7 min readConcept sheetA 3-mark question on dispersion or the rainbow, with a diagram; the order of colours comes as a 1-mark MCQ

Dispersion of white light and the rainbow

A prism splits white light into seven colours (VIBGYOR) because each colour bends by a different angle. Raindrops do the same to sunlight, making a rainbow.

DispersionSplitting of white light into its component colours

SpectrumThe band of colours: violet, indigo, blue, green, yellow, orange, red

Bends most / leastViolet bends the most, red the least

RainbowSeen opposite the Sun, with the Sun behind you

Dispersion and the spectrum

When a narrow beam of white sunlight passes through a glass prism, it comes out as a band of colours. This band is called a spectrum. The colours, in order, are Violet, Indigo, Blue, Green, Yellow, Orange and Red, remembered as VIBGYOR. The splitting of white light into its colours is called dispersion. It happens because each colour bends through a different angle in the prism. Red bends the least, so it is at the top of the band. Violet bends the most, so it is at the bottom.

Newton's two-prism experiment

1
Isaac Newton passed sunlight through one glass prism and got a spectrum of colours.
2
He tried to split one colour further with a second, similar prism. No new colours appeared.
3
He then placed the second prism upside down (inverted) next to the first.
4
All the colours passed through the second prism and recombined. A beam of white light came out on the other side.
5
Conclusion: sunlight is made of seven colours. Light that splits into the same band of colours as sunlight is called white light.

How a rainbow forms

A rainbow is a natural spectrum. To see one, stand with your back to the Sun after a shower.

How to draw dispersion by a glass prism

1
Draw a triangle for the prism, with the top corner up and the base at the bottom.
2
Draw a single ray coming from the left and hitting the left face. Label it white light.
3
Inside the prism, draw a narrow fan of lines spreading out from the point of entry.
4
Where the fan leaves the right face, draw it widening. Label the top ray red and the bottom ray violet, with the other colours in VIBGYOR order between them.
5
Draw a screen on the right and write the colour names on it. Label the band spectrum.

How to draw a rainbow forming in a raindrop

1
Draw a large circle for a raindrop.
2
Draw a ray of sunlight from the left entering the upper part of the drop. Label it sunlight.
3
At the point of entry, split it into two rays (red and violet) bending into the drop. Label this refraction and dispersion.
4
Draw both rays striking the back (right side) of the drop and bouncing back. Label this internal reflection.
5
Draw them leaving the lower part of the drop, bending again and heading down to the left, towards the observer. Label this refraction, and mark red and violet.

In real life

Stand with the Sun behind you and look at the spray of a garden hose or a fountain: you may see a small rainbow. You can see them near waterfalls such as Jog Falls or Athirappilly too.

Exam angle

1 marks: “Which colour of white light bends the most when it passes through a prism?” Violet.

3 marks: “What is dispersion? Why do the colours separate in a prism? How can they be combined back into white light?” Dispersion is the splitting of white light into its colours. Different colours bend through different angles (violet most, red least). A second, inverted prism recombines them into white light.

5 marks: “Explain with a labelled diagram how a rainbow is formed. In which direction is it seen?” Raindrops act as small prisms: refraction and dispersion on entering, internal reflection at the back, refraction on leaving. Diagram of a drop with red and violet rays. Seen in the direction opposite to the Sun.

Remember it

VIBGYOR from the bottom up in a prism: Violet bends very much, Red bends rarely.