Quick answer: A glass prism splits white light into a band of colours called a spectrum (VIBGYOR); this is dispersion. Violet bends the most and red the least. Isaac Newton found that a second prism could not split any one colour further. When he placed a second identical prism in an inverted position, the colours recombined and white light came out. This shows that white light is a mixture of colours and that a prism separates colours rather than creating them.
Dispersion by a prism
When white light enters a glass prism, each colour bends (refracts) by a different amount, because each colour travels at a slightly different speed in glass.
| Colour | Bending (deviation) |
|---|---|
| Red | Least |
| Orange, yellow, green, blue, indigo | In between |
| Violet | Most |
The colours spread out into the order violet, indigo, blue, green, yellow, orange, red (VIBGYOR), with red at the top and violet at the bottom of the spectrum on the screen.
Newton's experiments
Step 1: Newton used a glass prism to obtain the spectrum of sunlight.
Step 2: He tried to split the colours further with another similar prism, but he could not get any more colours. Each colour stayed the same colour.
Step 3: He then placed a second identical prism in an inverted position with respect to the first.
Step 4: All the colours passed through the second prism, and a beam of white light emerged from its other side.
white prism 1 spectrum prism 2 white
light -> /\ ---> R...V ---> \/ ---> light
(upright) (inverted)
The inverted prism bends each colour back by the same amount the first prism bent it, but in the opposite direction, so the colours come together again.
What the experiment shows
| Observation | Conclusion |
|---|---|
| A second prism cannot split one colour further | The colours are the basic parts of white light |
| An inverted prism recombines the colours into white | White light is a mixture of the seven colours |
| The glass adds no new colour | A prism only separates colours already present |
Link to the rainbow
A rainbow is a natural spectrum seen in the sky after rain, in the direction opposite to the Sun. Tiny water droplets act like small prisms:
Step 1: Sunlight is refracted and dispersed as it enters a droplet.
Step 2: It is internally reflected at the back of the droplet.
Step 3: It is refracted again as it comes out, reaching the observer's eye as separate colours.
Summary table
| Idea | Key fact |
|---|---|
| Dispersion | Splitting of white light into colours |
| Spectrum | The band of colours VIBGYOR |
| Least deviated | Red |
| Most deviated | Violet |
| Second inverted prism | Recombines the colours into white light |
| Rainbow | Refraction, dispersion, internal reflection, refraction |
Exam tips
- Draw the second prism upside down compared with the first, and label the emerging beam "white light".
- Label red at the top and violet at the bottom of the spectrum from the first prism.
- Quote NCERT: the second prism was "identical" and placed "in an inverted position".
Common mistakes
- Drawing both prisms the same way up. That spreads the colours further instead of recombining them.
- Saying the prism creates the colours. It only separates them.
- Saying red bends the most. Violet bends the most.
- Forgetting the internal reflection step when explaining a rainbow.
Frequently Asked Questions
What is dispersion of white light?
It is the splitting of white light into its component colours when it passes through a prism.
What did Newton show with two prisms?
He showed that a second inverted prism recombines the spectrum into white light, proving that white light is made of these colours.
Why must the second prism be inverted?
The inverted prism bends each colour by the same amount in the opposite direction, so the colours come back together.
Which colour bends the most in a prism?
Violet bends the most and red the least.
How is a rainbow formed?
Water droplets refract and disperse sunlight, reflect it internally and refract it again as it leaves, producing a spectrum in the sky.