Human eye and the colourful world.
As you all know, light is important for us as it enables us to see. Likewise, eye is very important as it produces a sensation of vision in us .
Eyes are called as jewels of our body.its a famous quote.
Human eye
It is a naturally occurring optical instrument. As you all know that we have a pair of eyes and its function is to enable us to see. Without it the whole world would have been a dark place for us.
Structure of eye
It is a spherical ball with a small bulge in the front part.
It is located in the eye socket.
It has two layers that cover it: scleroid and choroid.
The function of choroid: It is a grey membrane attached to choroid from inner side. Its function is to darken the eye from inside so that no internal reflection takes place.
Apart from these layers, it consists of :
Cornea
Iris
Pupil
Ciliary muscle
Lens
Retina
Blind spot
Optic nerve
Cornea:
It is the white part of the eye that allows light to enter.
It acts as a window to the world.
Iris
It is colored part of the eye.
It holds the pupil and also adjust the size of pupil according to the intensity of light.
Pupil:
It is black in color and absorbs all the light rays falling on it.
It gets constricted when the intensity of light is high.
It gets expanded when the intensity of light is low.
Ciliary muscles
They hold the lens.
They adjust the focal length of the lens.
Convex lens
A cellular structure resembling convex lens (diverging lens).
Retina :
It is the screen of an eye where image is formed.
It consists of two types of cells:
Cone cells: those cells which respond to colours.
Rod cells: those cells which respond to the intensity of light.
Yellow spot:
A point on the retina where the most clear image is formed.
Blind spot: It is that point on the retina where no image is formed.
Optic nerve: A nerve that connects the eye to the brain.
The fluid which is present between cornea and lens is called aqueous humour.
Function : It is a watery fluid present in the interior part of the eye and its function is to protect the exterior part of the eye from collapsing when there is a sudden change in the atmospheric pressure. Also, it is the fluid that flows out from the eye when we wink our eyes. So, it washes the eye and also keeps it moist.
The fluid is present between lens and retina and is called vitreous humor.
Function: It is a dense jelly – like fluid present in the posterior part of the eye and its function is to protect the posterior part of the eye from collapsing when there is a sudden change in the atmospheric pressure. It also helps in focusing the image clearly on the retina.
Adjustment of the size of the pupil according to the intensity of light: When we are exposed to bright light the iris constricts the pupil partially or we can say that the pupil shortens so that the right amount of light enters and a clear image is formed. Whereas in a dark room, Pupil expands itself to gather more light in order to obtain a clear image.
SPECIAL NOTE POINTS-
1.To see any object comfortably and distinctly you must hold it at about 25 cm from your eyes.from this distance you can see any object without strain .it is also called the near point of your eyes
2 The farthest point up to which the eye can see objects clearly is called the far point of eye .it is infinity for a normal eye.
Defects of vision and their corrections-
Sometime eyes may lose the power of accomodation due to the refractive defects or errors.Refractive error means that the shape of your eye does not bend light correctly, resulting in a blurred image
- THERE are major 3 types of refractive defects
1.MYOPIA(NEAR SIGHTEDNESS)
2 HYPERMETROPIA (FAR SIGHTEDNESS)
3.PRESBYOPIA
A)MYOPIA OR NEAR SIGHTEDNESS--->
A person with myopia can see nearby objects clearly.but he is unable to see the far object clearly.their far point is nearer than infinity. Image of the object is made infront of retina not on retina.
Causes of myopia-
A)Due to excessive curvature of eye lens
B)elongation of eye ball.
Remedy-corrected by using concave lens of suitable power.concave lens will bring the image back on retina.
2.HYPERMETROPIA(FAR SIGHTEDNESS)
Person can see far object clearly but can't see the distant objects clearly.the near point of such person is larger than normal .image is formed beyond the retina
Causes-
1.focal length of the eye is too long.
2 The eyeball has become too small.
Remedy- person may use convex lens of appropriate power .
Causes-due to weakening of ciliary muscles and decreasing flexibility of eye lens.
Sometime old person may suffer from both myopia and HYPERMETROPIA both.so both the lenses used together in a single frame.
The Lens formula,
In this case,
Object distance, u = ∞
Image distance, v =
person’s far point Focal length, f =?
Hence, lens formula becomes
- In case of a concave lens, the image is formed in front of the lens i.e., on the same side of the
object.
Focal length = -Far point
Now, power of the required lens (P) = 1/f - Power of the correcting convex lens: Lens formula,
1v−1u=1f can be used to calculate focal length f and hence, power P of the correcting convex lens, where,
Object distance, u = -25 cm, normal near point
Image distance, v = defective near point
Hence, the lens formula is reduced to - i/v -1/25=1/f
REFRACTION OF LIGHT THROUGH PRISM
What is Refraction?
The change in direction or bending of a light wave passing from one transparent medium to another caused by the change in wave’s speed is the refraction.
The extent of bending of light rays entering from one medium to another is the refractive index and is denoted by the ‘n’.
It is represented as n = c/v, where c = velocity/speed of light of a certain wavelength in the air and v = velocity of light in any medium.
What is a Prism?
It is a solid figure having two triangular bases and three rectangular surfaces and is the closed surface. The angle between each surface is the angle of the prism. Here the opposite surfaces are equal surfaces and are parallel. We notice that here there are two refracting surfaces which means the surface where refraction of light takes place.
- When a ray of light moves from one surface to another it bends. This is called Refraction
- The ray of light from air is incident on the glass surface AB. On hitting the surface AB, it bends towards the normal.
- At the surface AC, the light ray moves from glass to air. It again bends.
- T he angle of incidence (i), the angle of refraction (r) and the angle of emergence (e) is as shown in the figure.
- PQ is the incident ray
- QR is the refracted ray
- RS is the emergent ray.
- angle A is called the angle of prism.
- angle d is called angle of deviation.
- 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.this coloured bend is called the spectrum of lights.
- v
Composition of white light: White light consists of seven colours i.e., violet, indigo, blue, green, yellow, orange and red.
Monochromatic light: Light consisting of single colour or wavelength is called monochromatic light, example; sodium light.
Polychromatic light: Light consisting of more than two colours or wavelengths is called polychromatic light, example; white light.
Recombination of white light: Newton found that when an inverted prism is placed in the path of dispersed light then after passing through the prism, they recombine to form white light.
Issac Newton: He was the first, who obtained spectrum of sunlight by using glass prism. He tried to split the spectrum of white light more by using another similar prism, but he could not get any more colours.
He repeated the experiment using second prism in inverted position with respect to the first prism. It allowed all the colours of spectrum to pass through second prism. He found white light emerges on the other side of second prism.
He concluded that Sun is made up of seven visible colour VIBGYOR.
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.
At ‘A’ – Refraction and dispersion take place.
At ‘B’ – Internal reflection takes place.
At ‘C’ – Refraction and dispersion take place.NCERT EXCERCISE AND INTEXT QUESTIONS.
Page No: 190
1. 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.
2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of corrective lens used to restore proper vision?
Answer-
An individual with a myopic eye should use a concave lens of focal length 1.2 m so that he or she can restore proper vision.
3. 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.
4. A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?
Answer-
The student is suffering from short-sightedness or myopia. Myopia can be corrected by the use of concave or diverging lens of an appropriate power.
Page No: 197
Exercise
1. The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to
(a) presbyopia
(b) accommodation
(c) near-sightedness
(d) far-sightedness
Answer-
(b) accommodation
Due to accommodation the human eye can focus objects at different distances by adjusting the focal length of the eye lens.
Page No: 198
2. The human eye forms an image of an object at its
(a) cornea
(b) iris
(c) pupil
(d) retina
Answer –
(d) retina
The retina is the layer of nerve cells lining the back wall inside the eye. This layer senses light and sends signals to the brain so you can see.
3. The least distance of distinct vision for a young adult with normal vision is about
(a) 25 m
(b) 2.5 cm
(c) 25 cm
(d) 2.5 m
Answer –
(c) 25 cm
25 cm is the least distance of distinct vision for a young adult with normal vision.
4. The change in focal length of an eye lens is caused by the action of the
(a) pupil
(b) retina
(c) ciliary muscles
(d) iris
Answer-
(c) ciliary muscles
The action of the ciliary muscles changes the focal length of an eye lens
5. 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?
Answer-
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.
6. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
Answer-
The individual is suffering from myopia. In this defect, the image is formed in front of the retina. Therefore, a concave lens is used to correct this defect of vision.
Object distance (u) = infinity = ∞
Image distance (v) = – 80 cm
Focal length = f
According to the lens formula,
A concave lens of power – 1.25 D is required by the individual to correct his defect.
7. Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.
Answer-
An individual suffering from hypermetropia can see distinct objects clearly but he or she will face difficulty in clearly seeing objects nearby. This happens because the eye lens focuses the incoming divergent rays beyond the retina. This is corrected by using a convex lens. A convex lens of a suitable power converges the incoming light in such a way that the image is formed on the retina, as shown in the following figure.
The convex lens creates a virtual image of a nearby object (N’ in the above figure) at the near point of vision (N) of the individual suffering from hypermetropia.
The given individual will be able to clearly see the object kept at 25 cm (near point of the normal eye), if the image of the object is formed at his near point, which is given as 1 m.
Object distance, u= – 25 cm
Image distance, v= – 1 m = – 100 m
Focal length, f
Using the lens formula,
A convex lens of power +3.0 D is required to correct the defect.
8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Answer-
A normal eye is not able to see the objects placed closer than 25 cm clearly because the ciliary muscles of the eyes are unable to contract beyond a certain limit.
9. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Answer-
The image is formed on the retina even on increasing the distance of an object from the eye. The eye lens becomes thinner and its focal length increases as the object is moved away from the eye.
10. 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.
11. 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).
12. 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.
13. 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.
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