CBSE Class 10 Science Light reflection and refraction Notes

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CHAPTER - 10

LIGHT – REFLECTION AND REFRACTION

LIGHT

An object reflects light that falls on it. This reflected light when received by our eyes, enables us to see things.

Reflection of light

Reflection of light is the phenomenon of bouncing back of light in the same medium on striking the surface of any object.

There are two types of reflection:

1. Regular reflection or Specular Reflection

2. Irregular reflection or Diffuse Reflection

Regular Reflection: When the reflecting surface is smooth and well polished, the parallel rays falling on it are reflected parallel to one another, the reflected light goes in one particular direction. This is Regular reflection or Specular reflection see below figure.

Irregular reflection: When the reflecting surface is rough, the parallel rays falling on it reflected in different direction, as shown in below fig. Such a reflection is known as diffuse reflection or irregular reflection.

LAWS OF REFLECTION OF LIGHT

According to the laws of Reflection of light,

(i) The angle of incidence is equal to the angle of reflection, and

(ii) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane. 

These laws of reflection are applicable to all types of reflecting surfaces including spherical surfaces.

OBJECTS

Anything which gives out light rays either its own or reflected by it is called an object.

LUMINOUS OBJECTS: The objects like the sun, other stars, electric bulb, tubelight etc. which emit their own light are called luminous objects.

NON – LUMINOUS OBJECTS: The objects which do not emit light themselves but only reflect or scatter the light which falls on them, are called non-luminous objects.
A flower, chair table, book, trees, etc are all non-luminous objects.

IMAGES

Image is an optical appearance produced when light rays coming from an object are reflected from a mirror (or refracted through lens).

REAL IMAGE

The image which can be obtained on a screen is called a real image. In a cinema hall, we see the images of actors and actress on the screen. So, the images formed on a cinema screen is an example of real images.

VIRTUAL IMAGE

The image which cannot be obtained on a screen is called a virtual image. A virtual image can be seen only by looking into a mirror. The image of our face in a plane mirror is an example of virtual image.

LATERAL INVERSION

When an object is placed in front of a plane mirror, then the right side of object appears to become the left side of image; and the left side of object appears to become the right side of image. This change of sides of an object and its mirror image is called lateral inversion.
The phenomenon of lateral inversion is due to the reflection of light.

CHARACTERISTICS OF IMAGES FORMED BY PLANE MIRRORS

The characteristics of images formed by plane mirrors are:

1. The image of real object is always virtual. Such image cannot be taken on a screen.
2. The image formed in a plane mirror is always erect.
3. The size of the image in a plane mirror is always the same as the size of the object.
4. The image formed in a plane mirror is as far behind the mirror, as the object is in front of the mirror.
5. The image formed in a plane mirror is laterally inverted i.e. the left side of the objects becomes the right side of the image and vice-versa.

SPHERICAL MIRROR

A spherical mirror is that mirror whose reflecting surface is the part of a hollow sphere of glass. The spherical mirrors are of two types: Concave mirror and Convex mirror.

CONCAVE MIRROR: A concave mirror is that spherical mirror in which the reflection of light takes place at the concave surface (or bent-in surface).

CONVEX MIRROR: A convex mirror is that spherical mirror in which the reflection of light takes place at the convex surface (or bulging –out surface).

TERMS RELATED TO SPHERICAL MIRRORS

Centre of Curvature(C): The centre of curvature of a spherical mirror is the centre of the hollow sphere of glass of which the spherical mirror is a part. It is represented by letter ‘C’.

Pole(P): The pole of a spherical mirror is the centre of the mirror. It is represented by letter ‘P’.

Radius of Curvature(R): The radius of curvature of a spherical mirror is the radius of the hollow sphere of glass of which the spherical is a part. It is represented by the letter ‘R’.

Principal axis: The principal axis of a spherical mirror is the straight line passing through the centre of curvature C and pole P of the spherical mirror, produced on both sides.
Aperture: The aperture of a spherical mirror is the diameter of the reflecting surface of the mirror.

PRINCIPAL FOCUS OF A SPHERICAL MIRROR

The principal focus of a concave mirror is a point on its principal axis to which all the light rays which are parallel and close to the axis, converge after reflection from the concave mirror. A concave mirror has a real focus. The focus of a concave mirror is in front of the mirror. Since a concave mirror converges a parallel beams of light rays, it is also called converging mirror.

The principal focus of a convex mirror is a point on its principal axis from which a beam of light rays, initially parallel to the axis, appears to diverge after being reflected from the convex mirror. A convex mirror has a virtual focus. The focus of a convex mirror is situated behind the mirror. Since a convex mirror diverges a parallel beams of light rays, it is also called diverging mirror.

Focal Length: The focal length of a spherical mirror is the distance between its pole and principal focus. It is denoted by the letter ‘f’.

Relation between Radius of curvature and focal length of a spherical mirror The focal length of a spherical mirror is equal to half of its radius of curvature.
f = R/2
In other words, for spherical mirrors of small apertures, the radius of curvature is found to be equal to twice the focal length.
R = 2f

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