1. The human eye is like a camera. Its lens system forms an image on a light sensitive screen called the retina.
2. The eye ball is approx. spherical in shape with a diameter of 2.3cm.
3. The human eye has the following parts:-
(a) Cornea:-The transparent spherical membrane covering the front of the eye.
(b) Iris:-The coloured diaphragm between the cornea and lens.
(c) Pupil:-The small hole in the iris.
(d) Eye lens:-Its is a transparent lens made of jelly like material.
(e) Ciliary muscles:-These muscles hold the lens in position.
(f) Retina:-The back surface of the eye.
(g) Blind spot:-The point at which the optic nerve leaves the eye. An image formed at
this point is not sent to the brain.
(h) Aqueous humour:-A clear liquid region between the cornea and the lens.
(i) Vitreous humour:-The space between eye lens and retina is is filled with another
liquid called Vitreous humour.
4. In the eye, the image is formed on the retina by successive refraction at the cornea,
the aqueous humour, the lens and the vitreous humour. Electrical signals then travel
along the optic nerve to the brain to be interpreted.
In good light, the yellow spot is most sensitive due to presence of large
number of rods and cones to detail and the image automatically formed there.
5. Accommodation:-The ability of the eye to focus both near and distant objects, by
adjusting the focal length, is called the accommodation of the eye
or,
The ability of the ciliary muscles to change the focal length of the eye lens is called
accommodation.
When we see distant objects Ciliary muscles get relax. This decreases the
curvature of the eye lens and the lens becomes thin to increase focal length.
When we see nearby objects ciliary muscles get contract. This increases the
curvature of the eye lens and the lens becomes thicker to decrease focal length.
6. Defects of the eye:- Although the eye is one of the most remarkable organs in the
body, it may have several abnormalities, which can often be corrected with eye glasses,
contact lenses, or surgery.
The various defects from which an eye can suffer are (i) Hypermetropia or
long shortsightedness. (ii) Myopia or short sightedness and (iii) *Astigmatism. (iv)
Presbyopia.
7. Hyper metropia : A person suffering from this defect can see distant objects clearly
but cannot see nearby objects clearly. In this defect, the near point lies further away
from 25cm.
Hyper metropia (far sightedness)- the image of nearby objects is focused beyond the retina) is corrected by using a convex lens of suitable power. The eye losses its power of accommodation at old age.
8. Hyper metropia is due to the following reasons:-
(i) Either the hyperopic eye ball is too short or
(ii) The Ciliary muscle is unable to change the shape of the lens enough to properly focus the image that is the focal length of the eye lens increases.
9. Myopia or short sightedness or near sightedness:-A person suffering from myopia or short sightedness can see nearby objects clearly but cannot see the faraway objects clearly.
Myopia (shortsightedness-the image of distant objects is focused before the retina) is
corrected by using a concave lens of suitable power.
10. Presbyopia is the defect of human eye due to which an old person cannot read and write comfortably. That is why presbyopia is also called old sight.
11. Astigmatism:- The defect by which the
person is notable to differentiate horizontal and vertical position, is called
astigmatism. It is a defect in which a person cannot see in
all directions equally well. It can be rectified by using
cylindrical lenses.
(a) Corneal astigmatism - the cornea has an irregular shape
(b) Lenticular astigmatism - the lens has an irregular shape
Astigmatism is an eye condition
with blurred vision as its main symptom. The front surface of the eye (cornea)
of a person with astigmatism is not curved properly – the curve is irregular -
usually one half is flatter than the other - sometimes one area is steeper than
it should be.
When light rays enter the eye they do not focus correctly on the retina, resulting in a blurred image. Astigmatism may also be caused by an irregularly shaped lens, which is located behind the cornea.
Astigmatism is a type of refractive error. A refractive error means that the shape of the eye does not bend light properly, resulting in a blurred image. Light has to be bent (refracted) by the lens and the cornea correctly before it reaches the retina in order to see things clearly.
The two most common types of astigmatism are:
When light rays enter the eye they do not focus correctly on the retina, resulting in a blurred image. Astigmatism may also be caused by an irregularly shaped lens, which is located behind the cornea.
Astigmatism is a type of refractive error. A refractive error means that the shape of the eye does not bend light properly, resulting in a blurred image. Light has to be bent (refracted) by the lens and the cornea correctly before it reaches the retina in order to see things clearly.
The two most common types of astigmatism are:
(a) Corneal astigmatism - the cornea has an irregular shape
(b) Lenticular astigmatism - the lens has an irregular shape
In astigmatism, images focus in
front of and beyond the retina, causing both close and distant objects to
appear blurry (see below figure).
11. As our two eyes are separated by a few centimeters, each eye observes a
slightly different image. Our brain combines the two views into one and we get to know
how close or far away the things seen are.
12. The smallest distance, at which the eye can see objects clearly without strain, is called the near point of the eye or the least distance distinct vision. For a young adult with normal vision, it is about 25cm.
13. The distance between far point and near point of the eye is called range of vision
of the eye.
14. Spectrum is a band of distinct colors we obtain when white light is split by a prism.
15. Causes of dispersion:- Every colour has its own characteristic wave length or
frequency. Different colors move with same speed in air/vacuum. But their speeds in
refracting media like glass are different. Therefore, refractive index of the medium for different colors is different.
As a result, different colors undergo different deviations on passing through the prism. Hence, different colors emerge from the prism along different directions.
16.
In any medium other than air/vacuum red light travels the fastest and violet
light travels the slowest.
light travels the slowest.
17. When we pass white light through two identical prisms held side by side with their
refracting edges in opposite direction; the first prism disperses white light into
seven colors and the second prism recombines the seven colors into white light. Thus
light emerging from second prism is white.
18. A rainbow is formed due to dispersion of light by tiny droplets of water which act
as prisms.
19. Scattering of light causes the blue colour of the sky and the reddening of the sun
at sunrise and sunset.
20.We can see the sun about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction.
By actual sunrise, we mean the actual crossing of the
horizon by the Sun.
When the sun just below the horizon its rays enters earth’s atmosphere and refracted towards the earth. Refracted ray that reach us making it appears the sun reaches to horizon. This is the apparent sunrise. But actual sunrise occur when sun reach to horizon. Conversely apparent sunset occur slightly latter than actual sun set. Since light from sun already below the horizon it refract through atmosphere enabling us to see apparent sun set even sun has already set.
When the sun just below the horizon its rays enters earth’s atmosphere and refracted towards the earth. Refracted ray that reach us making it appears the sun reaches to horizon. This is the apparent sunrise. But actual sunrise occur when sun reach to horizon. Conversely apparent sunset occur slightly latter than actual sun set. Since light from sun already below the horizon it refract through atmosphere enabling us to see apparent sun set even sun has already set.
21. When the sun is overhead at noon, then the light coming from the sun has to travel a relatively shorter distance through the atmosphere to reach us. As a result, only a little of the blue colour of the white light is scattered (most of the blue light remains in it). Since the light coming from the overhead sun has almost all its components colors in the right proportion, therefore, the sun appears white.
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