How a rainbow forms
Raindrops act as tiny prisms. A primary rainbow needs two refractions and one internal reflection; the fainter secondary rainbow needs two reflections.
Path of sunlight in a raindrop (primary rainbow)
The secondary bow adds a second internal reflection, which turns the order of colours over.
Why we see a coloured bow
After a shower, the air is full of drops. Each drop disperses sunlight like a prism. For each colour, the drops send out the most light at one angle, the angle of minimum deviation. For the primary bow, rays turn back through about 137°, so they reach you at about 42° to the sunlight. Drops at about 43° from the line through the Sun and your eye send you red light; drops at about 41° send you violet. All the drops at the same angle lie on a circle round that line, so you see a coloured arc with red outside and violet inside. The ground cuts off the lower part. The bow is seen with the Sun low and behind you, in the morning or late afternoon. When the Sun is higher than about 42°, the bow falls below the horizon.
Primary and secondary rainbows
| Feature | Primary rainbow | Secondary rainbow |
|---|---|---|
| Internal reflections in each drop | One | Two |
| Brightness | Brighter | Fainter, as some light escapes at the extra reflection |
| Outer colour | Red | Violet |
| Inner colour | Violet | Red |
| Angle at the eye | About 41° (violet) to 43° (red) | About 51° (red) to 54° (violet) |
| Position | Lower | Above the primary, with a darker band of sky between them |
How to draw the formation of a primary rainbow
- 1Draw an eye at the lower left. Draw parallel arrows of sunlight coming from behind the eye and travelling to the right.
- 2Draw two circles (raindrops) to the right, one higher than the other.
- 3In each drop, show a ray entering the upper part and bending in. Show it reflecting once at the back, then leaving from the lower part towards the eye.
- 4From the upper drop, draw the red ray reaching the eye. Mark 43° between it and the direction of the sunlight.
- 5From the lower drop, draw the violet ray reaching the eye. Mark 41°.
- 6Label the bow: red on the outside (top), violet on the inside.
After a late-afternoon monsoon shower, turn your back to the Sun and look east to spot a rainbow. You can make one at home by spraying a fine mist from a garden hose with the Sun behind you.
3 marks: “Explain the formation of the primary and secondary rainbows. Why is the order of colours reversed in the secondary rainbow?” Primary: two refractions and one internal reflection in raindrops; red reaches the eye at about 43° and violet at about 41°, so red is outside. Secondary: two refractions and two internal reflections; the extra reflection reverses the order, so violet is outside (about 54°) and red inside (about 51°); it is fainter because light escapes at each reflection.
2 marks: “Why is a rainbow not seen at noon on a clear summer day after rain?” The bow lies at about 41° to 43° from the line joining the Sun to the eye, on the side away from the Sun. With the Sun high overhead this line points steeply down, so the bow falls below the horizon.
1 marks: “Which colour is on the outer edge of the primary rainbow?” Red.
Primary: one reflection, red outside. Secondary: two reflections, colours swapped.