Light

6.1 REFRACTION OF LIGHT

The change in direction of light when it passes from one medium to another obliquely is called refraction of light. In other words, the bending of light when it travelsfrom one medium to another obliquely is called refraction of light.

For example, printed matter placed below a thick glass slab appears raised. A pen dipped partially in water also appears to be displaced. This is due to difference in the speed of light in the two media in which the pen is dipped simultaneously.
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6.2 CAUSE OF REFRACTION

The refraction of light is due to the change in the speed of light on going from one medium to another.

6.3 OPTICALLY RARER MEDIUM AND OPTICALLY DENSER MEDIUM

Optically Rarer Medium: A medium in which the speed of light is more is known as optically rarer medium. Air is an optically rarer medium as compared to glass and water.

Optically Denser Medium: A medium in which the speed of light is less, is known as optically denser medium.

There are two points that is to be kept in mind:

1. When a ray of light goes from a rarer medium to a denser medium, it bends towards the normal.
2. When a ray of light goes from a denser medium to a rarer medium, it bends away from the normal.

6.4 EFFECTS OF REFRACTION OF LIGHT

(i) On partly immersing and obliquely holdinga stick (or pencil) in water, a bent at the water surface is observed.
(ii) On placing an object under water, the object appears to be raised.
(iii) a pool of water appears to be less deep than it actually is.

6.5 LAWS OF REFRACTION OF LIGHT

1. First Law of Refraction:

According to the first law of refraction of light, the incident ray, the refracted ray and the normal at the point of incidence, all lie in the same plane.

2. Second Law of Refraction:

The ratio of sine(sin i) of angle of incidence to the sine of angle of refraction(sin r) is constant for a given pair of media.The second law is called Snell’s law of refraction.

Refractive index of medium: The value of the constant 

for a ray of light passing from air into a particular medium.So, refractive index is represented as:

Refractive index,

6.6 REFRACTIVE INDEX AND SPEED OF LIGHT

• The refractive index depends on the nature of the material of the medium and on the wavelength (or colour) of the light used.
• The refractive index (n) can also be written as a ratio of speeds of light in the two media.

Where

= Refractive index of medium 2 with respect to medium1

= Speed of light in medium 1

= Speed of light in medium 2

6.7 REFREACTION BY SPHERICAL LENSES

Lenses that are formed by bounding two spherical transparent surfaces are called spherical lenses. If the lenses are formed by binding two spherical surfaces bulging outwards then, it is known as convex lens and if the lenses are formed by binding two spherical surfaces such that they are curved inward then it is known as concave lens.

Convex lenses are known as converging lenses since light rays falling on these lenses converge or meet at a point. Similarly, concave lenses are known as diverging lenses since light rays falling on these lenses diverge or go away from a point.

6.8 PRINCIPAL FOCUS AND FOCAL LENGTH OF A CONVEX LENS

• Principal Focus of convex lens: Itis a point on convex lens principal axis to which light rays parallel to the principal axis converge after passing through the lens.
• A lens has two foci. The two foci of a lens are at equal distances from the optical centre, one on either side of the lens.
• The focal length of a lens is the distance between optical centre and principal focus of the lens.
• A convex lens is also known as a converging lens because it converges (brings to point), a parallel beam of light rays passing through it.

6.9 PRINCIPAL FOCUS AND FOCAL LENGTH OF A CONCAVE LENS

The principal focus which of a concave lens is a point on its principal axis from which light rays, originally parallel to the axis, appear to diverge after passing through the concave lens.