# CBSE Class 10th Science

## Ch- 10 Light Reflection and Refraction

REFLECTION

Reflection of Light: The phenomenon of bouncing back of light into the same medium by the smooth surface is called reflection.

Incident light: Light which falls on the surface is called incident light.

Reflected light: Light which goes back after reflection is called reflected light.

The angle of incidence: The angle between the incident ray and the normal.

An angle of reflection: The angle between the reflected ray and the normal.

Mirror: The surface which can reflect the light is a mirror.

Plane Mirror: If the reflecting surface is a plane then the mirror is plane.

Spherical Mirror: If the reflecting surface is part of the hollow sphere then the mirror is a spherical mirror.
The spherical mirror is of two types:

• Convex mirror: In this mirror reflecting surface is convex. It diverges the light so it is also called a diverging mirror.
• Concave mirror: In this mirror reflecting surface is concave. It converges the light so it is also called converging mirror.

### Plane mirror

Any flat and polished surface that has almost no irregularities on its surface that reflect light is called as a plane mirror.

### Characteristics of images

• Images can be real or virtual, erect or inverted, magnified or diminished. A real image is formed by the actual convergence of light rays. A virtual image is the apparent convergence of diverging light rays.
• If an image formed is upside down then it is called inverted or else it is an erect image. If the image formed is bigger than the object, then it is called magnified. If the image formed is smaller than the object, then it is diminished.

### Image formation by a plane mirror

• The image formed by a plane mirror is always virtual and erect.
• Object and image are equidistant from the mirror.

### Spherical mirror

Consider a hollow sphere with a very smooth and polished inside surface and an outer surface with a coating of mercury so that no light can come out. Then if we cut a thin slice out of the shell, we get a curved mirror, which is called a spherical mirror.

### Relationship between focus and radius of curvature

Focal length is half the distance between pole and radius of curvature.

F = R/2

### Curved Mirror

A mirror (or any polished, reflective surface) with a curvature is known as a curved mirror.

### Important terms related to spherical mirror

• Pole (P): The midpoint of a spherical mirror.
• Centre of curvature (C): The centre of the sphere that the spherical mirror was a part of.
• The radius of curvature (r): The distance between the centre of curvature and the spherical mirror. This radius will intersect the mirror at the pole (P).
• Principal Axis: The line passing through the pole and the centre of curvature is the main or principal axis.
• Concave Mirror: A spherical mirror with the reflecting surface that bulges inwards.
• Convex Mirror: A spherical mirror with the reflecting surface that bulges outwards.
• Focus (F): Take a concave mirror. All rays parallel to the principal axis converge at a point between the pole and the centre of curvature. This point is called as the focal point or focus.
• Focal length: Distance between pole and focus.

Sign Conventions of Spherical Mirror

• All the distances are measured from the pole of the mirror as the origin.
• Distances measured in the direction of incident rays are taken as positive.
• Distances measured opposite to the direction of incident rays are taken as negative.
• Distances measured upward and perpendicular to the principal axis are taken as positive.
• Distances measured downward and perpendicular to the principal axis are taken as negative.
…where f, v and u are focal length, image distance, object distance

### Image formation by spherical mirrors

For objects at various positions, the image formed can be found using the ray diagrams for the special two rays. The following table is for a concave mirror.

Linear Magnification: This is the ratio of the height of the image to the height of the object.
$m=frac { { h }^{ ' } }{ h }$ …where m = magnification, h = height of image, h’ = height of object

Use of Convex Mirror: Convex mirror used as rear view mirror in vehicles, as shop security mirrors, etc.

REFRACTION

Refraction of Light: The bending of light at the interface of two different mediums is called Refraction of light.

• If the velocity of light in medium is more, then medium is called optical rarer.
Example, air or vacuum is more optical rarer.
• If the velocity of light in medium is less, then medium is called optical denser.
Example, glass is more denser than air.

Refractive Index: It represents the amount or extent of bending of light when it passes from one medium to another.
There are two types of refractive index

• Relative refractive index and
• Absolute refractive index.

Incident ray: It is incoming ray on the refracting surface.

Refracted ray: It is an outgoing ray from the refracting surface.

An angle of incidence (i): It is the angle between incident rays and perpendicular line (normal) at the point of incidence.

An angle of refraction (r): It is the angle between refracted rays and perpendicular line (normal) at the point of incidence.

Law of Refraction: According to this law

• “The incident ray, refracted ray and normal at the point of incidence all lie in the same plane.”
• “The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant.”
$frac { sinquad i }{ sinquad r }$ = constant (µ)

## Lens

The transparent refracting medium bounded by two surfaces in which at least one surface is curved is called lens.
Lenses are mainly two type

• Convex lens and
• Concave lens.

### Important terms related to spherical lenses

• Pole (P): The midpoint or the symmetric centre of a spherical lens is known as its Optical Centre. It is also called as the pole.
• Principal Axis: The line passing through the optical centre and the centre of curvature.
• Paraxial Ray: A ray close to principal axis and also parallel to it.
• Centre of curvature (C): The centres of the spheres that the spherical lens was a part of. A spherical lens has two centres of curvatures.
• Focus (F): It is the point on the axis of a lens to which parallel rays of light converge or from which they appear to diverge after refraction.
• Focal length: Distance between optical centre and focus.
• Concave lens: Diverging lens
• Convex lens: Converging lens

### Image formation by spherical lenses

The following table shows image formation by a convex lens.

The following table shows Image formation by a concave lens.

### Lens formula and magnification

Lens formula: 1/v = 1/u = 1/f, gives the relationship between the object-distance (u), image-distance (v), and the focal length (f) of a spherical lens.

### Uses of spherical lens

Applications such as visual aids: spectacles, binoculars, magnifying lens, telescopes.

### Power of a Lens

Power of a lens is the reciprocal of its focal length i.e 1/f (in metre). The SI unit of power of a lens is dioptre (D).