JEE Main 2022 · 25 July · morning · Single correct · +4 / −1
Which of the following statement is correct?
- (a) In primary rainbow, observer sees red colour on the top and violet on the bottom
- (b) In primary rainbow, observer sees violet colour on the top and red on the bottom
- (c) In primary rainbow, light wave suffers total internal reflection twice before coming out of water drops.
- (d) Primary rainbow is less bright than secondary rainbow.
Answer and solution
Answer (a) In primary rainbow, observer sees red colour on the top and violet on the bottom
Solution
Physics sir · 3 steps
Picture the angles from the drop: red leaves the primary bow at 42°, violet at 40°, so red sits higher.
- Inside one drop. In a primary rainbow, sunlight refracts into a raindrop, reflects once off the back surface, and refracts out again. Water bends violet more than red, so the colours leave at slightly different angles.
- Which colour on top. Measured from the line through the observer's head and away from the Sun, red returns at about 42° and violet at about 40°. A larger angle means higher in the sky, so red forms the outer, top edge and violet the inner, bottom edge.
Feature Primary rainbow Secondary rainbow Reflections inside the drop one two Angle for red and violet red ≈ 42°, violet ≈ 40° red ≈ 50°, violet ≈ 53° Colour order red on top, violet at the bottom violet on top, red at the bottom Brightness brighter fainter, because more light is lost at the extra reflection - So. Only (a) is correct: in the primary rainbow the observer sees red at the top and violet at the bottom.
The idea to keep. Primary bow: one reflection, red outside, brighter. Secondary bow: two reflections, colours reversed, fainter. Every extra reflection costs light and flips the colour order.
Answer from the NTA final answer key. Solution by PrepWiser.