CBSE Class 8 Science • Chapter 10 • Detailed Master Notes
Chapter Overview
We know that light generally travels in straight lines. But what happens when passing from air into water, or from air into glass? It bends! In this chapter, we will explore the science of Refraction, study different types of Lenses, and examine how white light splits into seven gorgeous colours through a process called Dispersion.
You might have noticed that a coin placed at the bottom of a bowl of water appears slightly raised, or a pencil kept obliquely in a glass of water appears bent at the surface. These optical illusions are caused by the phenomenon of refraction.
Definition: When a ray of light passes obliquely from one transparent medium to another (e.g., from air to water, or glass to air), it changes its direction or bends exactly at the boundary separating the two media. This bending of light is called Refraction.
Light travels astonishingly fast in a vacuum or in air (approx. $3 \times 10^8$ m/s). However, when it enters a denser medium like water or solid glass, it slows down because optical density offers resistance to its path. Conversely, when it goes from glass to air, it speeds up. This sudden change in the speed of light perfectly causes the bending!
The bending of light follows strict rules based on the optical density of the two media:
AI Image Prompt: A stunning, crisp 3D vector diagram on a white background showing a thick rectangular glass slab. A bright red laser beam strikes the top surface from the air at an angle, bending downwards sharper (towards the normal) when inside the glass, and then bending back outwards parallel to the original path when exiting the bottom. Label the 'Normal' with dashed lines, and mark incident and emergent rays.
This simple bending of light is responsible for several brilliant optical illusions in daily life:
Q1. Why does a lemon kept in a glass tumbler of water appear to be larger than its actual size?
Ans: The lemon appears magnified due to the refraction of light. The curved surface of the glass tumbler filled with water acts like a convex lens, which bends the light rays outward and makes the lemon inside appear larger.
You must have seen a magnifying glass, or people using spectacles for reading. A beautifully polished piece of transparent glass or plastic bound by at least one curved surface forms a lens. Lenses function entirely based on the refraction of light, bending rays to form images.
| Feature | Convex Lens | Concave Lens |
|---|---|---|
| Physical Shape | Thick in the middle and thin at the edges. (Bulges outwards). | Thin in the middle and thick at the edges. (Caved inwards). |
| Action on Light Rays | Converges (bends inward) the parallel light rays hitting it to a single sharp focal point. | Diverges (spreads outward) the parallel light rays hitting it so they never meet. |
| Image Formed | Can form real AND virtual images. Usually magnifies objects. | Always forms virtual, erect, and smaller (diminished) images. |
| Practical Uses | Used as a magnifying glass, in microscopes, telescopes, camera lenses, and to correct far-sightedness. | Used in door peepholes to make a wider area visible, in flashlights, and to correct short-sightedness. |
AI Image Prompt: A brilliant educational illustration on a white background. Top half: a shining convex lens (oval/lens shaped) bending 3 parallel yellow laser beams inward to intersect at a perfect bright dot (Focus). Bottom half: a beautiful concave lens (hourglass shaped profile) spreading 3 parallel green laser beams widely apart.
Did You Know? The human eye inherently contains a natural convex lens! This flexible crystalline lens focuses light coming from external objects directly onto the retina at the back of the eye, enabling us to see clearly.
Sunlight is often referred to as 'white light'. However, in 1665, Sir Isaac Newton discovered that white sunlight actually consists of seven distinct colours combined together. We can prove this using a glass prism.
Definition: The magical phenomenon of splitting white light into its seven constituent colours when passing through a transparent solid glass prism is vividly called Dispersion.
The Magical Spectrum: These seven colours bend at varying angles when striking the angled glass of the prism. Because each colour travels at a slightly different speed inside the glass, they separate to display a glorious colourful band prominently called a Spectrum.
A natural rainbow is the most magnificent example of dispersion in nature. It occurs after a rain shower when millions of tiny spherical water droplets effectively hang suspended in the atmosphere. These tiny water drops act exactly like miniature glass prisms.
When sunlight enters these water droplets, the light is simultaneously refracted and dispersed into seven colours. It then reflects internally at the back of the droplet, and finally refracts again on exiting. This spectacular dispersion creates the massive colourful rainbow arc seen across the gorgeous sky!
Q2. State Newton's Colour Disc experiment briefly.
Ans: Newton painted the seven colours of the spectrum (VIBGYOR) in equal sectors on a circular cardboard disc. When this disc was rotated at an extremely high speed, the seven distinct colours blended together and the disc magically appeared uniformly white. This proved that white light is composed of these seven colours.