Rainbow formation is as a result of the combination of which of the following phenomena? (i) Reflection (ii)…

Analyze the question: You are asked which phenomena combine to form a rainbow. Several are listed, and you must pick the right combination.

Explain the concept: A rainbow forms when sunlight passes through spherical raindrops. Four things happen inside each drop:

1. Refraction (iv): Sunlight enters the raindrop and bends as it passes from air into water.
2. Dispersion (ii): Because each colour has a different refractive index in water, the white light splits into its component colours (red bends least, violet most).
3. Reflection / total internal reflection: At the back surface of the drop, the light is reflected internally. In textbook treatments this is often called total internal reflection (iii), since the light strikes the water-air boundary at an angle larger than the critical angle.
4. Refraction again (iv): The light leaves the drop, bending once more as it goes from water to air, which spreads the colours further.

Step-by-step solution:

• Refraction is definitely involved, at entry and exit.
• Dispersion is definitely involved, since it separates the colours.
• Internal reflection at the back of the drop is involved, and the standard JAMB treatment calls it total internal reflection.
• Ordinary reflection (i) is not listed separately from the internal reflection in the standard answer. The key reflection step is the total internal reflection (iii).

So the combination is dispersion, total internal reflection and refraction: (ii), (iii) and (iv) only.

Note on the options: Option D, "(i), (ii) and (iv)", is arguably also defensible in a loose sense, since the internal bounce is a reflection. So C is the expected answer.

Answer: C

Exam tip: Remember the rainbow sequence as R-D-R-R: Refraction in, Dispersion, Reflection (total internal) at the back, Refraction out. When "total internal reflection" is among the options, JAMB usually wants it for rainbow questions.