Grade 7 → Lighting and Optics ↓
Dispersion of light and formation of spectrum
Have you ever seen a rainbow in the sky after a rain? This beautiful arch of colors spread across the sky is a great example of dispersion. But what is dispersion and how does it relate to light? Let's learn about this fascinating phenomenon.
What is light?
Before diving into dispersion, it's important to understand what light is. Light is a form of energy that enables us to see things around us. It travels in straight lines and is made up of tiny particles called photons. The visible light we can see is just a small part of the electromagnetic spectrum, which includes other types of light such as radio waves, microwaves, and X-rays.
Nature of light
Light can have different properties such as intensity, frequency and wavelength. The wavelength of light determines its colour. Red light has a longer wavelength, while violet light has a shorter wavelength. The different wavelengths combine to form white light, the kind of light we see coming from sources such as the sun or a bulb.
What is dispersion?
Dispersion occurs when white light is separated into its individual component colours. This separation or dispersion occurs because different colours of light bend different amounts as they pass through an object. This bending of light is also called refraction.
Exploring the dispersal
The credit for the discovery of dispersion goes to Sir Isaac Newton. He conducted an experiment by passing sunlight through a glass prism. A prism is a transparent object with a flat, polished surface that refracts light. Newton observed that sunlight, which appeared white, splits into a band of many colours. This band of colours is called the spectrum.
How does dispersion occur?
Let's look in detail at how dispersion actually works. When white light enters a prism, it slows down and bends. This bending occurs because the light enters from a less dense medium (air) into a denser medium (the glass of the prism). Here's a simple representation:
// light enters the prism
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/ <- Prism
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Different colours of light bend different amounts because each colour has a different wavelength. Violet light bends the most, and red light bends the least. This is how the colours separate and form a spectrum.
Spectrum
The spectrum is the range of colors that light waves can assume. In the spectrum formed by the dispersion of light, the colors appear in the following order:
- Red
- orange
- Yellow
- Green
- Blue
- Indigo
- Purple
These colors appear in this specific order because of their wavelength. Red light with the longest wavelength bends the least, and violet light with the shortest wavelength bends the most.
Real-life examples of dispersion
Besides rainbows, we encounter dispersion in daily life through a variety of objects and phenomena:
- CDs and DVDs: When light falls on the surface of a CD or DVD, the grooves split the light into different colors, creating an effect similar to a spectrum.
- Bubbles and oil streaks: Have you ever noticed colorful flashes on bubbles or oil streaks? These colors are caused by the interference and dispersion of light waves, giving a rainbow-like appearance.
- Sunset: The colour of the sky changes due to the scattering and dispersion of light, causing different colours to appear at different times.
How is a rainbow formed?
Rainbows are a natural example of dispersion. After rain, the air is filled with tiny water droplets. When sunlight hits these droplets, the light scatters, reflects, and refracts. This process creates a circular spectrum of colors that we see as a rainbow.
Light enters a drop, bends and splits into its component colors. It then reflects off the back of the drop and refracts once more as it exits the drop. Each drop scatters sunlight in the same way, creating a full spectrum of colors.
Understanding the science of bending and refraction
When light strikes a substance, its speed changes. This change in speed bends the light. The angle at which the light bends depends on the refractive index of the substance. The refractive index (n) can be calculated using the formula:
Refractive index (n) = Speed of light in vacuum (c) / Speed of light in medium (v)
Each medium has a different refractive index, which is why light may bend differently when passing from one medium to another.
Beyond visible light
The spectrum we see when light is scattered comprises only a small part of the electromagnetic spectrum. Other waves such as ultraviolet light, infrared light, gamma rays, etc. also exist outside the range of visible light.
Conclusion
The dispersion of light and the formation of a spectrum reveal the beautiful complexity of what we see as simple sunlight. By understanding dispersion, we gain insight into the nature of light, color, and many atmospheric phenomena. Whether through art or science, the study of light is an area of endless curiosity and wonder.
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