It is a wonderful gift of nature to the human body. Human eye is nearly spherical in shape of diameter about 2.5 cm.
Parts of Human Eye:
- Cornea: It is the protective and front layer of the eye. It is made by a transparent membrane. Light enters the eye through the cornea.
- Iris: Dark and a colourful muscular diaphragm is called iris. It is responsible for colour of the eye.
- Pupil: Small circular hole in the centre of iris. It regulates the amount of light entering the eye by adjusting the size of the iris.
- Ciliary Muscles: It holds the eye lens at its proper position. It changes the size of eye lens.
- Eye lens: The eye lens is a convex lens made by the transparent jelly like material.
- Retina: It is the screen of the eye. A real and inverted image form on the retina.
- Rods and Cones: These are colour sensitive rods and cones shaped cells. Rods are responsible for the vision in dim light while cones are responsible for colour.
- Optic Nerve: It converts information of the image into a corresponding electric signal and passes it to the brain.
- Blind Spot: The junction of the optic nerve and retina, where no rods and cones cells are present is called the blind spot. It is insensitive to light.
Near Point: The nearest point from eye at which the eye can see clearly without strain is called near point. For normal eye it is 25 cm.
Far Point: The farthest point, upto which the eye can see the object clearly is called far point. For normal eye it is infinity.
The range of Vision: Distance between near point and far point of eye is called range of vision.
Power of Accommodation: The ability of the eye to see near as well as far objects clearly is called Power of Accommodation.
What is Myopia (Nearsightedness):
In this defect, the eye is unable to see far off objects clearly but is able to see near objects clearly.
- Due to increase in size of eye ball
- Due to excessive curvature of cornea
- Due to increase in power (or decrease in focal length) of eye lens.
Correction: It is corrected by using concave lens of suitable focal length.
What is Hypermetropia (Far sightedness):
In this defect eye is unable to see nearby objects clearly but is able to see far objects clearly.
- Due to decrease in size of eye ball
- Due to decrease in power (or increase in focal length) of eye lens.
Correction: It is corrected by using a convex lens of suitable focal length.
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Dispersion of Light:
Splitting of white light into seven colours is called dispersion. Example, the formation of Rainbow (VIBGYOR). Violet deviates the most but red deviates least.
1. The human eye is one of the most valuable and sensitive sense organs. It enables us to see the wonderful world and the colours around us.
2. The eyeball is approximately spherical in shape with a diameter of about 2.3 cm.
3. Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea. The crystalline lens merely provides the finer adjustment of focal length required to focus.
The human eye is like a camera. Its lens system forms an image on a light-sensitive screen called the retina.
• The eyeball is approximately spherical in shape with a diameter of about 2.3 cm.
• The eye lens forms an inverted real image of the object on the retina.
RETINA: The retina is a delicate membrane having enormous number of light-sensitive cells.
CORNEA: Light enters the eye through a thin membrane called the cornea. It is the eye’s outermost layer. It is the clear, dome shaped surface that covers the front of the eye. It plays an important role in focusing your vision.
PUPIL : The pupil is a hole located in the centre of the iris of the eye. It appears black because light rays entering the pupil are either absorbed by the tissues inside the eye directly, or absorbed after diffuse reflections within the eye. The pupil regulates and controls the amount of light entering the eye.
IRIS : It is a dark muscular diaphragm that controls the size of the pupil and thus the amount of light reaching the retina.
AQUEOUS HUMOUR: It is fluid which fills the space between cornea and eye lens.
EYE LENS: It is a convex lens made of transparent and flexible jelly like material. Its curvature can be adjusted with the help of ciliary muscles.
CILIARY Muscle : These are the muscles which are attached to eye lens and can modify the shape of eye lens which leads to the variation in focal lengths.The light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals.
OPTICAL NERVES: These are the nerves which take the image to the brain in the form of electrical signals.
• When the light is very bright: The iris contracts the pupil to allow less light to enter the eye.
When the light is dim: the iris expands the pupil to allow more light to enter the eye. Thus, the pupil opens completely through the relaxation of the iris.
• A human being has a horizontal field of view of about 150° with one eye and of about 180° with two eyes.
Accommodation power: The ability of eye to change the focal length of eye lens with the help of ciliary muscles to get the clear view of nearby objects (about 25 cm) and far distant objects (at infinity).
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Deffects Of Vision And Their Corrections:
Myopia (Short sightedness):
It is a kind of defect in human eye due to which a person can see near objects clearly but he cannot see the distant objects clearly. Myopia is due to
(i) Excessive curvature of cornea.
(ii) Elongation of eye ball.
Correction: Myopia or short-sightedness can be corrected by wearing spectacles containing concave lens.
Hypermetropia (Long sightedness):
It is a kind of defect in human eye due to which a person can see distant objects properly but cannot see the nearby objects clearly. It happens due to
(i) Decrease in power of eye lens i.e., increase in focal length of eye lens.
(ii) Shortening of eye ball.
Correction: The near-point of an eye having hypermetropia is more than 25 cm. The condition of hypermetropia can be corrected by putting a convex lens in front of the eye.
It is a kind of defect in human eye which occurs due to ageing. It happens due to
(i) Decrease in flexibility of eye lens.
(ii) Gradual weakening of cilliary muscles.
Correction: The near-point of an old person having presbyopia is much more than 25 cm. Presbyopia can be corrects by wearing spectacles having convex lens.
Important note : A person can have both myopia and hypermetropia. In such a condition, spectacles having bifocal lens are worn. The upper part of bifocal lens is concave and lower part consists of convex lens.
Astigmatism: It is a kind of defect in human eye due to which a person cannot see (focus) simultaneously horizontal and vertical lines both.
Cataract: Due to the membrane growth over eye lens, the eye lens becomes hazy or even opaque. This leads to decrease or loss of vision.
Correction: The problem is called cataract. It can be corrected only by surgery.
Other Text book Questions of Human Eye and the Colorful World
Lens is a transparent structure found behind the pupil.
What is meant by power of accommodation of the eye?
Answer- The ability of the lens of the eye to adjust its focal length to clearly focus rays coming from distant as well from a near objects on the retina, is known as the power of accommodation of the eye.
What is the far point and near point of the human eye with normal vision?
Answer- The minimum distance of the object from the eye, which can be seen distinctly without strain is called the near point of the eye. For a normal person’s eye, this distance is 25 cm.
The far point of the eye is the maximum distance to which the eye can see objects clearly. The far point of a normal person’s eye is infinity.
Why do stars twinkle?
Answer- The twinkling of a star is due to atmospheric refraction of starlight. The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth. The atmospheric refraction occurs in a medium of gradually changing refractive index.
Explain why the planets do not twinkle?
Answer- Unlike stars, planets don’t twinkle. Stars are so distant that they appear as pinpoints of light in the night sky, even when viewed through a telescope. Since all the light is coming from a single point, its path is highly susceptible to atmospheric interference (i.e. their light is easily diffracted).
Why does the Sun appear reddish early in the morning?
Answer- White light coming from the sun has to travel more distance in the atmosphere before reaching the observer. During this, the scattering of all colored lights except the light corresponding to red color takes place and so, only the red colored light reaches the observer. Therefore, the sun appears reddish at sunrise and sunset.
Why does the sky appear dark instead of blue to an astronaut?
Answer- The sky appears dark instead of blue to an astronaut, as scattering of light does not take place outside the earth’s atmosphere.
A person needs a lens of power -5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?
The power (P) of a lens of focal length f is given by the relation
Power (P) = 1/f
(i) Power of the lens (used for correcting distant vision) = – 5.5 D
Focal length of the lens (f) = 1/P
f = 1/-5.5
f = -0.181 m
The focal length of the lens (for correcting distant vision) is – 0.181 m.
(ii) Power of the lens (used for correcting near vision) = +1.5 D
Focal length of the required lens (f) = 1/P
f = 1/1.5 = +0.667 m
The focal length of the lens (for correcting near vision) is 0.667 m.
Dispersion of white light by a glass prism
The phenomenon of splitting of white light into its seven constituent colours when it passes through a glass prism is called dispersion of white light. The various colours seen are Violet, Indigo, Blue, Green, Yellow, Orange and Red. The sequence of colours remembers as VIBGYOR. The band of seven colours is called the spectrum. The different component colour of light bends at a different angle with respect to the incident angle. The violet light bends the least while the red bends most.
Refraction of light through a prism
When a ray of light is incident on a rectangular glass slab, after refracting through the slab, it gets displaced laterally. As a result, the emergent ray comes out parallel to the incident ray.
Unlike a rectangular slab, the side of a glass prism are inclined at an angle called the angle of prism.
Prism: A prism has two triangular bases and three
Angle of Prism: Angle between two lateral faces is
Angle of Deviation: The angle between the incident deviation.
Rainbow: It is the spectrum of sunlight in nature. It is formed due to the dispersion of sunlight by the tiny water droplet, present in the atmosphere.
Formation of the rainbow
The water droplets act like small prism. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. Due to the dispersion of light and internal reflection, different colours reach the observer’s eye.
Red colour appears on top and violet at the bottom of rainbow.
A rainbow is always formed in a direction opposite to that of Sun.
What is Atmospheric Refraction
The refraction of light caused by the Earth’s atmosphere (having air layers of varying optical densities) is called Atmospheric Refraction.
Twinkling of Star
It is also due to atmospheric refraction.
Distant star act like a point source of light. As the beam of starlight keeps deviating from its path, the apparent position of star keeps on changing because physical condition of earth’s atmosphere is not stationary.
Hence, the amount of light enters our eyes fluctuate sometimes bright and sometime dim. This is the “Twinkling effect of star.
When a beam of light strikes, the minute particle of earth’s atmosphere, suspended particles of dust and molecule of air the path of beam become visible. The phenomenon of scattering of light by the colloidal particle gives rise to Tyndall Effect.
It can be observed when sunlight passes through a canopy of a dense forest.
The colour of the scattered light depends on the size of the scattering particles.
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