Question

17. Two lenses x and y are placed in close contact with
each other. If the power of y-lens is -10 D and focal
length of this combination is -16.6 cm, then x and
y are respectively
(1) Concave lens (f = 20 cm), convex lens
(f = 10 cm
(2) Convex lens (f = 25 cm), concave lens
(f = 10 cm)
(3) Concave lens (f = 20 cm), concave lens
(f = 10 cm)
(4) Convex lens (f = 25 cm), convex lens
(f = 10 cm)​

Answer 1

Answer:

option 2 is correct

when two lenses placed very close to each other then combination of focal length is 1/Fcom.=1/Fx+1/Fy let x lens focal length is Fxand y lens focal length is Fy here power of y lens is -10D using power of lens formula Fy(in cm)=100/power then Fy(cm)=100/-10 then Fy is -10 cm and we know that concave lens focal length in negative then y lens is concave and using lens combination formula ..Fcom=-16.6cm ,Fy =-10cm ,Fx=? then -1/16.6=1/Fx+(-1/10) ,, -1/16.6+1/10=1/Fx , 1/Fx= -10+16.6/166 ,,, 1/Fx =6.6/166 then Fx=166/6.6 so Fx=25.15 cm in positive so X lens is convex lens