(a) A monochromatic beam of light is travelling from a rarer to a denser medium. Giving reasons, explain the following : (i) Is the frequency of the reflected and the refracted light the same as the frequency of incident light ? (ii) Does the decrease in speed in denser medium imply a reduction in the energy carried by the light wave ? OR (b) (i) The lower half of a concave mirror's reflecting surface is covered with an opaque non-reflecting material. What effect will this have on the image of an object placed in front of the mirror ? (ii) A lens made of glass of refractive index $1\cdot5$ disappears, when immersed in a trough of liquid. What is the refractive index of the liquid ? Give reasons for your answer in each case.

Year	Paper Code	Set	Position
CBSE 2026	Series 55-7	Set 1	Question 18
CBSE 2026	Series 55-7	Set 2	Question 19
CBSE 2026	Series 55-7	Set 3	Question 20

• Step 1: Stating the fundamental formula / law statement (+1 Mark)
• Step 2: Intermediate mathematical substitution or proof steps (+1 Mark)
• Step 3: Correct final value or conclusion with mandatory SI units (+1 Mark)