(a) Why do we use red colour as a danger signal on the top of a skyscraper?

(a) Red colour is used as a danger signal on the top of a skyscraper because red light has the longest wavelength among visible light, so it is scattered the least by air molecules and dust and remains visible from a long distance and in low visibility, making it ideal for danger signals.

(b)

Here, the refracted ray PQ is normal to the surface AB of the prism (i.e., ∠r = 0°), so the incident ray at the point P should also be normal to the surface AB so that ∠i = 0°.

At point Q, the angle of incidence for the ray PQ = 133° - 90° = 43°

Now, this angle should be equal to the critical angle because ray PQ is refracted at 90°, i.e., it is refracted along QC. Hence, the critical angle of the prism is 43°.

2. In △AРQ

∠APQ + ∠PQA + ∠PAQ = 180°

⇒ 90° + 47° + ∠PAQ = 180°

⇒ ∠PAQ = 180° - 90° - 47°

⇒ ∠ PAQ = 43°

Hence, ∠A= 43°

3. As the light used and its wavelength determine the angle of deviation since with a decrease in the light wavelength, the angle of deviation increases so the beam will experience total internal reflection if we swap out the blue light with lower wavelength i.e., indigo.