Human Eye
|
|
|
|
|
|
|
|
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 is. For a normal
person’s eye, this distance is 25cm.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.
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) Power
of 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.
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 cm25 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
relationPower (P) = 1/f(i) Power of the lens (used for correcting
distant vision) = - 5.5 DFocal length of the lens (f) = 1/Pf=
1/-5.5 = -0.181 mThe 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 DFocal
length of the required lens (f) = 1/Pf = 1/1.5 = +0.667 mThe
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 natureand 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 theretina. 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,
We know Power P= 1/f(in meters)P=1/0.33
= +3.0DA 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. For this 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. Because all the light is coming from a single point, its path is
highly susceptible to atmospheric interference (i.e. their light is easily
diffracted).
Planets shine more steadily because … they're closer to Earth
and so appear not as pinpoints, but as tiny disks in our sky
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 to 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.
Comments
Post a Comment