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प्रश्न
Due to gradual weakening of ciliary muscles and diminishing flexibility of the eye lens a certain defect of vision arises. Write the name of this defect. Name the type of lens required by such persons to improve the vision. Explain the structure and function of such a lens.
उत्तर
The defect caused due to gradual weakening of ciliary muscles and diminishing flexibility of the eye lens is presbyopia. Presbyopia is the defect of eye in which a person cannot see nearby objects comfortably and distinctly without corrective eye-glasses. A presbyopic eye has its near point greater than 25 cm and it gradually increases as the eye becomes older.
The type of lens required by such person to improve the vision is bifocal lens.
A bifocal lens consists of both convex lens and concave lenses. The convex lens used in bifocal lens is used to correct hypermetropia (far sightedness) and concave lens is used to correct myopia (short sightedness).
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संबंधित प्रश्न
Where must the object be placed for the image formed by a converging lens to be:
real, inverted and same size as the object?
A converging lens is used to produce an image of an object on a screen,object on a screen. What change is needed for the image to be formed nearer to the lens?
(a) increase the focal length of the lens
(b) insert a diverging lens between the lens and the screen
(c) increase the distance of the object from the lens
(d) move the object closer to the lens
Copy and complete the diagram below to show what happens to the rays of light when they pass through the concave lens:
When an object is placed 10 cm in front of lens A, the image is real, inverted, magnified and formed at a great distance. When the same object is placed 10 cm in front of lens B, the image formed is real, inverted and same size as the object.
What is the focal length of lens A?
When a fork is seen through lenses A and B one by one, it appears as shown in the diagrams. What is the nature of (i) lens A, and (ii) lens B? Give reason for your answer.
A concave lens, if kept at a proper distance from an object, can form its real image.
The power of a lens is negative. State whether it is convex or concave.
State two applications of a concave lens.
A concave lens has a focal length 30 cm. Find the position and magnification (m) of the image for an object placed in front of it at distance 30 cm. State whether the image is real or virtual?
Give two characteristics of the image formed by a concave lens.
