Chapter name – Human Eye and Colourful world
Topic name – Human Eye and Defects of Vision
Subjective Questions: 07
Total Marks: 15
Question 1: In an eye, where is image formed?
Ans. In an eye, image is formed on the retina.
Question 2: What is the cause of cataract?
Answer: Cataract is caused due to the formation of an opaque membrane over the crystalline lens of eye.
Question 3: What is iris? [1 Mark]
Answer: Iris is a dark muscular diaphragm that controls the size of the pupil of the eye.
Question 4: Calculate the maximum power of accommodation of a person having normal vision. [2 Marks]
Answer: The least distance of distinct vision ‘d’ for a person, having normal vision is 25 cm. Therefore, the maximum power of accommodation is –
Question 5: What happens to the image distance in an eye if we increase the distance of an object from the eye? [2 Marks]
Answer: For a normal eye, the image distance in the eye is fixed. When we increase the distance of an object from the eye, the focal length of the eye lens changes on the account of accommodating power of the eye to keep the image distance constant.
Question 6: Why does it take some time to see objects in a dim room when you enter the room from bright sunlight outside? [3 Marks]
Ans. In the bright sunlight, iris causes the pupil to become smaller so that only a small portion of light enters the eye and rods of the retina are also adjusted. When a person enters into a dim lighted room, each iris take some time to increase the diameter of the pupil, so that more amount of light can enter to see the objects clearly and rod cells of the retina also take some time to adjust themselves to get the picture of the object in the dim light.
Question 7: What is short sightedness or myopia? What causes myopia? How is myopia corrected?
What is presbyopia? How is it corrected? [5 Marks]
Ans. Myopia: Short sightedness or myopia is the defect of the eye vision due to which a person can see the near objects clearly but cannot see the far objects clearly.
Myopia is caused due to —
- The elongation of the eyeball.
- Decrease in the focal length of the eye lens.
Myopia can be corrected by using a concave lens of suitable power which will bring the image back on to the retina.
esbyopia: The accommodation of the eye decreases with ageing. It occurs due to the gradual weakening of the ciliary muscles and decreasing flexibility of the crystalline lens. The near point of the eye increases with age. This defect is known as presbyopia. Such a person may suffer from myopia and hypermetropia. This defect can be corrected by using bifocal lenses of suitable focal lengths. The upper part of the lens is concave lens for correction of myopia to see the distant objects and the lower part of the lens has convex lens to correct the hypermetropia to see the nearly objects clearly.