Why does a rainbow appear after a rainstorm?
Ever wondered why the sky sometimes paints itself with a perfect arc of colors? The answer lies in the same trick that makes a prism split a flashlight beam into a rainbow.
💡 In Simple Words: When white light passes through a glass prism, each colour bends a little differently. This bending, called dispersion, spreads the light into a spectrum – a continuous band of colours from red to violet.
What is a Light Spectrum?
A spectrum (pronounced "SPEK-truhm") is simply a range of something. In physics, a light spectrum is the spread‑out collection of all the colours (or wavelengths) that make up white light. Think of it like a piano keyboard: each key is a different note, and together they form a complete scale. Similarly, each colour is a different wavelength, and together they form the visible spectrum.
Types of Spectra
- Continuous spectrum: A smooth, unbroken band of colours, like the one you see after a prism. Sunlight and incandescent bulbs give this.
- Line spectrum: Only a few discrete colours appear, like bright lines on a dark background. This is what you get from excited gases (e.g., a neon sign).
How Does Dispersion Happen?
Dispersion (pronounced “dis-PER-zhun”) means the separation of light into its component colours because each colour travels at a slightly different speed in a medium other than vacuum. Imagine water flowing through a garden hose. If the hose narrows at one point, the water speeds up there. In glass, the “speed limit” for each colour is a bit different, so they bend by different amounts when they enter or leave the glass.
The Physics Behind It
Light’s speed in a material is given by v = c/n, where c is the speed of light in vacuum and n is the refractive index of the material. The refractive index isn’t the same for every wavelength – it’s higher for blue/violet light and lower for red light. Higher n means slower speed and stronger bending.
Prism Experiment – Step by Step
The classic classroom experiment shows dispersion in action. Here’s how it works, broken down into a simple flowchart:
When the white beam enters the prism (step A), it slows down and bends (step B). Inside the glass, each colour continues at its own speed, so they start to fan out (step C). By the time the light exits (step D), the separation is complete and you see a colourful spread (step E) that you can project onto a screen (step F).
Key Observations
- The angle between red and violet rays is about 2°–3° for a typical glass prism.
- If you rotate the prism, the spectrum also rotates – the order of colours stays the same.
- Water droplets in the sky act like millions of tiny prisms, giving us a natural rainbow.
Comparison Table: Continuous vs. Line Spectrum
| Feature | Continuous Spectrum | Line Spectrum |
|---|---|---|
| Source | Sun, incandescent bulb, prism experiment | Excited gases, neon signs, discharge tubes |
| Appearance | Unbroken band of colours | Bright isolated lines on dark background |
| Explanation | All wavelengths present | Only specific wavelengths emitted |
Why Does Dispersion Matter in Everyday Life?
Besides the pretty rainbows, dispersion is behind many technologies. Fiber‑optic cables use the principle to send light signals, and spectrometers (devices that measure spectra) help astronomers figure out what stars are made of.
Quick Revision Bullet Points
- Dispersion = separation of white light into colours because different wavelengths refract differently.
- Refractive index varies with wavelength – higher for blue/violet, lower for red.
- Prism experiment: white light → refraction → separation → spectrum.
- Continuous spectrum = all colours; line spectrum = only certain colours.
- Real‑world examples: rainbows, spectrometers, fiber optics.
📝 Likely Exam Questions
- Explain why a prism separates white light into a spectrum.
Answer: White light contains all visible wavelengths. When it enters glass, each wavelength slows down by a different amount because the glass’s refractive index depends on wavelength. This causes each colour to bend by a different angle, spreading them out into a spectrum – a process called dispersion. - What is the difference between a continuous spectrum and a line spectrum?
Answer: A continuous spectrum shows an unbroken range of colours (e.g., sunlight). A line spectrum shows only specific, discrete colours (bright lines) emitted by excited atoms or gases. - Describe the steps of the prism experiment that demonstrate dispersion.
Answer: (1) Direct a narrow beam of white light onto a glass prism. (2) Light refracts (bends) as it enters the prism. (3) Inside the prism, different wavelengths travel at different speeds, causing them to separate. (4) Light refracts again when exiting, further spreading the colours. (5) A spectrum appears on a screen. - How does the refractive index vary with wavelength, and what effect does this have?
Answer: The refractive index is higher for shorter wavelengths (blue/violet) and lower for longer wavelengths (red). Higher index means more bending, so blue light deviates more than red, leading to the spread of colours. - Give two practical applications of light dispersion.
Answer: (i) Spectroscopy – identifying elements in stars by their line spectra. (ii) Fiber‑optic communication – using dispersion management to transmit data over long distances.