Question

Many optical instruments consist of a number of lenses. They are combined to increase the magnification and sharpness of the image. The net power(P) of the lenses placed in contact is given by the algebraic sum of the individual powers P₁, P₂, P₃……... as: 

P = P₁ + P₂ + P₃ + ......

i. Opticians use powers, instead of focal lengths, for prescribing glasses because: 

a) It is convenient 

b) It is cumbersome 

c) Focal length calculation is lens system is complex 

d) Both a and c. 

ii. Lens system (consisting of several lenses in contact) is commonly used in the design of lenses of: 

a) Cameras

b) Microscopes 

c) Telescopes 

d) All 

iii. Lens system increases: 

a) Brightness 

b) Magnification 

c) Sharpness 

d) Both b and c. 

iv. Power of a concave lens is: 

a) Positive 

b) Negative

c) Neutral 

d) None 

v.  A combination of two lenses of power +2.0 D and +0.25 D is equivalent to a single lens of power; 

a) +2 D 

b) +3 D 

c) +2.25D 

d) None 

vi. Lens of power +2.0 D has focal length, equal to: 

a) +1 m 

b) +0.50 m 

c) 2 m 

d) None

Answer 1

(i) (d) 

(ii) (d) 

(iii) (d) 

(iv) (b) 

(v) (c) 

(vi) (b)