Part (a) Majority of the candidates merely defined critical angle without adding the necessary additional information so they lost some marks. Performance was above average

Part (b) majority of the candidates had no difficulty in answering this question.

Part (c) most candidates assumed the angle of the prism to be 60°. this is not correct. They are expected to ca;culate it out from the given information in the question i.e

A= 2r = 2 x 29 = 58° . Few candidates did this and scored full marks

Part (d) (i) and (ii) posed no problem to majority of the responding candidates. However in (iii)

and (IV) candidates again assumed the problem to be minimum deviation problem which is not

correct. Performance was average. -

The expected answers are:

(a) Critical angle is the angle of incidencc in the denser medium when the angle of refraction in the less dense medium is 90°

Additional information

e.g correct diagram

(b) Factors affecting the deviation of a rav in a triangular glass prism

angle of incidence

refracting angle of the prism

refractive index of the material of the prism

(C) n =

A = 2r

= 2 x 29

= 58

Sin ½ (A+D) = n Sin ½ A

½ (A+Dm) = (1.5 x Sin½ x 58)

= 0.7272 = 46.67

58 + Dm = 93.32

Dm= 35.32

(d) (i) Angle P = 45-20

= 25°

Angle Q = 180 – 60

= 120°

P + Q + R = 180

R = 180 - (P +Q)

= 180 – 145

= 35°

- n =

= =

= 1.673

- n =

1.673 =

Sin e = 1.673 x Sin 35

= 0.9595

E = sin-1(0.9595)

=73.7°

- D = (i+e) – A

= (45 + 73.3) – 60

= 58.3°