Lakhmir Singh Physics Class 10 Solutions Page No:281
Question 47: In a certain murder investigation, it was important to discover whether the victim was long-sighted or short-sighted. How could a detective decide by examining his spectacles ?
Solution : If the spectacle lenses are convex, the person was long sighted and if the spectacle lenses are concave, the person was short sighted.
Question 48: The picture given here shows a person wearing ‘half-moon’ spectacles. What sort of eye-defect do you think he has ? Why are these particular spectacles useful to him ?
Solution : Half moon spectacles are used for reading. so, the person has long-sightedness.
These particular spectacles are useful to him since the convex lenses of spectacles form the image of nearby object (like a book in hand) at the near point of his eye.
Question 49: A short-sighted person has a near point of 15 cm and a far point of 40 cm.
(a) Can he see clearly an object at a distance of:
(i) 5 cm ? (ii) 25 cm ? (iii) 50 cm ?
(b) To see clearly an object at infinity, what kind of spectacle lenses does he need ?
Solution : (a) (i) No, (ii) Yes, (iii) No
(b) Concave lenses
Question 50: The near point of a long-sighted person is 50 cm from the eye.The near point of a long-sighted person is 50 cm from the eye.
(a) Can she see clearly an object at :
(i) a distance of 20 cm ?
(ii) at infinity ?
(b) To read a book held at a distance of 25 cm, will she need converging or diverging spectacle lenses ?
Solution : (a) (i) No, (ii) Yes
(b) Converging lenses
Question 51: A person can read a book clearly only if he holds it at an arm’s length from him. Name the defect of vision :
(a) if the person is an old man
(b) if the person is a young man
Solution : (a) Presbyopia