(a) Critical angle is defined as the angle of incidence in the denser medium for which the angle of refraction in the less dense medium is 90°.

(b) Antinodes are created in a stationary wave when an incident and its reflected waves are superposed and 180° out of phase.

(c) n = \(\frac{Sin\frac{(A + Dm)}{2}}{\sin (\frac{A}{2})}\)
= \(\frac{Sin\frac{(60 + 46.2)}{2}}{\sin (\frac{60}{2})}\)
= \(\frac{sin 53.1}{sin 30}\)
= 1.6
(d) No image is obtained on the screen because
- Object, screen and mirror are not coaxial;
- Object is placed at focal point of mirror;
- Object is placed between the focus and the pole;
- Screen is located at a distance less than or greater than actual image distance.

(e) (i) \(\frac{1}{u} + \frac{1}{v} = \frac{1}{f}\)
\(\frac{1}{v} = \frac{1}{f} - \frac{1}{u} = \frac{1}{30} - \frac{1}{7}\)
= \(\frac{5 - 2}{150} = \frac{3}{150}\)
U = 75 + 25 = 100cm
\(\frac{1}{u} + \frac{1}{v} = \frac{1}{f}\)
\(\frac{1}{100} + \frac{1}{50} = \frac{1}{50} = \frac{1}{50}\)
f = 50cm