawing board, few drawing pins or cello-tape, a glass prism, a few sharp pointed alpins, three sheets of white paper, a measuring scale, a pencil and a protractor.

THEORY

  • Let ABC be the principal section of a glass prism of refractive index ‘n’ The angle ‘A’ is the refracting angle of the prism.
  • When a ray of light PQ from rarer medium (such as air) is incident on a refracting surface AB, it gets refracted and bends towards the normal to the plane of face AB. Here, ray of light is entering from air to glass and refracted along QR.
  • At the second surface AC, the refracted light ray QR has entered from glass to air, i.e. denser to rarer medium and emerges out, along RS as it has bent away from the normal i.e. bends towards the base of prism shown in the adjacent figure.
    CBSE Class 10 Science Practical Skills – Refraction Through Prism 4
  • In the figure:
    PQ – Incident ray
    QR – Refracted ray
    RS – Emergent ray
    ∠A – Angle of prism
    ∠i – Angle of incidence
    ∠r – Angle of refraction
    ∠e – Angle of emergence
    ∠δ – Angle of deviation
    Thus, the incident ray PQ suffers two refractions in passing through the prism.

PROCEDURE

  1. Take a sheet of white paper and fix it on the drawing board with the help of a drawing pins or cello- tape.
  2. Place a glass prism along its triangular base at middle on the paper. Mark the boundary ABC of the glass prism and then remove the prism.
  3. Draw a thin dotted line XY along one of the refracting surfaces (say AB) as shown in the figure given below.
  4. Draw a normal NFN1, i.e. perpendicular to line AB at F. Also, draw a line EF making an angle 30° to the normal. EF will act as the incident ray.
  5. Place the glass prism back to its original position ABC.
  6. Fix two alpins P1 and Q1 on line EF vertically by gently pressing their heads with thumb into the drawing board about 6 cm apart from each other.
  7. Look the images of P1 and Q1 from the other refracting face AC. The images of pins P1 and Q1 appear to be at I1 and I2.
  8. Fix two more alpins P2 and Q2 vertically such that the feet of the pins P2 and Q2 appear to be on the same straight line as the feet of the images I1 and I2 respectively. Confirm it by moving the head slightly to either side while looking them. All the pins P2, Q2 and image of P1 and Q1 would appear to move together if they are on the same straight line.
  9. Remove the prism and all alpins. Mark the position of feet P2 and Q2 and also P1 and Q1 [like Θ],
  10. Join the pin pricks P2 and Q2 and produce P2Q2 so that it meets the refracting side AC of the prism boundary at G. The line HG represents the path of the emergent ray.
  11. Join F and G. The line FG represents the path of the refracted ray.
  12. Repeat the experiment on different paper sheets by taking different values of angle of incidence, viz. 35°, 45°, 50° and 55°.

To trace the path of the rays of light through a glass prism - Lab Work

OBSERVATIONS

  • The ray of light entering from air to glass at the first refracting surface bends towards the normal after refraction. At the second refracting surface, the ray of light entering from glass to air bends away from the normal.
  • Thus, the ray of light suffers two refractions on passing through a prism and hence deviates through a certain angle from its original path.

Refractive Index Of Different Liquids Using Hollow Prism | Physics Astronomy Project Topics

RESULT

EFGH represents the path of light through the prism as shown in the figure drawn.

PRECAUTIONS

  1. The refracting faces of the glass prism should be smooth, transparent and without any air bubble or broken edge. It must be a triangular.
  2. Use a sharp pencil to draw boundary of the prism and rays of light.
  3. The alpins should have sharp tip and should be fixed exactly vertical to the plane of the paper.
  4. The distance between the alpins should be atleast 6 cm. It provides the greater accuracy in finding the direction of incident ray and refracted ray.
  5. Always adjust the feet and not the heads of alpins to lie along the same straight line.
  6. The pin pricks should always be dotted and circled.
  7. The angle of incidence should be in between 30° and 60°.
  8. Proper arrows should be drawn to indicate direction of the incident ray, the refracted ray and the emergent ray.

SOURCES OF ERROR

  1. The feet of alpins may not lie on the same line.
  2. The alpins may not be exactly vertical.
  3. The pin pricks may not be marked properly.
  4. The lines which indicate the path of ray of light may not be drawn with accuracy.

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