Light

Efforts by :- Aagman Saini

Light is a form of energy which enables us to see everything around us. It does not require a medium and can travel even in vacuum.

Introduction

The property of light travelling in straight lines in a medium is known as rectilinear propagation of light.

Rectilinear propagation of light

The bouncing of light from a surface is called reflection.

Reflection of light is of two kinds :-

Regular reflection

Irregular reflection

Reflection of light

When the reflecting surface is smooth & well polished, the light that falls on it is reflected regularly. This phenomenon is known as regular reflection or specular reflection.

Regular reflection

When the reflecting surface is irregular, the rays of light that fall on it are scattered in all directions, it is known as irregular reflection or diffused reflection.

Irregular reflection

There are two types of images:-

Real images

Virtual images

Types of images

A real image is formed by the actual intersection of light rays & can be formed on the screen.

Real images

Real image

A virtual image is formed by the intersection of the light rays when produced behind the mirror & not by their actual intersection.

Virtual images

Virtual image

Spherical mirrors :- concave & convex

Plane mirror

Types of mirror

The pole of a spherical is the centre (p) of the spherical mirror.

The centre of curvature of a spherical mirror is the centre (c) of the sphere of which the mirror forms a part.

The radius of curvature of a spherical mirror is the radius (r) of the sphere of which the mirror forms a part.

The principal axis of a spherical mirror is the line joining the pole & the centre of curvature of the mirror.

Spherical mirrors

In a concave mirror, the image is erect & enlarged if the object is very close to the mirror. Concave mirrors are used as shaving mirrors & compact mirrors erect, enlarged images of the face are formed.

concave mirror

In convex mirrors, the image is always erect & diminished, at whatever distance the object may be in front of the mirror. The convex mirror has a wide field of view & hence is used as a rear view or driving mirror in vehicles, to view the traffic behind.

Convex mirror

Concave mirror are used as reflector in torchlight , shaving mirrors and compact mirrors.

Uses of concave mirror

Convex mirrors are used in :-

Rear view mirrors

Vigilance mirrors in stores

Uses of convex mirror

Images formed by spherical mirrors & plane mirror

Nature of image Plane mirror Concave mirror Convex mirror

Size Same as object 1.Enlarged, when the object is closer to mirror than C2.Same size as the object, when the objet is at c3.Diminished when the object is beyond c

Diminished for all positions of the object.

Real or Virtual Virtual 1.Virtual when the object is within F2.Real,for all other positions of the object

Virtual for all positions of the object

Distance from the mirror

Same as the object

As the object comes closer to the mirror

Within F for all positions of the object

inversion lateral 1.Erect when object is within F2.Inverted for all the other positions of the object

Erect

A lens is a piece of transparent material bound by curved surfaces.

Lenses are of two types :-

Lenses which are thicker in the middle than at the edges are called convex lens(converging lens).

Lenses which are thinner in the middle than the edges are called concave lenses(diverging lenses).

Types of lenses

The principal axis is the line joining the centres of the two spheres of which the lens forms a part.

The optical centre is the centre of the lens(o). The principal focus (F) of a concave lens is a pointy of the

principal axis to which rays parallel axis converge after refraction .

The principal focus(F) of a concave lens is a point on the principal axis from which rays parallel to the principal axis appear to diverge after refraction.

The focal length (f)of a lens is the distance between the optical centre and principal focus.

lenses

Lens Object Location of the image

Nature of the image

Convex lens Beyond 2FAt 2FBetween F & 2fBetween F & O

Between F & 2FAt 2FBeyond 2FOn the same side

Real, Diminished, Inverted

Real, small, inverted Real, magnified, invertedVirtual, magnified, erect

Concave lens All positions Within F on the same side

Virtual,diminished, erect

Images formed by lens

The famous scientist Isaac Newton showed that light is made of seven colours.

The splitting of white light is known as dispersion.

The band of colours obtained because of dispersion of light is called spectrum

The seven colours in the spectrum of white light are violet, indigo, blue, green, yellow, orange & red (VIBGYOR).

Spectrum