Note on Lens

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Light

Light is a form of energy which gives sensation of vision. It gives the ability to see the object. It shows unique properties.Being an electromagnetic wave, it undergoes reflection,refraction,absorption,transmission,etc.Here we discuss about refraction of light.

Refraction of light:

The phenomenon of bending of light as it passes from one medium to another medium is called refraction of light. The refraction is caused due to change in velocity of light as it passes from one medium to another medium.

Lens:

a curved piece of glass,or other transparent material, used in cameras, glasses, and scientific equipment, which makes objects seem closer, larger, smaller, etc is called the lens.

Convex lens

A lens, which is thicker at center and thinner at edges, is called convex lens.

When theparallel beam of light incident on convex lens, after refraction, it converges at a single point. So it is called converging lens.

Concave lens:

The lens that is thicker at the edges and thinner at centre is known as concave lens.

When a parallel beam of light incident on concave lens, refracts and appears to diverse from a point. So, the concave lens is called diverging lens.

Some related terms

Center of curvature

Centers of curvature of a lens are the centers of the spheres, from which the lens is formed. In figure c1 and c2 are the center of curvature.

The radius of thesphere whose parts form a lens is known as radius of curvature.

Optical center

The geometrical center of the lens is known as theoptical center. In the following figure, 'o' is the optical center

Principal focus or focal point

When a parallel beam of light incident on a lens after refraction, it appears to meet from a point or it meet at a point. That point is called principal focus or focal length.

Principal focus is real for theconvex lens and virtual for theconcave lens.

Principal axis:

The line passing through the centre of curvature of both spheres is called principal axis.

Focal length:

The distance between theoptical center and principal focus is called focal length.

Rules of refraction in convex and concave lens:

1. An incident ray parallel to the principal axis of a convex lens passes through the principal focus of the lens after refraction. Incaseofconcave lens, the ray appears to be diverging from the focus.

2. 2. An incident ray, passing through the principal focus of a convex lens, becomes parallel to the principal axis after refraction.

3. An incident ray passing through the optical center of a convex lens and concave lens goes straight without deviation after refraction.

Power of a lens:

The ability of a lens to converge or to diverge a beam of light is called power of a lens. The unit of power of a lens is Dioptre (D)

Mathematically,
Power of a lens: $$\frac{1}{focal\;length}$$
Note: focal length should be in "meter".

Characteristics of image when placed at different places:

1. Convex lens

2. Concave lens

When object is placed anywhere between optical center and infinity, the image is formed between optical center and principal focus, which has following characteristics:

• The image is virtual, erect and diminished.

Some important definition:

1. One dioptre of a lens: One diopter power is defined as the power of lens whose focal length is 1 m.
2. Lens formula: A formula which gives the relationship between the object distance (u), image distance (v) and the focal length(f) is called the lens formula.
3. Magnification: The magnification (m) produced by a lens is defined as the ratio of theheight of theimage to the height of theobject.It is also equal to the ratio of image distance to theobject distance.

• Light is a form of energy which gives the sensation of vision. It gives the ability to see the object.
• Lens is an optical device, which is made up of from transparent and spherical medium. There are two types of lens. They are convex lens and concave lens.
• Convex lenses are thicker at center and thinner at edges.
• Concave lenses are thicker at the edges and thinner at center.
• The instruments which are used to produce the image of an object by using a lens or a combination of lenses are called optical instrument. Examples are Photographic camera, microscope, telescope, Human Eye etc.
.

Very Short Questions

Lens is a portion of refracting medium bounded by one or two spherical surfaces, that is, two surfaces may be curved or one curved surface and another plane surface. Lenses are divided into two types. They are:-

• Convex lens and
• Concave lens

Convex lens is called a converging lens because after passing through this lens, the emergent rays come closer to one another. A parallel incident rays meets or converges at its focus.

Concave lens is called a diverging lens because after refraction through this lens, the emergent rays go further apart from one another. A parallel incident rays appeared to diverge from its focus.

The differences between convex and concave lens are:

 Convex lens Concave lens a) It is thick at the middle and thin at the edges. a) It is thick at the edges and thin at the middle. b) It converges all parallel rays of light falling on its surface at a point. b) It diverges all parallel rays of light falling on its surface away from a point. c) Depending upon the object distance, the image may be real or virtual, magnified or diminished and erect or inverted. c) It always forms the image which is virtual, erect and diminished.

The ability of a lens to converge or to diverge rays of light is called the power of lens. In other words, the reciprocal of the focal length of a lens is called power of lens. It is denoted by the letter "p". The unit of power of lens is diopter (D).

The distance between the optical center and the principal focus of a lens is called focal length. It is denoted by f.

The three uses of the lens are as follows:

1. They are used as spectacles.
2. They are used in telescope, camera, slide projector, etc.
3. Convex lens having short focal length is used as magnifying glass.

A lens is a denser medium than air. Hence, the light rays refract from lens when the light rays passes from lens. The refraction occurs due to change in speed of light in medium.
In other word, when light travels from air to lens medium, due to changes in speed, the light rays are refracted.

1. Center of curvature: it is the center of the sphere of which the lens forms a part. A lens has two centers of curvatures, which are denoted by C1 and C2.
2. Principal axis: The line passing through the two centers of curvature of is known as principal axis.
3. Optical center: The geometrical center of a lens is called its optical center. A ray of light passing through it does not bend even after refraction. It is denoted by "O".
4. Principal focus: When narrow parallel rays of light fall on the surface of lens parallel and close to the principal axis, the rays of light converge to a point or appear to come from a point after refraction. The point is known as principal focus. It is denoted by "F".
5. Focal length: The distance between principal focus (F) and optical center (O) is known as focal length. It is denoted by "f". It is taken as positive (+ve) for convex lens and negative (-ve) for concave lens.

The image formed is virtual and magnified when the object is placed within its focal length, so convex lens can be used as magnifying lens.

The main uses of convex lens are as follows:

1. It is used to correct the defects of vision of eyes called long-sightedness.
2. It is used to make simple microscope, compound microscope, camera and telescope, etc.
3. It is used to make the paper burn from sunlight.
The main uses of concave lens are as follows:
1. It is used to correct the defects of vision of eyes called short-sightedness.
2. It is used to combine with convex lens to get clear image on camera, telescope, microscope, etc.

The power of lens is called one dioptre, if the focal length of a lens is one meter.

The ration between size of the image formed to the size of the object is called magnification. The magnification is generally represented by letter "M".

In other words, the magnification can be defined as the ratio between the image distance to object distance.

1. Real, inverted and diminished - Convex lens between 2F and infinity)
2. Virtual, upright and diminished - Concave lens (between optical center and infinity)
3. Virtual, upright and enlarged - Convex lens (between F and optical center)

Here,
We have,
or, or,= 1m
f = 1m
This type of lens must be convex in nature.

Solution,
Here,
Power of lens (P) = +2D
Focal length (f) = ?
Now using formula,

or,
or, = 0.5m
f = 0.5m
Hence, the focal length of the lens is 0.5m (50 cm).
Here, the power of lens is positive i.e. the lens is convex lens.
So, the man is suffering from the long-sightedness or hypermetropia.

Solution,
Here,
Focal length (f) = 25cm = 0.25m
Power of lens (P) = ?
We know that,

or, = 4D
or, P = 4D Hence, the power of lens is 4 dioptre.

0%

absorbtion

reflection

polarization

refraction

• The image formed by a plane mirror is always ______.

virtual and erect

real and inverted

real and diminished

real and magnified

• An image formed by a concave mirror is always _______.

virtual, erect and diminished

real, inverted and magnified

real, inverted and diminished

virtual, erect and magnified

• An incident ray, passing through the optical center (O) _______.

emerges without deviation after refraction

emerges with deviation after refraction

emerges without deviation after reflection

emerges without deviation before refraction

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prkash aryal

It doesn't have nepali edition ??