Explain with a labelled diagram how a rainbow is formed in the sky.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Open in one line. A rainbow is a spectrum made by nature. After a shower, tiny water droplets in the air disperse sunlight into its colours.
2 · Explain the drop as a prism.
Sunlight is refracted and dispersed as it enters a droplet.
The colours are reflected internally at the back of the droplet.
They are refracted again as they leave the droplet.
3 · Draw the raindrop diagram. [Diagram: raindrop; sunlight enters at the top; splits into red and violet; internal reflection at the back; both leave towards the observer; labels: refraction and dispersion, internal reflection, refraction]
4 · State the direction. A rainbow is always seen in the direction opposite to the Sun, so the Sun must be behind the observer.
5 · Daily-life link. The spray of a garden hose or a waterfall on a sunny morning shows the same effect.
6 · Close in one line. So each raindrop acts as a tiny prism, and together they make a rainbow.
Scoring tip: Name all three processes in order: refraction with dispersion, internal reflection, refraction. Leaving out internal reflection is the most common slip.
