Chapter 10 · Question 5
What is dispersion of white light? Explain why different colours bend through different angles when passing through a prism.
Q5
What is dispersion of white light? Explain why different colours bend through different angles when passing through a prism.
Answer Revealed
Direct Answer:
Dispersion is the splitting of white light into its seven constituent colours (VIBGYOR — Violet, Indigo, Blue, Green, Yellow, Orange, Red) when it passes through a glass prism. Different colours bend by different angles because each wavelength has a different refractive index in the prism material.
Simple Explanation
White light is actually a mixture of seven colours. When it passes through a prism, the colours separate and spread out like a rainbow because each colour bends by a slightly different amount. Violet bends the most, red bends the least. This happens because the prism material slows down different colours by different amounts.
Exam-Ready Structure
Dispersion of white light is the phenomenon in which a beam of white light splits into its seven constituent colours (Violet, Indigo, Blue, Green, Yellow, Orange, Red — remembered as VIBGYOR) when it passes through a glass prism. The band of distinct colours produced is called the spectrum. The explanation for why different colours bend through different angles is as follows: 1. White light is composed of light of multiple wavelengths, each corresponding to a different colour. 2. The refractive index of glass (or any medium) is different for light of different wavelengths. Shorter wavelengths (like violet) experience a higher refractive index, while longer wavelengths (like red) experience a lower refractive index. 3. According to Snell's law, a higher refractive index means greater bending (refraction). Therefore, violet light bends the most on entering and leaving the prism, while red light bends the least. 4. This differential bending causes the colours to emerge at different angles and spread out into a spectrum. 5. Isaac Newton demonstrated that this spectrum can be recombined into white light using a second inverted prism, proving that white light is a mixture of colours. 6. In nature, a rainbow is produced by the dispersion of sunlight through water droplets in the atmosphere.
Key Points
- Dispersion: splitting of white light into seven colours (VIBGYOR)
- Different colours have different wavelengths
- Refractive index varies with wavelength — higher for shorter wavelengths
- Violet bends most, red bends least in a prism
- Rainbow is a natural example of dispersion
Common Mistakes
- Saying red bends the most (red bends the least, violet bends the most)
- Confusing dispersion with simple refraction