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Refraction

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

In one line: Change medium, change speed, change direction.

Understand it

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.

Example

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

Formula and rule

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

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1When light goes from air to glass, it slows down and bends towards the normal.
2That is why the image of a coin in water seems shifted from its actual position.

Method you can reuse

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

Sketch it in your answer

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer

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

Common wrong answer

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

Don't mix this up

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

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

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

1
Enough for 1 mark
Refraction is the bending of light when it passes from one medium to another because its speed changes.
2
Add for 2 marks
Example: A straw partly dipped in water appears broken at the water surface.
3
Add for 3 marks
The ray bends towards the normal when it enters a denser medium and away from the normal when it enters a rarer medium.

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