Class 10 Science


Refraction of Light

The change of direction of light when light goes from one medium to another medium is known as Refraction or Refraction of Light. The ray of light changes its direction because of difference in speed in different media.

These notes are based on the chapter Light: Reflection and Refraction from class 10 science NCERT book and CBSE syllabus

  • Refraction through glass slab
  • Laws of refraction

Many interesting phenomena happen because of refraction of light. When a pencil is kept in a glass full of water, the pencil appears to be bent near the junction of water and air. It happens because of refraction of light.

If you will keep a coin in a bowl filled with water, the coin would appear shallower than its actual depth. It happens because of refraction of light.

Causes of Refraction of Light

Light travels at faster speed in rare medium and at slower speed in denser medium. The nature of media (rarity or density) is taken as relative. For example air is a rarer medium than water or glass.

When ray of light enters from a rarer medium into a denser medium, it bends towards normal at the point of incidence. On the contrary, when ray of light enters into a rarer medium from a denser medium it bends away from the normal.

Ray emerging after the denser medium goes in the same direction and parallel to the incident ray.

The angle between incident ray and normal is called Angle of Incidence and it is denoted as ∠i. The angle between refracted ray and normal is called the Angle of Refraction. Angle of refraction is denoted by ∠r.

Refraction through Glass Slab

refraction through glass slab

Fig: Refraction of Light

This figure shows a glass slab ABCD. The point O is the point of incidence. Point O’ on edge DC is the point of emergence. NN’ and MM’ are normal at surface of incidence and surface of emergence respectively. When the ray of light enters the glass slab, it bends towards the normal. Similarly, when the ray of light emerges from the glass slab, it bends away from the normal. The emergent ray is parallel to the original ray, as shown by dotted lines. This happens due to refraction of light.

Laws of Refraction

  1. The incident ray, refracted ray and normal to the interface of given two transparent media, all lie in same plane.
  2. The ratio of sine of angle of incidence and sine of angle of refraction is always constant for the light of given colour and for the pair of given media.

The Second Law of Refraction is also known as Snell’s Law of Refraction.

That is, `text(Sin i)/text(Sin r)=text(constant)`

The constant is called refractive index of the second medium in relation to the first medium.


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