Explain with a diagram how a primary rainbow is formed. How does a secondary rainbow differ from it?
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Name the cause. Raindrops disperse sunlight, acting like tiny prisms.
2 · Trace the path of light.
Refraction and dispersion as light enters the drop
One internal reflection at the back of the drop
Refraction again as light leaves
3 · Draw the diagram. [Eye with the Sun behind it; two raindrops; red ray from the upper drop at 43° and violet ray from the lower drop at 41°]
4 · Explain the bow. Drops at the same angle from the line through the Sun and the eye lie on a circle, so we see an arc with red outside and violet inside.
5 · Describe the secondary bow. Two internal reflections: fainter and higher (51° to 54°), with violet outside and red inside.
6 · Real life. Rainbows appear opposite the Sun in the morning or evening after monsoon showers.
Scoring tip: Draw the Sun behind the observer and write the two angles; most marks go to the diagram.
