CBSE Class 10 Science The Human Eye and the Colourful World Notes

POWER OF ACCOMMODATION

The process by which the ciliary muscles change the focal length of an eye lens to focus distant or near objects clearly on the retina is called the accommodation of the eye.

How Does an Eye Focus Objects at Varying Distances?

To focus on distant objects the ciliary muscles relax making the eye lens thin. As a result the focal length of the eye lens increases and we see the distant objects. But to focus on nearby objects the ciliary muscles contract making the eye lens thick. As a result the focal length of the eye lens decreases and we see the nearby objects. In short it is the adjustment of the focal length of the eye lens which enables us to focus on objects situated at different distances.

Near point or Least Distance of Distinct Vision Near point or least distance of distinct vision is the point nearest to the eye at which an object is visible distinctly. For a normal eye the least distance of distinct vision is about 25 centimetres. However, it varies with age of the person. For example, for infants it is only 5 to 8 cm.

Far Point
Far point of the eye is the maximum distance up to which the normal eye can see things clearly. It is infinity for a normal eye.

Range of Vision

The distance between the near point and the far point is called the range of vision.

DEFECTS OF VISION

A normal eye can see all objects over a wide range of distances i.e., from 25 cm toninfinity. But due to certain abnormalities the eye is not able see objects over such a wide range of distances and such an eye is said to be defective. Some of the defects of vision are

• Hypermetropia or long sightedness

• Myopia or short sightedness and

• Presbyopia

• Astigmatism

HYPERMETROPIA Hypermetropia is also known as far-sightedness. Hypermetropia or hyperopia is the defect of the eye due to which the eye is not able to see clearly the nearby objects though it can see the distant objects clearly. The near point, for the person, is farther away from the normal near point (25 cm). Such a person has to keep a reading material much beyond 25 cm from the eye for comfortable reading. This is because the light rays from a closeby object are focussed at a point behind the retina as shown in below figure. This defect arises either because (i) the focal length of the eye lens is too long, or (ii) the eyeball has become too small. This defect can be corrected by using a convex lens of appropriate power. This is illustrated in below figure. Eyeglasses with converging lenses provide the additional focusing power required for forming the image on the retina.

MYOPIA Myopia is also known as near-sightedness. A myopic person cannot see distant objects clearly because the far point of his eye is less than infinity. Myopia is the defect of the eye due to which the eye is not able to see the distant objects clearly. Myopia is due to: 

• the elongation of the eye ball, that is, the distance between the retina and eye lens is increased. 

• decrease in focal length of the eye lens. 

In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself. This defect may arise due to (i) excessive curvature of the eye lens, or (ii) elongation of the eyeball. This defect can be corrected by using a concave lens of suitable power. This is illustrated in below figure. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.

Objective Questions

Question. The human eye forms the image of an object at its
(a) cornea.
(b) iris.
(c) pupil.
(d) retina.
Answer : D

Question. 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

Question. The change in the focal length of human eye is caused due to
(a) ciliary muscles
(b) pupil
(c) cornea
(d) iris
Answer : A

Question. The persistence of vision for normal eye is
(a) (1/10)th of a second
(b) (1/16)  th of a second
(c) (1/6)  th of a second
(d)(1/18) th of a second
Answer : B

Question. The phenomenon of light responsible for the working of the human eye is
(a) reflection
(b) refraction
(c) power of accommodation
(d) persistence of vision.
Answer : B

Question. The coloured light that refracts most while passing through a prism is
(a) Yellow
(b) Violet
(c) Blue
(d) Red
Answer : B

Question. Which part of the eye refracts light entering the eye from external objects?
(a) Lens
(b) Cornea
(c) Iris
(d) Pupil
Answer : B

Question. The focal length of human eye lens can be changed due to
(a) Iris
(b) Ciliary muscles
(c) Contact lens
(d) Spectacles
Answer : B

Question. Long-sightedness or hypermetropia can be corrected by
(a) Contact lens
(b) Concave lens
(c) Convex lens
(d) Bifocal lens
Answer : C

Question. The part of eye that determines the colour of the eye of a person is
(a) Pupil
(b) Cornea
(c) Retina
(d) Iris
Answer : D

Question. The colour of sky is blue during day time, red during sunset and black at night. This is due to
(a) Scattering of light
(b) Small particles present in atmosphere
(c) Atmospheric refraction
(d) All of the above.
Answer : D

Question. The following illustration represents the

(a) Correction of eye defect
(b) working of a simple microscope
(c) both (a) and (b)
(d) none of the above
Answer : B

Very Short Answer Questions

Question. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?
Answer : A person with a myopic eye can use concave lens to restore proper vision.

Question. 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 : A student is suffering with the eye defect named myopia, in this defect person can see nearby objects clearly but cannot see far off objects distinctly. It can be corrected by using concave lens.

Question. What is the diameter of human eye?
Answer : 2.3 cm

Question. What is the function of iris?
Answer : Iris controls the size of the pupil.

Question. What type of image is formed on the retina?
Answer : Real, inverted image.

Question. What type of signals are generated and sent to the brain by light sensitive cells of retina?
Answer : Electrical signals.

Question. What is the function of crystalline lens of human eye?
Answer : The crystalline lens provides the proper focal length required to focus objects at different distances on the retina.

Question. Which part of the human eye helps in changing the thickness of lens?
Answer : Ciliary muscles.

Question. Which is the range of vision of normal eye?
Answer : 25 cm to infinity.

Question. What is persistence of vision?
Answer : When the image is formed on retina it remains there for 1/16th of a second and this property of eye is called persistence of vision.

Question. In which type of eye defect far point of the eye gets reduced?
Answer : Myopia.

Question. What is presbyopia?
Answer : It is the defect of eye in which one cannot see nearby as well as far objects clearly.

Question. What is dispersion of light?
Answer : The splitting of light into its various components (i.e., 7 colours) is called dispersion of light.

Question. What is spectrum?
Answer : The band of seven colours obtained due to the dispersion of white light is called spectrum.

Question. Give one main difference between the lens of human eye and lens of camera.
Answer : Lens of human eye has flexible aperture, its focal length can be changed. In camera focal length can not be changed for a lens.

Short Answer Questions

Question. 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 : For the myopic eye
u = – α
ν = –80 cm
f = 1
1/ν - 1/u = 1/f  lens formula
1/−80cm = 1/− α = 1/f

∴ f = –80 cm = –0.80 m
Power of the lens is P = 1/f
∴ P = 1/−0.80 m
= –1.25 D
A concave lens; P = –1.25 D.

Question. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Answer : Ciliary muscles can contract the lens of human eye to a certain limit because of which a person with normal vision can see the nearby objects clearly only if placed at 25 cm but if the object is placed closer to the eye than it cannot see the objects clearly.

Question. Why do stars twinkle?
Answer : Stars twinkle due to atmospheric refraction of light from the stars and changing density of air around the earth.

Question. Why does the sun appear reddish early in the morning?
Answer : When the sun rises early in the morning (or set in the evening), the light from sun travels through the thicker layer of air and larger distance of the atmosphere surrounding the earth. Hence the blue light scatters the most but red light does not scatters and reaches our eyes.