Describe the dispersion of light as illustrated by the refraction of white light by a glass prism

What is a Thin Lens?

A thin lens is a piece of glass or plastic with two curved surfaces that is thin compared with the distance between the object and the image. It can be converging (convex) or diverging (concave).

🔍 Key formula: $$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$$ where f is the focal length, u the object distance, and v the image distance.

Refraction in a Lens

When light passes from one medium to another (e.g., air to glass), its speed changes, bending the ray. The amount of bending depends on the refractive index n of the material.

📐 For a thin lens, the curvature of the surfaces and the refractive index determine how strongly the lens converges or diverges light.

Dispersion of Light by a Glass Prism

🌈 A prism separates white light into its component colours because different wavelengths have slightly different refractive indices in glass. This phenomenon is called dispersion.

Think of light as a traffic jam of cars of different colours. The prism is a traffic cop that slows down the blue cars more than the red ones, causing them to spread out into a rainbow.

✨ The higher the refractive index, the more the light bends. Since n is larger for violet than for red, violet light is bent more, creating the spread of colours.

Exam Tips

• 🔎 Remember: Dispersion occurs because n varies with wavelength.
• 📐 For lens questions, always use the thin lens formula and check the sign conventions.
• 🌈 In prism problems, identify the order of colours (V–O–R–J–G–B–V) and explain why red is the least deviated.
• 📝 When drawing ray diagrams, label u, v, f, and indicate the direction of light.
• 💡 Use the analogy of a traffic cop to explain why different colours separate.