Category : 10th Class
Light and Human Eye
Light is an electromagnetic wave which do not require a material medium for their propagation. Light is composed of particles which travel in a straight line at very high speed. Light has a dual nature i.e. waves and particles. Speed of light is different in different mediums. Speed of light in vacuum is \[3\times {{10}^{8}}m/s.\]
Reflection of Light
The process of sending back the light rays which fall on the surface of an object, is called reflection of light.
Rules for obtaining images formed by concave mirrors
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Object Position |
Image Position |
Nature of Image |
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(a) |
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at infinity |
at the focus F |
real and point-sized |
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(b) |
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between infinity and the center of curvature C |
between F and C |
real, smaller than the object, inverted |
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(c) |
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at C |
at C |
real, same size, inverted |
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(d) |
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between C and F |
between C and infinity |
real, enlarged inverted |
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(e) |
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at F* |
at infinity |
real, infinitely large, inverted |
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(f) |
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between the pole |
behind the P and F |
virtual, enlarged mirror erect |
Rules for obtaining images formed by convex mirrors
Images Formed by Convex Mirrors
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Object Position |
Image Position |
Nature of image |
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(a) |
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between infinity and the pole |
between the focus and the pole |
virtual, smaller and erect |
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(b) |
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At Infinity |
At the focus F |
Virtual, pointy sized |
Mirror Formula
\[\frac{1}{v}+\frac{1}{u}=\frac{1}{f}\]
Where,
v = distance of images from mirror
u = distance of object from mirror
f = focal length of the mirror
Magnification Produced by Mirrors
The ratio of the height of image to the height of object is known as linear magnification.
\[m\frac{{{h}_{2}}}{{{h}_{1}}}\]
Where, m = magnification
\[{{h}_{2}}\]= height of image
\[{{h}_{2}}\]= 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.
\[m=\frac{-v}{u}\]
So, \[\frac{{{h}_{2}}}{{{h}_{1}}}=\frac{-v}{u}\]
Refraction of Light
The bending of light when it goes from one medium to another medium is called refraction of light. The refraction of light occurs due to the change in the speed of light on going from one medium to another.
Laws of Refraction of Light
\[\frac{\sin i}{\sin r}=Cons\tan t\]
Refractive Index
Refractive index is ratio of speeds of light in the two mediums. The refractive index of medium 2 with respect to medium 1 is equal to the ratio of speed of light in medium 1 to the speed of sit in medium 2.
Refraction of Light by Spherical Lenses
The Working of a lens is based on the refraction of light rays when they pass through it. A lens is a piece of transparent glass bound by two spherical surfaces. There are two types of lenses:
Rules for Obtaining Images Formed by Convex Lenses
Images Formed by a Convex Lens
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|
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Position of the object |
Position of the Images |
Nature of Size |
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(a) |
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at infinity |
at \[{{F}_{2}}\] |
real point - size |
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(b) |
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between infinity and \[2{{F}_{1}}\] |
between \[{{F}_{2}}\] and \[2{{F}_{2}}\] |
real, smaller, inverted |
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(c) |
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at \[2{{F}_{1}}\] |
at \[2{{F}_{2}}\] |
real, same size, inverted |
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(d) |
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between \[2{{F}_{1}}\] and \[{{F}_{1}}\] |
between infinity |
real, enlarged, inverted |
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(e) |
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at |
\[{{F}_{1}}\] |
at infinity real infinity large, inverted |
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(f) |
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between \[{{F}_{1}}\]and \[O\] |
on the side of the object |
virtual, larger, erect |
Rules for Obtaining Images Formed by Concave Lenses
A ray of light which is parallel to the principal axis of a concave lens, appears to be coming from its focus after refraction through the lens.
A ray of light passing through the optical centre of a concave lens goes straight after passing through the lens.
A ray of light going forwards the focus of a concave lens, becomes parallel to its principal axis after refraction through the lens.
Images Formed by a Concave Lens
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Object Position |
Image Position |
Nature of image |
|
(a) |
|
between infinity and the pole |
between the focus and the pole |
virtual, smaller and erect |
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(b) |
|
At Infinity |
At the focus F |
Virtual, pointy sized |
Lens Formula
\[\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\]
Where, v = image distance
u = object distance
f= focal length
Magnification produced by lenses
\[Mgnification=\frac{hieght\,\,of\,\,image}{hieght\,\,of\,\,object}\]
\[m=\frac{{{h}_{2}}}{{{h}_{1}}}\] … (i)
Where, m = magnification
\[{{h}_{2}}\] = height of image
\[{{h}_{1}}\] = height of object
The linear magnification produced by a lens is equal to the ratio of image distance to the object distance:
\[m=\frac{v}{u}\] … (ii)
Where, m = magnification
v= image distance
u = object distance
from (i) and (ii)
\[m=\frac{{{h}_{2}}}{{{h}_{1}}}=\frac{v}{u}\]
Power of a Lens
The power of a lens is defined as the reciprocal of its focal length in metres.
\[P=\frac{1}{F}\]
Where, P = Power of the lens
F = Focal length of the lens
The unit of the power of a lens is dioptre, which is denoted by the letter D.
Eye
The human eye is one of the most important living material which enables us to see things around us.
The main parts of the human eye are: Cornea, Iris, Pupil, Ciliary muscles, eye lens, retina and optic nerve.
Working of Human Eye
The light rays coming from the object kept in front of us enter our eyes through the cornea, pass through the pupil of the eye and fall on the eye-lens.
The eye-lens is a convex lens, so it converges the light rays and produces a real and inverted image of the object on the retina. The image formed on the retina is conveyed to the brain by the optic nerve and gives rise to the sensation of vision.
Defects of Vision
There are mainly three common refractive defects of vision. These are myopia or near-sightedness, Hypermetropia or farsightedness and Presbyopia. These defects can be corrected by lenses.
Myopia (Short-sightedness)
A person suffering from myopia can see nearby objects clearly but cannot see distant objects. This defect may arise due to thinking of eye-lens or due to eye-ball being too long. This defect ran be corrected by using a concave lens of suitable power.
Hypermetropia (Far-sightedness)
It is also known as far-sightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. This defect arises either because the focal length of length eye lens is too long or the eyeball has become too small. This defect can be corrected by using a convex lens of appropriate power.
Presbyopia (Old-sight)
The people find difficult to see nearby objects. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens with ageing.
Dual Eye Defect
Sometimes, a person may suffer from both myopia and hypermetropia. Such people often require bifocal lenses. A common type of bifocal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision.
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