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Refraction

Refraction is the bending of light when it passes from one medium to another because its speed changes.

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Student-friendly explanation

The ray bends towards the normal when it enters a denser medium and away from the normal when it enters a rarer medium. This is why a pencil in water appears bent and why lenses can focus light.

How to write this in exams

  1. 1

    Start with the exact idea

    Refraction is the bending of light when it passes from one medium to another because its speed changes.

  2. 2

    Then show how to use it

    1. Identify the two media. 2. Decide whether speed increases or decreases. 3. State whether the ray bends towards or away from the normal.

  3. 3

    Add one concrete example

    A straw partly dipped in water appears broken at the water surface.

  4. 4

    Avoid this incomplete answer

    Saying the ray bends because the object itself is bent. The bending happens only in the light ray.

Definition

Refraction is the bending of light when it passes from one medium to another because its speed changes.

Example

A straw partly dipped in water appears broken at the water surface.

Rule to remember

Refraction occurs because light changes speed on entering a different medium; bending towards the normal means the new medium is optically denser.

Memory hook

Change medium, change speed, change direction.

Examples and method

Worked example

When light goes from air to glass, it slows down and bends towards the normal. That is why the image of a coin in water seems shifted from its actual position.

Method to apply

1. Identify the two media. 2. Decide whether speed increases or decreases. 3. State whether the ray bends towards or away from the normal.

Diagram support

Draw the incident ray, normal, refracted ray, and label the angle change at the boundary between two media.

How CBSE asks it

Questions ask for the reason behind bent objects in water, the direction of bending, or the medium comparison.

Avoid common mistakes

Common confusion

Students think the light ray bends because the object itself changes shape. Actually the light changes direction at the boundary.

Common wrong answer

Saying the ray bends because the object itself is bent. The bending happens only in the light ray.

Exam tip

Always mention both speed change and boundary crossing when explaining refraction.

Quick check

Why does a pencil partly placed in water look bent to our eyes?

It looks bent because light from the submerged part refracts at the water-air boundary before reaching our eyes.

Answer writing and exam use

1-mark answer

Refraction is the bending of light when it passes from one medium to another because its speed changes.

2-mark answer

Refraction is the bending of light when it passes from one medium to another because its speed changes. Refraction occurs because light changes speed on entering a different medium; bending towards the normal means the new medium is optically denser. A straw partly dipped in water appears broken at the water surface.

3-mark answer

The ray bends towards the normal when it enters a denser medium and away from the normal when it enters a rarer medium. This is why a pencil in water appears bent and why lenses can focus light. Refraction occurs because light changes speed on entering a different medium; bending towards the normal means the new medium is optically denser. When light goes from air to glass, it slows down and bends towards the normal. That is why the image of a coin in water seems shifted from its actual position. Questions ask for the reason behind bent objects in water, the direction of bending, or the medium comparison. Saying the ray bends because the object itself is bent. The bending happens only in the light ray.
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