Q. What is meant by power of accommodation of the eye?
Ans: The ability of the eye lens to adjust its focal length is called accommodation of the eye. Power of accommodation of an eye is also defined as the maximum variation in the power or focal length of the eye lens.
Q. What is the far point and near point of the human eye with normal vision?
Ans: The farthest distance at which an object clearly can be seen clearly. It is infinity for normal eyes.
The nearest distance at which an eyes focus image clearly. of an object. It is 25 cm from the eye.
The nearest distance at which an eyes focus image clearly. of an object. It is 25 cm from the eye.
Q. 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?
Ans: A student has difficulty reading the blackboard while sitting in the last row so he is suffering from near sightedness or myopia as far point of eye is infinity. This defect can be corrected by using a concave lens of suitable focal length. This defect is cased due to decrease in size of eye ball and increase in focal length of eye lens.
Q. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Ans: A lens of eye cannot focus image clearly on retina if object is placed closer to 25cm and form blurred image .An eye lens can not decrease focal length below 25 cm Hence, eye cannot see it clearly.
Q. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Ans: The image is formed on the retina even on increasing the distance of an object from the eye. For this eye lens becomes thinner and its focal length increases as the object is moved away from the eye
Q. Why do stars twinkle?
Ans: As we go higher and higher, the density of layers and refractive index decreases regularly. As the light from a star enters the different layers of the atmosphere and travel large distances before reaching the earth, it scattered none uniformly and seems twinkling where as light from planets travel shorter distance and don’t seem twinkling.
Q. Why does the Sun appear reddish early in the morning?
Ans: At the time of morning, the position of the sun is very far away from us. The sunlight travels longer distance through the atmosphere of the earth before reaching our eyes. Scattering of blue light is more than the scattering of red light due to less velocity. Due to this more red light reaches to our eyes. Hence sun appears reddish in the morning.
Q. 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.
Ans: In the problem it is given that the near point of the normal eye is 25 cm, hence u = - 25 cm.
The lens used forms its Virtual image at near point of hypermetropic eye i.e., v = - 1m = - 100 cm.
v= near point of hypermetropic eye = -1m = -100 cm
u = near point of normal eye = -25cm.
1/f = 1/v -1/u = -1/100 + 1/25 = (-1 + 4)/100 =3/100
f = 100/3 cm = 1/3 m
Power = 1/f = 1/(1/3) =3 dioptre
Q. 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?
Ans:This defect is corrected by concave lens. D=1/f=1/80cm=100/80m =-1.25D
Or, The far point is 80 cm the object distance is ¥
so applying lens formula 1/v - 1/u = 1/f
v = - 80 cm u = ¥
-1/80 + 1/¥ = 1/f
1/f = -1/80 (1/¥ = 0)
f = -80 cm
or -0.8 m power is = -1/0.8 = -10/8 = -1.25 D
So it would be a concave lens as it has a negative power
Q. Why a chicken can only see in bright colour?
An: The retina of chicken’s eyes has only few rod cells .Its retina has mostly cone cells. Due this it can see only in bright colour. Hence wake up late in the morning and sleep early in evening.
Q.Why does the sky appear dark instead of blue to an astronaut?
Ans. Colour of the sky is on account of scattering of light of shorter wavelengths by particles in the atmosphere of earth. If the earth had no atmosphere, there would not have been any scattering and the sky would have looked dark. When an
astronaut in his spacecraft goes above the atmosphere of earth, sky appears dark to him because there is no scattering of light.
Q. Which part in the eye provides the most of refraction of the light entering the eye?
Q. Which part in the eye provides the most of refraction of the light entering the eye?
Ans. Cornea and Aqueous humor.
Q. What is Presbyopia? How can it be corrected?
Ans. Presbyopia is an eye defect of old age. Elasticity of ciliary muscles becomes weak and it is not able to change shape and focal length of eye lens effectively to see near and far away objects.
To correct Presbyopia, bifocal lenses are used in spectacles. Bifocal lenses are in spectacles. Bifocal lenses have both converging as well as diverging lenses.
Q. State two main causes of person developing near sightedness.
Ans. A person develops near sightedness when
i. The eye lens has very high converging power
ii. The eyeball is too long so that image is not made on the retina but in front of the retina.
Q. A star seem higher than it actually is. Why?
Ans. The air higher up towards the outer space has a lower optical density than the air closer to the surface of the earth. So the light traveling towards the earth bends more due to dense air and hence it
appears to be higher than its actual height.
Q. Why do we see the Sun 2 minutes before the Sunrise?
Ans. We see the Sun 2 minutes before the Sunrise because of atmospheric refraction. The air near the earth is optically denser than that at higher altitude. The sun rays travel from a rarer to denser medium. So the light rays coming from sun to earth get refracted and bends towards the normal. As a result, the sun appears 2 minute before the actual sunrise.
Q. Why do we have two eyes for vision and not just one?
Ans:There are several advantages of our having two eyes instead of one. It gives a wider field of view. A human being has a horizontal field of view of about 150° with one eye and of about 180° with two eyes. The ability to detect faint objects is, of course, enhanced with two detectors instead of one.
Ans:There are several advantages of our having two eyes instead of one. It gives a wider field of view. A human being has a horizontal field of view of about 150° with one eye and of about 180° with two eyes. The ability to detect faint objects is, of course, enhanced with two detectors instead of one.
Q. Why ‘danger’ signal lights are red in colour?
Ans: The red colour light rays is least scattered by fog or smoke. Therefore, it can be seen in the same colour at a distance.So,‘danger’ signal lights are red in colour.