Unit 14: Light — Short Questions

When light passes from air (rarer medium) into water (denser medium), it slows down and bends towards the normal. This bending of light is called refraction.

The refractive index of a medium is the ratio of the speed of light in air (or vacuum) to the speed of light in that medium.

Formula n = c/v

The refractive index of air is approximately 1.00. The refractive index of water is 1.33.

Optical fibres work on the principle of total internal reflection. When light travels from the denser core to the rarer cladding at an angle greater than the critical angle, it is totally reflected back into the core. This repeated reflection allows light to travel long distances with very little loss of energy.

A convex lens forms a real image when the object is placed beyond its principal focus. The image formed is inverted and can be obtained on a screen. When the object is placed between the lens and its focal point, the lens forms a virtual image that is upright, magnified, and cannot be projected on a screen.

The power of a lens is its ability to bend light and is the reciprocal of its focal length (in metres).

Formula P = 1/f

Unit Dioptre (D). A shorter focal length means greater power (stronger bending). A convex lens has positive power, and a concave lens has negative power.

Snell's

The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a specific pair of media and equals the refractive index of the second medium relative to the first.

Formula n = sin i / sin r

Or n₁sin i = n₂sin r

The reflection of light follows two basic laws:

(i) First Law: The incident ray, the reflected ray, and the normal at the point of incidence, all lie in the same plane.

(ii) Second Law: The angle of incidence (i) is always equal to the angle of reflection (r). This is written as ∠i = ∠r.

Regular Reflection

Reflection of light from smooth and polished surfaces in which parallel incident rays remain parallel after reflection is called regular reflection. It forms a clear image e.g. the reflection from a mirror.

Diffused Reflection

Reflection of light from rough surfaces in which reflected rays scatter in different directions is called diffused reflection. It does not form a clear image e.g. reflection from a wall.

Characteristics

(i) The image is erect, meaning it appears upright like the object.

(ii) The image is of the same size as the object.

(iii) The image is laterally inverted, so left and right sides are reversed.

(iv) The image is virtual and cannot be obtained on a screen.

Lateral Inversion

The phenomenon in which the left side of an object appears as the right side in its image, and vice versa, when seen in a plane mirror is called lateral inversion.

Explanation

This occurs because the image is formed behind the mirror, causing a left-right reversal while the image remains upright.

Refraction of Light

The bending of light when it passes from one medium to another due to a change in its speed is called refraction of light.

Condition

Refraction occurs when light travels between media of different optical densities such as air and water or air and glass.

Snell's Law

States that for a given pair of media, the ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is constant. This constant is the refractive index (n) of the second medium with respect to the first.

Formula n = sin i / sin r

Refractive Index

The ratio of the speed of light in vacuum to its speed in a given medium is called the refractive index of that medium.

Formulas

(i) n = C / v

(ii) n = sin i / sin r

Critical Angle

The angle of incidence in the denser medium for which the angle of refraction in the rarer medium becomes 90° is called the critical angle.

Condition

It occurs only when light travels from a denser to a rarer medium.

Total Internal Reflection

The complete reflection of light back into the same medium when it travels from a denser medium to a rarer medium at an angle greater than the critical angle is called total internal reflection.

Conditions

(i) Light must travel from a denser to a rarer medium.

(ii) The angle of incidence must be greater than the critical angle.

Principle

Optical fibres work on the principle of total internal reflection.

Explanation

Light entering the fibre strikes the core-cladding boundary at an angle greater than the critical angle and undergoes repeated total internal reflection, allowing light to travel long distances.

Focal Length

The distance between the optical centre of a lens and its principal focus is called focal length.

Principal Focus

The point on the principal axis where parallel rays of light either converge or appear to diverge after refraction through the lens is called the principal focus.

Real and Virtual Images

Real Image Formed by actual meeting of light rays, Can be obtained on a screen, Usually inverted

Virtual Images Formed by apparent meeting of rays, Cannot be obtained on a screen, Always erect

Linear Magnification

The ratio of the height of the image to the height of the object is called linear magnification.

Formula

M = Image height / Object height

M = h₁ / h₀

Unit

It has no unit.

Power of a lens

The ability of a lens to bend light is called the power of a lens and is defined as reciprocal of focal length expressed in meters.

Formula

P = 1 / f

Unit

The SI unit of power of a lens is dioptre (D).

This phenomenon is explained by refraction of light.

Reason

When light travels from water to air, its speed changes and it bends away from the normal. This bending of light at the water-air boundary causes the submerged part of the pencil to appear bent.

Due to dispersion.

Reason

A prism has non-parallel surfaces. Different colours (wavelengths) of light refract by different amounts when entering and exiting, causing them to spread out. A plane glass slab has parallel surfaces, so colours refract upon entry but recombine upon exit, resulting in no net separation.

Total internal reflection occurs only when light travels from a denser to a rarer medium.

Reason

For total internal reflection, two conditions must be met: (1) Light must go from denser (higher n) to rarer (lower n) medium, and (2) The angle of incidence must be greater than the critical angle. When going from air to water, light bends towards the normal, so the angle of incidence can never exceed the critical angle for total internal reflection.

Total internal reflection would not occur, and light would escape from the core.

Reason

For total internal reflection, the core must have a higher refractive index than the cladding (n_core > n_cladding). If the cladding had a higher index, light would refract into the cladding instead of reflecting back into the core, causing signal loss.

The object must be placed between the lens and its principal focus (object distance < focal length).

Reason

For a convex lens (magnifying glass), a virtual, upright, and magnified image is formed only when the object is within one focal length of the lens. This causes the refracted rays to diverge, appearing to come from a larger, upright image on the same side as the object. This is the working principle of magnifying glass.