convex lens is f = dD d − D . . . .
. .( . ) Presbyopia Due to ageing, ciliary muscles become weak and the eye-lens become rigid (inflexible) andso the eye loses its power of accommodation. Because of this, an aged person cannot see the nearby objects clearly.
So, it is also called as ‘old age hypermetropia’. Some persons may have both the defects of vision - myopia as well as hypermetropia. This can be corrected by ‘bifocal lenses’. In which, upper part consists of concave lens (to correct myopia) used for distant vision and the lower part consists of convex lens (to correct hypermetropia) used for reading purposes.
Astigmatism In this defect, eye cannot see parallel and horizontal lines clearly. It may be inherited or acquired. It is due to the imperfect structure of eye lens because of the development of cataract on the lens, ulceration of cornea, injury to the refracting surfaces, etc. Astigmatism can be corrected by using cylindrical lenses.
. MICROSCOPE This is an optical instrument, which helps us to see tiny (very small) objects. It is classified as . Simple microscope .
Compound microscope Simple Microscope Simple microscope has a convex lens of short focal length. It is held near the eye to get enlarged image of small objects. Let an object (AB) is placed at a point within the principal focus (u < f) of the convex lens and the observer’s eye is placed just behind the lens. As per this position the convex lens produces an erect, virtual and enlarged image (A'B').
The image formed is in the same side of the object and the distance equal to the least distance of distinct vision (D) (For normal human eye D = cm). Uses of Simple microscope Simple microscopes are used a) by watch repairers and jewellers. b) to read small letters clearly. c) to observe parts of flower, insects etc.
d) to observe finger prints in the field of forensic science. Compound microscope Compound microscope is also