CBSE Class 10 Science HOTs Human Eye and Colourful World Set 02

TOPIC : The Human Eye and Power of Accommodation

Question. The muscular diaphragm that controls the size of the pupil is
(a) cornea
(b) ciliary muscles
(c) iris
(d) retina
Answer: (c) iris

Question. The black opening between the aqueous humour and the lens is called
(a) retina
(b) iris
(c) cornea
(d) pupil
Answer: (d) pupil

Question. Near and far points of a young person normal eye respectively are:
(a) 0 and infinity
(b) 0 and 25 cm
(c) 25 cm and infinity
(d) 25 cm and 150 cm.
Answer: (c) 25 cm and infinity

Question. The ability of eye lens to adjust its focal length to form a sharp image of the object at varying distances on the retina is called
(a) power of observation of the eye
(b) power of adjustment of the eye
(c) power of accommodation of the eye
(d) power of enabling of the eye
Answer: (c) power of accommodation of the eye

Question. The image formed on the retina of the human eye is
(a) virtual and inverted
(b) real and inverted
(c) real and erect
(d) virtual and erect
Answer: (b) real and inverted

Question. The near point and the far point are determined with regards to the function of which part of the eye?
(a) Pupil
(b) Retina
(c) Eye-ball
(d) Ciliary muscles
Answer: (d) Ciliary muscles

Question. Which of the following can be directly affected if the iris does not function properly? 
(a) Identification of colours
(b) The amount of light entering the eye
(c) Transmission of visual information to the brain
(d) Finer adjustments for focussing the objects
Answer: (b) The amount of light entering the eye

Question. When the eye is looking at a distant objects, the ciliary muscles are in a
(a) contract position
(b) relaxed position
(c) neutral position
(d) stretched position
Answer: (b) relaxed position

Question. Which of the following correctly gives the sequence of events that take place when human eye changes its focus from a distant object to an object closer to the eye?
(a) Ciliary muscles relax \( \rightarrow \) Curvature of eye lens increases \( \rightarrow \) Focal length of eye lens increases
(b) Ciliary muscles contract \( \rightarrow \) Curvature of eye lens decreases \( \rightarrow \) Focal length of eye lens increases
(c) Ciliary muscles relax \( \rightarrow \) Curvature of eye lens decreases \( \rightarrow \) Focal length of eye lens decreases
(d) Ciliary muscles contract \( \rightarrow \) Curvature of eye lens increases \( \rightarrow \) Focal length of eye lens decreases
Answer: (d) Ciliary muscles contract \( \rightarrow \) Curvature of eye lens increases \( \rightarrow \) Focal length of eye lens decreases

Very Short Answer Type Questions

Question. What is meant by least distance of distinct vision? How does this vary between the very young and old people?
Answer: The minimum distance at which an object can be seen most distinctly without strain by the eye, is termed as least distance of distinct vision.
For an infant, it is about 5 to 8 cm.
For a young adult with normal vision, it is about 25 cm.
For an old person, the power of accommodation of the eye decreases due to gradual weakening of ciliary muscles and the diminishing flexibility of the eye lens. Hence the least distance of distinct vision for him generally increases.

Question. The limitation of an eye is that the distance from the eye lens to where the image formed is always the same. How is then it possible for the eye to form focussed images from the objects that are located at varying distances from the eye?
Answer: The ability of eye lens to adjust its focal length with the help of ciliary muscles, it is possible for the eye to form focussed images from the objects that are located at varying distances from the eye. This mechanism is called accommodation.

Question. Why does it take sometime to see the objects in dim light when you enter the room from bright sunlight outside?
Answer: In bright sunlight, the iris contracts the pupil to allow less light to enter the eye and in dim light, the iris expands the pupil to admit more light to see the object clearly. Therefore, it takes sometime to increase the size of pupil in dim light.

Question. Why are we not able to see the things clearly when we come out of a darkroom?
Answer: In dim light, the iris expands the pupil to allow more light to enter the eye. So, when we come out of a darkroom into the bright sunlight, a large amount of light enters into our eyes and due to glare feeling, we are not able to see the things clearly. Gradually, the iris contracts the pupil to allow less light to enter the eye to see the objects clearly. It take some time for the pupil. For that time interval, person is unable to see the things.

Question. In which of the following two cases the focal length of the eye lens will be more–
(i) When ciliary muscles of a normal eye is most relaxed.
(ii) When ciliary muscles of a normal eye is in most contracted state.
Explain with reason.
Answer: The focal length of the eye lens will be more in case (i) i.e. when ciliary muscles of a normal eye is most relaxed.
Reason: When ciliary muscles are relaxed, the eye lens becomes thin. Thus, its focal length increases.
(ii) When ciliary muscles is in most contracted state, curvature of eye lens increases. Lens becomes thicker. This decreases the focal length of eye lens.

Question. What happens to the image distance in the normal human eye when we decrease the distance of an object, say 10 m to 1 m? Justify your answer. 
Answer: There is no change in the image distance in the eye. The eye lens has the ability to adjust its focal length called accommodation. When object distance decreases, ciliary muscles contract and lens becomes thick and its focal length decreases. It facilitate the near vision.

Short Answer Type Questions 

Question. (a) Which part of the eye has delicate membrane and containing large number of light sensitive cells?
(b) What happens to the size of pupil of our eye in (i) dim light, (ii) bright light?
(c) State the structure of eye lens and its function.
Answer: (a) Retina contains large number of light sensitive cells known as rods and cones.
(b) (i) Increases (ii) Decreases.
(c) The eye lens of human eye is in the form of convex lens which as thicker in the middle and thinner at the edge. It converge the rays incident on it and form the real, inverted and diminished image on the retina.

Question. What is meant by the term ‘power of accommodation’ of human eye? How does it help a person to see nearby as well as distant objects clearly?
Or
What is power of accommodation? How ciliary muscles help in accommodation?
Answer: The ability of eye lens to adjust its focal length to form a sharp image of the object at varying distances on the retina, is called its power of accommodation.
Help by the ciliary muscles in accommodation:
When we are looking at nearby object, the ciliary muscles contract or strained. This increases the curvature of eye lens but its radius of curvature decreases. The eye lens then becomes thicker. As a result, the focal length of the eye lens decreases in such a way that a clear sharp image of nearby object is formed on the retina. Thus, the object is seen clearly to us.
When we are looking at distant object, these muscles are in relaxed position. This decreases the curvature of eye lens but its radius of curvature increases. The eye lens becomes thinner. As a result, the focal length of the eye lens increases. Therefore, the parallel rays coming from the distant object are focussed on the retina and the object is seen clearly to us.
Thus, the accommodation power of an eye helps a person to see nearby as well as distant objects clearly.

PRACTICE QUESTIONS

Question. Most of the refraction for the light rays occurs
(a) at the outer surface of the cornea
(b) at the inner surface of the cornea
(c) at the eye lens
(d) by the iris behind the cornea
Answer: (a) at the outer surface of the cornea

Question. The amount of light entering the human eye is regulated and controlled by
(a) cornea
(b) iris
(c) pupil
(d) crystalline lens
Answer: (b) iris

Question. In human eye, the pupil opens completely through the
(a) relaxation of eye lens
(b) contraction of ciliary muscles
(c) relaxation of ciliary muscles
(d) relaxation of iris
Answer: (c) relaxation of ciliary muscles

Question. State the function of pupil and ciliary muscles.
Answer: Pupil: It regulates and controls the amount of light entering the eye by adjusting its size. Ciliary muscles: They adjust the focal length of the eye lens by changing its curvature, enabling the eye to focus on objects at different distances.

Question. (a) State one function each of iris and cornea.
(b) Calculate maximum power of accommodation of a person having normal vision.
Answer: (a) Iris: It controls the size of the pupil. Cornea: It acts as a protective layer and provides most of the refraction for the light entering the eye. (b) For a normal eye, the near point is 25 cm and the far point is infinity. Power \( P = \frac{1}{f} \). The power for distance vision is \( P_{\infty} = \frac{1}{\infty} = 0 \). The power for near vision is \( P_{25} = \frac{1}{0.25} = 4\text{D} \). The maximum power of accommodation is \( 4\text{D} - 0 = 4\text{D} \).

TOPIC : Defects of Vision and their Correction

Question. Which of these is a reason why a far-sighted person needs a convex lens to correct his vision?
(a) The image forms in front of his retina.
(b) The image forms behind the retina.
(c) The image forms below the retina.
(d) The image forms on the retina.
Answer: (b) The image forms behind the retina.

Question. Under which of these can myopia and hypermetropia be classified? 
(a) Breakdown of tissues.
(b) Incorrect bending of light in the eye.
(c) Incorrect reflection of light by surfaces around us.
(d) Incorrect coordination with brain for colour.
Answer: (b) Incorrect bending of light in the eye.

Question. Myopia and hypermetropia can be corrected by
(a) concave and plano-convex lens
(b) concave and convex lens
(c) convex and concave lens
(d) plano-concave lens for both defects.
Answer: (b) concave and convex lens

Question. Bi-focal lens are required to correct
(a) cataract
(b) hypermetropia
(c) myopia
(d) presbyopia
Answer: (d) presbyopia

Question. The basic cause of refractive error that makes far-away objects look blurry is
(a) decrease in the focal length of eye-lens.
(b) increase in the focal length of eye-lens.
(c) decreasing curvature of eye-lens.
(d) weakening of ciliary muscles.
Answer: (a) decrease in the focal length of eye-lens.

Question. Corrective lens used to correct the myopic defect of eye forms a virtual image of distant object at
(a) near point of myopic eye
(b) far point of myopic eye
(c) least distance of distinct vision
(d) any where between near point and far point of the eye.
Answer: (b) far point of myopic eye

Question. The defective eye of a person has near point 0.5 m and far point is at 3 m. The power for corrective lens required for (i) reading purpose and (ii) seeing distant objects, respectively are:
(a) 0.5 D and +3D
(b) +2D and \( – \frac{1}{3} \text{D} \)
(c) – 2D and \( + \frac{1}{3} \text{D} \)
(d) 0.5 D and – 3.0 D
Answer: (b) +2D and \( – \frac{1}{3} \text{D} \)

Question. The defect of vision in which image of nearby objects is formed behind the retina is 
(a) short-sightedness
(b) far-sightedness
(c) presbyopia
(d) All of the options
Answer: (b) far-sightedness

Question. If curvature of eye lens causes decrease in its focal length, then the refractive defect of vision will be
(a) myopia
(b) hypermetropia
(c) presbyopia
(d) cataract
Answer: (a) myopia

Question. A person’s near point is at 45 cm and far point is at 2 m. What kind of corrective lens is BEST suited for his vision defect? 
(a) Convex
(b) Concave
(c) Bifocal
(d) Plano-convex
Answer: (c) Bifocal

Question. The defect of vision in which the person is able to see distant object distinctly but cannot see nearby objects clearly is called
(a) Long sightedness
(b) Far-sightedness
(c) Hypermetropia
(d) All of the options
Answer: (d) All of the options

Question. Name the defect of vision due to
(i) Power of the eye is too long.
(ii) Focal length of the eye lens is too long.
Answer: (i) Short-sightedness
(ii) Long-sightedness.

Question. When a person is said to have developed cataract? How is the vision of such a person restored?
Answer: When the crystalline lens of eye becomes hazy (or even opaque) due to the formation of thin membrane over it, this causes partial or complete loss of vision. This defect of eye is called cataract. The vision of the defected eye can be restored by the cataract surgery.

Question. The near point of a hypermetropic eye is 50 cm. What is the nature and power of the lens required to enable him to read a book placed at 25 cm from the eye?
Answer: Given: Object distance, \( u = -25 \), Image distance, \( v = -50 \text{ cm} \), Power of lens, \( P = ? \)
Using lens formula,
\( \frac{1}{f} = \frac{1}{v} - \frac{1}{u} \)
\( \implies \frac{1}{f} = \frac{1}{-50} - \frac{1}{-25} \)
\( \implies \frac{1}{f} = -\frac{1}{50} + \frac{1}{25} = \frac{1}{50} \)
or \( f = + 50 \text{ cm} = + 0.5 \text{ m} \)
The nature of lens is convex.
and power, \( P = \frac{1}{f(\text{m})} = \frac{1}{0.5} = + 2.0 \text{ D} \)

Question. Name the type of defect of vision a person is suffering from, if he uses convex lenses in his spectacles for the correction of his vision. If the power of the lenses is +0.5 D, find the focal length of the lenses. 
Answer: The defect of vision is hypermetropia.
Focal length of lenses, \( f = \frac{1}{P} = \frac{1}{+0.5 \text{ D}} = +2 \text{ m} \)

Short Answer Type Questions

Question. (a) Name the defects of vision when a person cannot see clearly:
(i) the nearby objects
(ii) the distant objects
(b) Ritu needs a lens of power -2D for correct of her vision.
(i) What kind of defect in vision is she suffering from?
(ii) What are the possible cause of this defect?
(iii) What is the nature of corrective lens? [KVS]
Answer: (a) (i) Hypermetropia (ii) Myopia
(b) (i) Ritu is suffering from myopia or short sightedness.
(ii) Causes: • Increase in size of eye ball. • Decrease in focal length of eye lens.
(iii) Nature of corrective lens: Concave/diverging lens.

Question. You are given three lenses namely, bifocal lens, convex lens and concave lens. Which lens would you prefer to correct the myopia, hypermetropia, and presbyopia respectively?
Answer:

Question. Explain why?
(a) A myopic person prefer to remove his spectacles while reading a book.
(b) A hypermetropic person prefer to remove his spectacles while looking at the sky. 
Answer: (a) A myopic person does not need spectacles while reading a book as he has the near point at 25 cm. If such person reads the book with corrective lens (concave) he will have to keep the book at a distance greater than 25 cm so that the image of book will be formed by the concave lens at 25 cm and moreover, the size of the book appears to him is also smaller then actual size. Therefore, the person prefer to remove his spectacles while reading a book.
(b) A hypermetropic person does not need spectacles to see distant objects as he has the far point at infinity. If such person uses spectacles (convex lens) to see the distant objects, the image will be formed before the retina due to increase in converging power and hence the person cannot see distant objects distinctly. Therefore, such person prefers to remove his spectacles while looking at the sky.