Critical angle is the angle of incidence in the denser medium when the angle of refraction in the less dense medium is 90°.

(b) Deviation of a ray in a triangular prism is determined by the factors:
- Angle of incidence
- Refracting angle of the prism, and
- Refractive index of the prism material.
(c) A = 2r = 2 x 29 = 58${}^o$
But n = $\frac{sin\frac{1}{2}(A+Dm)}{sin\frac{1}{2}A}$
sin $\frac{1}{2}$(A + Dm) = n sin A $\frac{1}{2}$A
58 + Dm = 93.32 - 58 = 35.32${}^o$
(d) Angle P = 45° - 20° = 25°
But Angle Q = 180°- 60° = 120°
P +Q + R = 180°
Hence R = 180 - (P + Q) =180 - (25 + 120) = 35°
(ii) n = $\frac{Sini}{sinP}=\frac{sin{45}^o}{sin{45}^o}=1.673$
(iii) n = $\frac{sini}{sine}=\frac{sine}{sinR}=\frac{sine{e}^o}{sin{35}^o}$
= 1.673 = $\frac{sine}{sin{35}^o}$
e = sin${}^{-1}$ (1.673 x sin 35${}^o$) = 73.7${}^o$
(iv) D = (i + e) - A = (45 + 73.3) - 60${}^o$
= 58.3${}^o$