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Note on Light

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Light

http://www.kullabs.com/js/ckfinder/userfiles/images/Incandescent_light_1783785c(2).jpg

Light is a form of energy which gives sensation of vision. It gives the ability to see the object.

Refraction of light:

http://www.kullabs.com/js/ckfinder/userfiles/images/Picture18(2).png

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:

Lens is an optical device, which is made up of transparent and spherical medium. There are two types of lens:

 

  1. Convex lens:

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

    http://www.kullabs.com/js/ckfinder/userfiles/images/Lens2(2).png

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

  2. Concave lens:

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

    http://www.kullabs.com/js/ckfinder/userfiles/images/concave lens trans background(3).png

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

Some related terms

  1. Center of curvature

    http://www.kullabs.com/js/ckfinder/userfiles/images/centre-of-curvature-of-convex-lens(3).png

  2. Radius of curvature

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

    http://www.kullabs.com/js/ckfinder/userfiles/images/radius-of-curvature(2).JPG

     
  3. Optical center

    The geometrical center of the lens is known as optical center.

    http://www.kullabs.com/js/ckfinder/userfiles/images/Optics_gr_40(2).gif

    Here 'o' is the optical center

  4. Principal focus:

    http://www.kullabs.com/js/ckfinder/userfiles/images/Ray_1(2).gif

    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.

    Note: Principal focus is real for convex lens and virtual for concave lens.

  5. Principal axis:

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

    http://www.kullabs.com/js/ckfinder/userfiles/images/principal-axis(2).JPG

  6. Focal length:

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

    http://www.kullabs.com/js/ckfinder/userfiles/images/lens-focal-length(3).gif

     

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. In case of concave lens, the ray appears to be diverging from the focus.

    http://www.kullabs.com/js/ckfinder/userfiles/images/figure_12(1).JPG

     
  2. http://www.kullabs.com/js/ckfinder/userfiles/images/Figure 26_06_04(1).jpg

     

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

  3. http://www.kullabs.com/js/ckfinder/userfiles/images/Figure 26_06_06(1).jpg

    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: 1 / focal length

Note: focal length should be in meter.

Characteristics of image when placed at different places:

  1. Convex lens

    http://www.kullabs.com/js/ckfinder/userfiles/images/table7_6(1).jpg

  2. Concave lens

    http://www.kullabs.com/js/ckfinder/userfiles/images/u14l5eb2(1).gif

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 dioptre 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 the image to the height of object.

Optical instrument

The instrument which are used to produce image of an\ object by using a lens or a combination of lenses are called optical instrument. Examples are Photographic camera, microscope, telescope,etc.

Photographic camera

A photographic camera is an optical instrument that is used to record a permanent image of an object on a photographic film.

The construction of camera is based on the principle that, when an object is placed beyond 2F of a concave lens. A real, inverted and diminished image is formed between F and 2F on the other side of the lens.

Microscope:

http://www.kullabs.com/js/ckfinder/userfiles/images/Design-Industrial-design-British-optical-instruments-24.jpg

A microscope is an optical instrument that is used to see the details of very small objects which cannot be seen through our naked eyes.

A microscope forms highly magnified images of the tiny objects by a system of combined lenses due to which it becomes possible to see the details of very tiny objects through microscope.

Telescope

http://www.kullabs.com/js/ckfinder/userfiles/images/refracting-telescope.jpg

A telescope is an optical instrument that is used to see the details of the distant objects.

Construction: An astronomical telescope two convex lenses are used ( one objective and other eye-piece) are fitted at the outer ends of two metal tubes which can slide into one another. The objective of an astronomical telescope has a large focal length but the eye-piece has a short focal length.

In order to bring the telescope in normal adjustment, the telescope is focused at infinity that can be achieved by turning the adjustment knobs, In the normal adjustment, a person can view distant objects clearly.

Human Eye

http://www.kullabs.com/js/ckfinder/userfiles/images/human_eye_diagram(1).gif

Accommodation:

Accommodation is defined as the ability of an eye to focus the distant objects as well as the nearby objects on the retina by changing the focal length of its lens.

Far point:

The far point of an eye is defined as the farthest point from the eye which can be seen clearly. The far point of human eye is at infinity.

Near point:

The near point of an eye is defined as the nearest point from the eye upto which eye can see objects clearly. The nearest point of normal human eye is 25cm from eye.

Defects of vision of eye:

Long sightedness ( Hypermetropia )

http://www.kullabs.com/js/ckfinder/userfiles/images/images(1).jpg

Hypermetropia / long-sightedness is the defect of vision in which the eye cannot see the nearby objects clearly but can see the distant objects clearly.

 

Causes of hypermetropia

      1. Shortening of eye ball
      2. Increment of focal length

Shortsightedness (Myopia)

Myopia is the defect of vision in which an eye cannot see the distant objects clearly but can see nearby objects clearly.

http://www.kullabs.com/js/ckfinder/userfiles/images/352px-Myopia-2-3_svg.png

 

Causes of myopia

      1. Elongation of eye ball.
      2. decrement of focal length.

Ray diagram showing working mechanism of a compound microscope

http://www.kullabs.com/js/ckfinder/userfiles/images/compound-microscope.jpeg

Working mechanism of an astronomical telescope

http://www.kullabs.com/js/ckfinder/userfiles/images/astronomical-telescope-diagram.jpeg

  • This note illustrates the brief description of light which contains the definition of lens, optical instrument, human eye and their working mechanism.
  • Light is a form of energy which gives sensation of vision. It gives the ability to see the object.
  • Lens is an optical device, which is made up of transparent and spherical medium. There are two types of lens:
  • Convex lens: thicker at center and thinner at edges.
  • Concave lens: thicker at the edges and thinner at center.
.

Very Short Questions

When ray of light travels from one medium to another medium, it bends at separating media. That phenomenon of bending of light in this way is called refraction of light.

The process of adjusting the distance between the lens and the screen in order to produce a sharp and clear image is called focusing of lens.

The process of variation of the size of image produced by a lens, according to the position of the object is called magnification of lens.

When sunlight is made to strike a convex lens, the heat energy striking on total area of the lens is concentrated to a point. The heat concentrated at a point has a very high is concentrated to a point. The heat concentrated at a point has very high temperature. The temperature is enough to urn the paper. That is the reason, the paper burns when sunlight is focused on a piece of paper through a convex lens.

Solution,
Here,
Object distance (u) =10 m
Image distance (v) = 50cm = 0.5m
Magnification (M) = ?
We know,

or,
or,M = 0.05
The magnification produced is 0.05times, which shows that the size of the image will be smaller than that of the object.

0%
  • A converging mirror is also known as ________.

    concave mirror
    bi-focal mirror
    plane mirror
    convex mirror
  • Which mirror has a wider field of view?

    bi-focal mirror
    Convex mirror
    Concave mirror
    plane mirror
  • The lens that is thicker at the edges and thinner at centre is known a

    plane mirror
    convex mirror
    concave mirror
    bi-focal mirror
  • The nearest point of normal human eye is ______.

    25m
    30 m
    30cm
    25 cm
  • In which defect of vision an eye cannot see the distant objects clearly but can see nearby objects clearly?

    Hypermetropia
    Astigmatism
    Myopia
    Axis error
  • Far point of a normal eye is

    1 km
    25 cm
    infinite
    50 cm
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Jagdip prasad Tharu

Why power of lens is measured as reciprocal of focal length


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