3. Refraction of Light

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Refraction through a rectangular Glass slab

• The ray of light enters from rarer to denser medium at point O that is from air to glass and bends towards the normal.
• At ‘O’, the light ray enters from glass to air, that is, from a denser medium to a rarer medium. The light here bends away from normal.
• The emergent ray is parallel to the incident ray. However the light ray shifts slightly sideward.
• Refraction is due to change in speed of light when it enters from one medium to another.

 

Refraction of light: Bending of the light rays as it passes from one medium to another medium is known as refraction of light.
Laws of refraction

Two laws of reflection are:
(i) The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
(ii) The ratio of sine of angle of incidence to the sine of angle of refraction is constant for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction. 

Refractive index: The extent of the change in direction of light that takes place in a given pair of media is expressed in terms of the refractive index. This can be expressed in an equation form as

Absolute refractive index: Refractive index of medium 2 with respect to vacuum is called the absolute refractive index of the medium. It is represented as n₂.
Spherical lens: A transparent medium bound by two surfaces, of which one or both surfaces are curved is called a spherical lens.
Concave lens: It is a spherical lens in which two spherical surfaces bulge inwards. It is also called diverging lens.
Convex lens: It is a spherical lens in which two spherical surfaces bulge outwards. It is also called converging lens.

Refraction of light by spherical lens:

Image formation by Convex Lens
(i) When the object is at infinity

→ Image is formed at focus F₂
→ Image is highly diminished or point-sized
→ Image is real and inverted

(ii) When the object is beyond 2F₁

→ Image is formed between F₂ and 2F₂
→ Image is diminished
→ Image is real and inverted

(iii) When the object is at 2F₁

→ Image is formed at 2F₂
→ Image is same size as that of object
→ Image is real and inverted

(iv) When the object is between F₁ and 2F₁

→ Image is formed beyond 2F₂
→ Image is enlarged
→ Image is real and inverted

(v) When the object is at focus F₁

→ Image is formed at infinity
→ Image is infinitely large or highly enlarged
→ Image is real and inverted

(vi) When the object is between focus F₁ and optical centre O

→ Image is formed on the same side of the lens as the object
→ Image is enlarged
→ Image is virtual and erect