Refraction, Lenses and Dispersion

Refraction gives 1 to 2 questions and is the topic where students form the most misconceptions, so a teaching paper examines it carefully. Lens types with their uses, and the cause of dispersion, carry most of the marks.

Why Light Bends

Refraction is the bending of light as it passes from one transparent medium into another. It happens because the speed of light changes between media.

• Going from a rarer to a denser medium, such as air to glass, light bends towards the normal.
• Going from denser to rarer, such as glass to air, it bends away from the normal.
• A ray striking the surface along the normal does not bend at all.
• The speed of light is greatest in vacuum and slows in any material medium.

Everyday consequences are asked as reasoning questions: a pencil looks bent in water, a pool looks shallower than it is, a coin in a bowl becomes visible as water is poured, and stars twinkle because air layers of different densities refract their light continuously.

✗ A pencil appears bent in water because water is heavier than air  |  ✓ It appears bent because light changes speed and refracts at the surface

Lenses

A convex lens converges parallel rays to a focus; a concave lens diverges them. The simplest memory hook is that a convex lens is fat in the middle and gathers light, while a concave lens is thin in the middle and spreads it.

A convex lens used as a magnifying glass gives a virtual, erect and enlarged image when the object is inside the focal length. Held further away it gives a real inverted image, which is how a camera and the human eye both work.

Dispersion and the Spectrum

White light is a mixture of seven colours. When it passes through a prism it separates into a band called the spectrum, in the order remembered as VIBGYOR: violet, indigo, blue, green, yellow, orange, red.

Dispersion happens because different colours refract by different amounts. Violet bends the most and red the least, which is what spreads them apart.

• A rainbow forms when sunlight is refracted, reflected inside and refracted again by water droplets.
• The rainbow always appears on the side of the sky opposite the Sun.
• Passing the spectrum through a second inverted prism recombines it into white light, which proves that white light is a mixture.

The order VIBGYOR runs from most bent to least bent, so violet is at one end for the same reason it appears first in the mnemonic. Knowing why the order is what it is turns three possible questions into one fact.

Frequently Asked Questions

Why does a pencil look bent when placed in water?

Because light from the submerged part changes speed as it leaves the water and enters the air, so it refracts away from the normal. The eye traces the bent rays back in a straight line and sees the lower part displaced, making the pencil appear broken at the surface.

Which colour bends the most in a prism?

Violet bends the most and red the least. That difference in the amount of refraction is precisely what separates white light into its seven colours. It also explains the order of colours in a rainbow, so one fact covers both dispersion and rainbow questions.

What is the difference between a convex and a concave lens?

A convex lens is thicker in the middle and converges parallel rays to a focus, so it is used in magnifying glasses, cameras and for correcting long sight. A concave lens is thinner in the middle, diverges rays, and is used to correct short sight.

Why do stars twinkle but planets do not?

Because starlight passes through layers of air of varying density, and continuous refraction makes the apparent position and brightness fluctuate. Planets are much closer and appear as small discs rather than points, so the fluctuations from different parts average out and the light looks steady.