Power of Lens
Power of the lens is defined as the ability of the lens to converge or diverge the ray of light passing through it. The power of the convex lens is said to be more, if the ray of the light converges more strongly towards the optical center. The power of the lens depends on the focal length of the lens. It is defined as the reciprocal of the focal length in meters. It is expressed as \[P=\frac{1}{f}\].
The SI unit of power of the lens is diopter and is denoted by D. One diopter is defined as the power of the lens having focal length of one meter. The focal length of the convex lens is positive so the power is positive and that of the concave lens is negative as its focal length is negative.
Types of Simple Lenses
Power of Combination of Lens
When number of lens are placed in contact with each other, the power of combination of the lenses is equal to the algebraic sum of the powers of the individual lenses. If \[{{P}_{1}},\,\,{{P}_{2}},\,\,{{P}_{3}},\,---\,{{P}_{n}}\] are n lenses placed in contact with each other than the combined power of the lens is given by \[P={{P}_{1}}+{{P}_{2}}+{{P}_{3}}+---+{{P}_{n}}\]
The system of combination of lens is used in designing the optical instrumentlike cameras, microscope and telescope. It increases the sharpness of the image.
The speed of light in water is found to be:
(a) \[3\times {{10}^{8}}\,m/s\]
(b) \[3.2\times {{10}^{8}}\,m/s\]
(c) \[1.2\times {{10}^{8}}\,m/s\]
(d) \[2.25\times {{10}^{8}}\,m/s\]
(e) None of these
Answer: (d)
A stick partially immersed in water appears to be bent when seen from outside because of:
(a) Reflection
(b) Refraction
(c) Dispersion
(d) Scattering
(e) None of these
Answer: (b)
On which one of the following does the refractive index of the light depend?
(a) Intensity
(b) Frequency
(c) Wavelength
(d) Density of medium
(e) None of these
Answer: (c)
Where will the image forms if the object is placed at 2F in case of convex lens?
(a) At F
(b) Between F and 2F
(c) At 2F
(d) Beyond 2F
(e) None of these
Answer (c)
Find the power of lens whose focal length is +10 cm.
(a) -10 D
(b) +10 D
(c) +1D
(d) -1 D
(e) None of these
Answer: (b) 
Lens
A transparent object having two refracting surface is called lens. It is of two types, concave lens and convex lens. Working of lens is based on the refraction of light passing through it.
Convex Lens
Convex lens is the lens which is thicker at the center and thinner at the edge. It has two refracting surfaces. It is also called the converging lens as the ray of light converges at a point after refraction. The point, at which ray of light converges after refraction, is called the focus of the lens.
Concave Lens
It is the lens which is thinner at the center and thicker at the edge. It also has two refracting surfaces. It is called diverging lens as the ray of light diverges in different direction after refraction.
The centre of the lens is called the optical center. It is normally denoted by C. The ray of light passing through optical center do not undergo any’ deviation, and goes in a straight line. Thus, the line passing through the optical center of the lens and perpendicular to both the face of the lens, is called the principal axis. The ray of light that converges at a point on the principal axis is known as the principal focus. The distance between focus and optical centre is called the focal length.
Image Formation by a Convex Lens
I. When the object is at infinity
Properties of Image formed is
Properties of Image formed is
Properties of Image formed is
Properties of Image formed is
Properties of Image formed is
Properties of Image formed is
Refraction of Light
We know that light always travels in a straight line and never changes its direction. But this fact is true as long as it travels in one medium. As soon as it changes its medium i.e. as it goes from one medium to another, it changes its direction at the border of the two medium. This change in the direction of light as it goes from one medium to another is called refraction of light.
For example, when light travels from air to glass, glass to water, water to air, etc it undergoes refraction. Let us consider a rectangular glass slab ABCD as shown in the figure given below. A ray of light PQ is incidented on the glass slab from air at the point Q. The incident light passes through the glass slab. Since the glass slab is optically denser than air, so the ray of light changes its direction as it goes from air into the glass along the line QR, inside the glass slab.
This ray of light when emerges from the glass slab along RS, again it changes its direction, because of the difference in the density of both the medium. Here PQ is incident ray, QR is refracted ray and RS is emergent ray. The incident ray and the refracted ray are not along the same direction. \[{{N}_{1}}Q{{N}_{2}}\]and\[{{N}_{3}}R{{N}_{4}}\]are the normal at the point Q and R to the surface of the glass slab. The angle, between the incident ray and the normal is called angle of incidence and the angle, between the refracted ray and the normal at that point is called angle of refraction. The angle of incidence and angle of refraction are not usually same.
The real cause for the refraction of light is the difference in the density of the two medium. Due to difference in the density of the two medium, the speed of light changes as it goes from one medium to the another, and hence refraction takes places. The medium, in which the speed of light is more of the two medium, is called the rarer medium and the medium, in which the speed of light is less is called the denser medium. When the ray of light goes from rarer medium to the denser medium it bends towards the normal; and when it goes from denser medium to the rarer medium, it bends away from the normal. The distance between the emergent ray and the original direction of the ray of the light is called the lateral displacement.
Various Effect of Refraction of Light
A Pencil Appears Bent when Partially Immersed in a Glass of Water
When a pencil is partly immersed in water and held obliquely to the surface of water, it more... 
Image Formation by a Concave Mirror
In this part we will discuss about the nature, position, and relative size of the mage formed of the various position of the objects. The image may be re a or real or virtual. We will try to find out, with the help of a mirror, the position of the objects. The nature of image formed depends on the position o the object the position carnage is obtained by intersection of at least two of the reflected rays.
When the Object is at Infinity
When the object is at infinity, in the case of a concave mirror, the image formed has the following properties:
When the Object is Infinity
When the Object is Beyond C
When the object is beyond C, the image formed has following properties:
When the Object is placed beyond C
When the Object is at C
When the object is beyond C, the image formed has following properties:
When the Objects is Placed at the Centre of Curvature
When the Object is between Centre of Curvature and Focus
When the object is between C and F, the image formed has following properties:
When the Object is Between C and F
When the Object is at Focus
When the object is at F, the image formed has following properties:
more... 
Image formation by a convex mirror
When the object is placed between P and F
When the object is placed in front of convex mirror, the image formed has following properties:
| S. NO. | Position of Object | Position of Image | Nature of Image | Size of Image |
| 1 | In front of Mirror | Behind Mirror and between P and F | Diminished | Virtual and Erect |
| 2. | At infinity | Behind the mirror and at F | Highly diminished | Virtual and Erect |
Uses of Concave Mirror
Objects and Images
In this chapter we will use the term objects and image more often. Here the objects refers to the sources of light, which falls on the mirror and reflection takes places. It may be of two types, i.e. either very small or very large. The very small objects are called the point objects. The other term is the image which is obtained when the ray of light from an objects is reflected from a reflecting surface and an object like appearance is obtained. Image is of two types: real image and virtual image.
The real image are the images which can be obtained on the screen. For example the image which we see on the screen of the cinema hall is a real image. The image which cannot be obtained on the screen are called the virtual image. For example the image obtained from the reflection in the mirror are allvirtual image. It is just an illusion of which we can simply think of and which cannot be obtained in real sense. A plane mirror always forms the virtual image.
The image in the case of plane mirror is obtained only when we look into the mirror and cannot be obtained on the screen. It we place a screen behind the mirror it will not be possible for us to obtained the image of any object on the screen. The dotted line shown behind the mirror are only imaginary ray of lights.
There is no real ray of light behind the mirror as the surface of mirror is painted with silver paints, which do not allow the ray of light to pass through it. It is only the reflected light coming from the mirror, where the image is actually formed. Hence the image formed by the plane mirror is virtual and erect and size of the image is equal to the size of the object. The distance of the image is same as that of the object in front of the mirror.
Lateral Inversion
All of us might have observed that when we stand in front of the mirror our left appears right and right appears left and vice versa. It seems that as if our images have been reversed. This reversal of image is called the lateral inversion. Hence, we can say that the image formed in the plane mirror is
In the above figure, we may observe that a tree which has been placed in front of a plane mirror he left side of the tree appears right and right side of the tree appears left. We can, therefore say that the image formed in the plane mirror is laterally inverted. The most common phenomena of lateral inversion which we observe is that the word AMBULANCE on more... 
Reflection of Light
Whenever light falls on an object some of it is reflected and others are absorbed by the surface. If the light is not reflected by the surface, i.e. all the light is absorbed by the surface, then the object will appear black, like black board or black hole, which is not visible to us. If the light falls on a polished surface, it gets reflected back in the same plane from where it is incidented. This bouncing back of light, after striking a polished surface, is called reflection of light.
Silver metal is considered to be one of the best reflector of light.
Reflection of light shows the particle nature of light. For example, when a rubber cricket ball is strike against a wall it bounces back in the opposite direction, or in other words it is reflected back by the wall. Similarly, if we try to see objects in a dark room it is not visible to us, but if we switch on the light it becomes visible to us. This is because when we switch on the light, the light falls on the object and is reflected back and absorbed by our eyes, and hence we are able to visualize the objects.
Thus on the basis of above observation the laws of reflection has been formulated as below:
There are Two Laws of Reflection. These are
The Spherical Mirror is Again of Two Types
Concave Mirror
The mirror which is curved inwards is called a concave mirror. It is also known as converging mirror. It converges the ray of light falling on its surface at a point on the surface..
Convex Mirror
The mirror which is curved outward is called a convex mirror. It is also known as diverging mirror.
The centre of curvature of a spherical mirror is the centre of the hollow spherical surface of which mirror is consider to be the part. The centre of curvature does not more... 
Introduction
Whenever we hear the word light, many questions arises in our mind. But the most common question that comes in our mind is 'what is light?'. Why we are able to see the object in presence of light and not in the dark? What makes the things visible to us? And many such questions keeps wondering in our mind.
Light is can be defined as the form of energy which produce the sensation of vision. Whenever the light falls on a object it gets reflected by the object and received by our eyes, therefore enable us to see that object. There are many wonderful phenomenon associated with the light, such as image formation, refraction of light, twinkling of star, rainbow formation just after rain, etc. In this chapter we will discuss about all these phenomena associated with the light, in detail.
Light exhibits two properties known as dual property i.e. wave nature andparticle nature. Light consists of electromagnetic waves which do not require any medium to travel. According to particle nature of light, light travels in a straight line and cannot pass through the opaque wall or any other barrier.
Other phenomena like reflection, refraction, and casting of shadow also shows the particle nature of light. Thus the quantum theory of light explains both the nature of light according to the condition. 
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