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# LIGHT

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by

## Prateek Srivastava

on 21 December 2013

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#### Transcript of LIGHT

The centre of curvature of a spherical mirror is the centre of the hollow sphere of glass of which the mirror is the part.
Principle Focus and Focal Length of a Concave Mirror
The principle 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.
Rules for obtaining images formed by Concave Mirror
Regular Reflection and Diffuse Reflection of Light
In regular reflection, a parallel beam of incident light is reflected as a parallel beam in one direction.
Regular reflection of light occurs from smooth surfaces like that of a plane mirror(or highly polished metal surfaces).
Formation of Image in a Pane Mirror
The image formed by a plane mirror can be seen only by looking into mirror.
An image of this type, which cannot be received on a screen, is known as a virtual image.
Reflection Of Light
The process of sending back the light rays which fall on the surface of an object, is called reflection of light.
Objects and Images
Anything which gives out light rays
(either it's own or reflected by it)
is called an object.
Reflection of Light from Curved Surfaces :
Spherical

Mirrors

A spherical mirror is that mirror whose reflecting surface is the part of a hollow sphere of glass.
The image is formed at that point where at least two reflected rays intersect.
LIGHT
REFLECTION
Laws Of Reflection Of Light
The reflection of light from a plane surface or from a spherical surface takes place according to 2 laws, which are known as laws of reflection:-
1.First Law of Reflection:-
The incident ray, the reflected ray, and the normal, all lie in the same plane.
2.Second Law of Reflection:-
The angle of reflection is always equal to the angle of incidence.
In diffuse reflection, a parallel beam of incident light is reflected in different directions.
Diffuse reflection of light occurs from rough surfaces like that of paper, cardboard, chalk, and unpolished metal objects.
Example:- Bulb, candle, pin-head, arrow,etc..
In physics, image is an optical appearance produced when light rays coming from an object are reflected from a mirror
(or refracted through a lens)
.
Real and Virtual Images
The image which
can
be obtained on a screen is called a real image.
Example:-the image formed on a cinema screen.
The image which
cannot
be obtained on a screen is called a virtual image.
Example:-the image of our face in a plane mirror.
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 right side of image
.This change of side of an 'object' and it's 'mirror image' is called
lateral inversion
.
Spherical mirrors are of 2 types
:
Concave Mirrors, and
Convex Mirrors

A concave mirror is that spherical mirror in which the reflection of light takes place at the concave surface
(or bent-in surface)
.
A convex mirror is that spherical mirror in which the reflection of light takes place at the convex surface
(or bulging-out surface)
.
Centre of Curvature, Pole and Principle Axis of a Spherical Mirror
The centre of curvature of a concave mirror is in front of it but the centre of curvature of a convex of a convex mirror is behind it.
The centre of a spherical mirror is called it's
pole
.
The straight line passing through the centre of curvature and pole of a spherical mirror is called it's
principle axis.
That portion of a mirror from which the reflection of light actually takes place is called the
aperture of the mirror
.
The focus of a concave mirror is in front of the mirror.
Principle Focus and Focal Length of a Convex Mirror
The principle 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.
The focus of a convex mirror is situated behind the mirror.
Relation between radius of Curvature and Focal Length of a Spherical Mirror
The focal length of a spherical mirror
(convex or concave mirror)
is equal to half its radius of curvature.
f=R/2
Where
f=focal length
and
Rule 1:A ray of light which is parallel to the principal axis of a concave, passes through its focus after reflection from the mirror.
Rule 2: A ray of light passing through the centre of curvature of a concave mirror is reflected back along the same path.
Rule 3: A ray of light passing through the focus of a concave mirror becomes parallel to the principal axis after reflection.
Rule 4: A ray of light which is incident at the pole of a concave mirror is reflected back making the same angle with principal axis.
Formation of different types of images by a Concave Mirror
The type of image formed by a concave mirror depends on the position of object in front of the mirror.
CASE 1:
When an object is placed between the pole
(p)
and focus
(f)
of a concave mirror, the image formed is :-
(i)behind the mirror
(ii)virtual and erect
(iii)larger than the object(or magnified).
CASE 2:
When an object is placed at the focus of concave mirror, the image formed is:-
(i)at infinity
(ii)real and inverted
(iii)highly magnified
CASE 3:
When an object is placed between the focus and centre of curvature of a concave mirror, the image formed is:-
(i)beyond the centre of curvature
(ii)real and inverted
(iii)magnified
CASE 4:
CASE 5:
CASE 6:
When an object is placed at the centre of curvature(c) of a concave mirror, the image formed is:-
(i)at the centre of curvature
(ii)real and inverted
(iii)same size as the object
When an object is placed beyond the centre of curvature(c) of a concave mirror, the image formed is:-
(i)between the focus and centre of curvature
(ii)real and inverted
(iii)smaller than the the object
When an object is at infinity from a concave mirror, the image formed is:-
(i)at he focus
(ii)real and inverted
(iii)much smaller than the the object
Sign Convention for Spherical Mirrors
The object is always placed on the left side of the mirror.
All the distances measured from the pole(p) of mirror to the right side will be considered positive.
All the distances measured from the pole(p) of mirror to the left side will be considered negative.
Some Important Conclusions
(i)The object distance is always negative.
(ii)If an object is formed behind a concave mirror, the image distance is positive
but
if the image is formed in front of the mirror, then the image distance will be negative.
(iii)Image distance for a convex mirror will be always be positive.
(iv)The focal length of a concave mirror is considered negative.
(v)The focal length of convex mirror is positive.
(vi)If image formed above the principal axis, then its height is taken positive
but
if the image is formed below principal axis, then its height is taken negative.
(vii)The height of all virtual and erect images is considered positive.
(viii)The height of all real and inverted images is considered negative.
Rules for obtaining images formed by Convex Mirror
Rule 1:A ray of light which is parallel to the principal axis of a convex mirror, appears to be coming from its focus after reflection from the mirror.
Rule 2:A ray of light going towards the centre of curvature of a convex mirror is reflected back.
Rule 3:A ray of light going towards focus of a convex mirror becomes parallel to the principal axis after reflection.
Rule 4:A ray of light which is incident at he pole of a convex mirror is reflected back making the same angle with the principle axis.
Formation of image by a Convex Mirror
Case1:
When an object is placed anywhere between pole(p) and infinity in front of a convex mirror, the image formed is:-
(i)behind the mirror between pole and focus
(ii)virtual and erect
(iii)diminished
Case2:
When an object is at infinity from a convex mirror, the image formed is:-
(i)behind the mirror at focus
(ii)virtual and erect
(iii)highly diminished
MIRROR FORMULA
A formula which gives the relationship between image distance(v), object distance(u), and focal length(f) of a spherical mirror is known as the mirror formula.
1
--------------------
Image Distance
1
--------------------
Object Distance
1
----------------
Focal Length
+
=
Or
1
u
1
v
1
f
+
=
_
_
_
Where
v=distance of image from mirror
u=distance of object from mirror
and f=focal length of mirror
Linear Magnification Produced by Mirrors
The ratio of the height of image to the height of object is known as linear magnification.
Magnification=height of image
height of object
----------------------
Or m=h

2
h
1
--
Where
m=magnification
h2=height of image
and h1=height of object
The linear magnification produced by a mirror is equal to the ratio of the image distance to the object distance, with a minus sign, i.e:-
Magnification= Image Distance
--------------------
Object Distance
_
Or m= v
---
u
_
Where
m=magnification
v=image distance
and u=object distance
So since,
m=h

2
h
1
--
m= v
---
u
_
and
Therefore,
= v
---
u
_
h

2
h
1
--
Uses of Concave mirrors
Concave mirrors are used as shaving mirrors to see a large image of the face.
Concave mirrors used by dentists to see the large images of teeth of patients.
Concave dishes are in TV dish antennas to receive TV signals from the distant communications satellites.
Uses of Convex mirrors
Convex mirrors are used as rear-view mirrors in vehicles to see the traffic at rear side.
Big convex mirrors are used as 'shop security mirrors'.
Submitted to:-
Sanjeev Agarwal Sir