What is Dispersion of Light?

S6-SA3-0139

Grade Level:

Class 10

AI/ML, Physics, Biotechnology, Space Technology, Chemistry, Engineering, Medicine

Definition

What is it?

Dispersion of light is the phenomenon where white light splits into its constituent colours (like a rainbow) when it passes through a transparent medium, such as a glass prism. This happens because different colours of light travel at slightly different speeds in the medium, causing them to bend at different angles.

Simple Example

Quick Example

Imagine you have a group of friends, and you all start running together on a muddy track. Some friends are faster, some are slower. After some distance, you won't be in a straight line anymore; you'll be spread out. Similarly, when white light (a mix of colours) enters a prism, each colour 'runs' at a slightly different speed, causing them to spread out and separate.

Worked Example

Step-by-Step

Let's understand how a prism disperses white light.

1. A ray of white light (e.g., from a torch) is directed at one face of a triangular glass prism.
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2. As the white light enters the prism, it slows down. However, red light slows down slightly less than violet light.
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3. Because red light slows down less, it bends (refracts) less. Violet light slows down more, so it bends more.
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4. As the light exits the prism, this difference in bending becomes more pronounced. Red light deviates the least, and violet light deviates the most.
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5. The result is that the white light splits into a spectrum of colours: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR), appearing in that order.

Why It Matters

Understanding dispersion is crucial in fields like optical engineering for designing lenses and cameras that correct colour distortions. It's also vital in astronomy for analysing starlight to determine what stars are made of, and in medical diagnostics using spectroscopy. It even helps us understand how rainbows form!

Common Mistakes

MISTAKE: Thinking dispersion happens because the prism 'adds' colours to white light. | CORRECTION: The prism doesn't create colours; it only separates the colours that are already present in white light.

MISTAKE: Believing all colours of light travel at the same speed in all mediums. | CORRECTION: All colours of light travel at the same speed only in a vacuum. In any other transparent medium (like glass or water), different colours travel at slightly different speeds.

MISTAKE: Confusing dispersion with reflection or absorption. | CORRECTION: Dispersion is specifically about the splitting of white light into its constituent colours due to different refractive indices for each colour, not about light bouncing off a surface (reflection) or being taken in by a material (absorption).

Practice Questions

Try It Yourself

QUESTION: What is the main reason why white light splits into colours when it passes through a prism? | ANSWER: Different colours of light travel at different speeds in the prism, causing them to bend at different angles.

QUESTION: Which colour of light deviates the most when white light passes through a prism, and why? | ANSWER: Violet light deviates the most because it travels slowest in the prism, causing it to bend the most.

QUESTION: If a monochromatic (single colour) light beam, like a red laser, passes through a prism, will it show dispersion? Explain why or why not. | ANSWER: No, it will not show dispersion. Dispersion occurs because white light is a combination of many colours. A single-colour light beam has only one wavelength, so it cannot split into further colours.

MCQ

Quick Quiz

Which of the following phenomena is responsible for the formation of a rainbow?

Reflection of light

Refraction of light

Dispersion of light

Both refraction and dispersion of light

The Correct Answer Is:

Rainbows are formed when sunlight passes through raindrops. The raindrops act like tiny prisms, causing the sunlight to first refract, then disperse into its colours, and finally reflect internally before refracting again as it exits the drop.

Real World Connection

In the Real World

You see dispersion every time you admire a rainbow after monsoon rains, or when sunlight passes through the bevelled edge of a glass table or a diamond, creating tiny colourful sparks. In India, optical fibre cables, crucial for our mobile internet and high-speed data, use principles related to light's behaviour in different mediums, where managing dispersion is key for efficient data transmission over long distances.

Key Vocabulary

Key Terms

Dispersion: The splitting of white light into its constituent colours | Spectrum: The band of colours formed when white light is dispersed | Prism: A transparent optical element with flat, polished surfaces that refract light | Refraction: The bending of light as it passes from one medium to another | VIBGYOR: An acronym for the colours of the visible spectrum: Violet, Indigo, Blue, Green, Yellow, Orange, Red

What's Next

What to Learn Next

Great job understanding dispersion! Next, you should explore 'Total Internal Reflection' and 'Scattering of Light'. These concepts will help you understand more about how light behaves in different situations, from optical fibres to why the sky is blue.