Which of the following statements with regard to the phenomenon of the primary rainbow formation by water droplets is/are correct? 1. It involves refraction and one internal reflection of sunlight. 2. It involves refraction of sunlight only. 3. It is formed as the inner bow. 4. It may involve more than one internal reflection as well as refraction of sunlight. Select the answer using the code given below:
- (a)1 only
- (b)1 and 3
- (c)3 and 4
- (d)2 and 3
Correct — B, 1 and 3. In a primary rainbow, sunlight entering a spherical raindrop is refracted (bent, and split into colours by dispersion) as it enters, undergoes one internal reflection at the back of the drop, and is refracted again as it leaves — so statement 1 is correct. When a fainter secondary bow also appears, the primary is the lower, inner and brighter arc, so statement 3 is correct too. Statement 2 is wrong because refraction alone cannot return the light to the observer (the internal reflection is essential), and statement 4 (more than one internal reflection) describes the secondary rainbow, not the primary. Hence 1 and 3, option (b).
- (a)1 only — Statement 1 is correct, but this leaves out that the primary bow is also the inner arc (statement 3), so it is incomplete.
- (c)3 and 4 — Statement 3 is right, but statement 4 — more than one internal reflection — describes the secondary rainbow; the primary has exactly one internal reflection.
- (d)2 and 3 — Statement 3 is right, but statement 2 ('refraction only') is wrong, because the primary rainbow also needs one internal reflection, not refraction alone.
A rainbow forms when sunlight interacts with raindrops. In the primary bow, each drop refracts the incoming light (splitting it into colours by dispersion), reflects it once off its inner back surface, and refracts it again on exit. This returns light to the observer at about 40 to 42 degrees, with red on the outer edge and violet on the inner. A secondary bow, formed by two internal reflections, is fainter, larger (about 51 degrees) and has its colours reversed.
Read each statement against this standard model. 'One internal reflection' and 'inner bow' both describe the primary rainbow, so statements 1 and 3 are in. 'Refraction only' ignores the reflection step, and 'more than one internal reflection' belongs to the secondary bow, so statements 2 and 4 are out.
- The primary rainbow involves refraction, one internal reflection, then refraction again inside each raindrop.
- It appears at about 40 to 42 degrees from the antisolar point, with red on the outer edge and violet on the inner.
- When both bows appear, the primary is the inner and brighter one, while the secondary is outer and fainter.
- The secondary rainbow is produced by two internal reflections and has its colour order reversed.
- Thinking a rainbow needs refraction only — it also needs one internal reflection.
- Swapping the primary and secondary bows; the primary has one reflection and is the inner, brighter arc.
- Assuming both bows show the same colour order — it reverses in the secondary rainbow.
A statement-based question on how the primary rainbow forms — the keys are one internal reflection and the inner, brighter bow.
Rainbow is produced when sunlight falls on drops of rain. Which of the following physical phenomena are responsible for this? 1. Dispersion 2. Refraction 3. Internal reflection
- (a) 1 and 2 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer(d) 1, 2 and 3 — a rainbow forms by dispersion, refraction and internal reflection acting together inside raindrops.
Tests the very same optics — refraction, internal reflection and dispersion in raindrops — that this NDA question applies to the primary bow.
- practice — not a real PYQ
The secondary rainbow differs from the primary rainbow mainly because it involves:
- (a)no refraction at all
- (b)two internal reflections
- (c)only dispersion
- (d)reflection off clouds
Answer(b) two internal reflections — this makes it fainter, larger and reverses its colour order.
- practice — not a real PYQ
In a primary rainbow, which colour appears on the outer (upper) edge of the arc?
- (a)Violet
- (b)Green
- (c)Red
- (d)Blue
Answer(c) Red — the primary rainbow shows red on the outer edge and violet on the inner.