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What is refraction of light, and why does it happen?

Class 10 ScienceNCERT chapter: Light – Reflection and Refractionइसे हिंदी में पढ़ें

Exam-ready answer

Refraction of light is the change in direction of light when it travels from one transparent medium to another. It occurs because the speed of light changes in different media. When light enters a medium where its speed decreases, it bends towards the normal; if speed increases, it bends away from the normal.

Explanation

Refraction is the bending of light as it passes obliquely from one transparent medium to another. This happens because light travels at different speeds in different media. The change in speed causes the light ray to change direction at the boundary.

The speed of light is maximum in vacuum (or air) and less in denser media like water or glass. When light goes from air to glass, it slows down and bends towards the normal. When it goes from glass to air, it speeds up and bends away from the normal.

A common example is a pencil appearing bent when partly immersed in water. Another is the apparent flattening of a pool's bottom. The refractive index of a medium gives the ratio of the speed of light in vacuum to that in the medium: n = c/v.

Points to remember

  1. Refraction: Bending of light when it passes from one medium to another.
  2. Cause: Change in speed of light in different media.
  3. Laws of refraction: Incident ray, refracted ray, and normal lie in the same plane; Snell's law: sin i / sin r = constant (n).
  4. Refractive index (n) = speed of light in vacuum / speed of light in medium = c/v. No unit.
  5. Bending towards normal when entering denser medium; away when entering rarer medium.

Exam tip

Always draw a labelled ray diagram showing the incident ray, refracted ray, normal, and angles. Write the formula for refractive index with proper symbols and mention that it has no unit.

Questions students also ask

What is the difference between reflection and refraction?
Reflection is the bouncing back of light from a surface, while refraction is the bending of light when it passes from one medium to another. In reflection, light stays in the same medium; in refraction, it changes medium.
Why does a pencil look bent in water?
The light coming from the part of the pencil inside water refracts at the water-air boundary. It bends away from the normal, so the pencil appears bent at the surface.
What is Snell's law?
Snell's law states that for a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. This constant is the refractive index of the second medium with respect to the first.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.