Three thin lenses are combined by placing them in contact with each other to get more magnification in an optical instrument. Each lens has a focal length of 3 cm. If the least distance of distinct vision is taken as 25 cm, the total magnification of the lens combination in normal adjustment is

Question

Three thin lenses are combined by placing them in contact with each other to get more magnification in an optical instrument. Each lens has a focal length of 3 cm. If the least distance of distinct vision is taken as 25 cm, the total magnification of the lens combination in normal adjustment is (A) 9 (B) 26 (C) 300 (D) 3

Tardigrade Admin · Accepted Answer

Combined focal length ((1/F))=(1/F1)+(1/F2)+(1/F3) ⇒ (1/F)=(1/3)+(1/3)+(1/3) ⇒ F=1 cm Magnification of the lens combination in normal adjustment is M=1+(D/F)=1+(25/1)=26

Q. Three thin lenses are combined by placing them in contact with each other to get more magnification in an optical instrument. Each lens has a focal length of 3cm. If the least distance of distinct vision is taken as 25cm, the total magnification of the lens combination in normal adjustment is

9

26

300

3

Combined focal length (F1​)=F1​1​+F2​1​+F3​1​ ⇒F1​=31​+31​+31​ ⇒F=1cm Magnification of the lens combination in normal adjustment is M=1+FD​=1+125​=26

Combined focal length $(\frac{1}{F}) = \frac{1}{F _{1}} + \frac{1}{F _{2}} + \frac{1}{F _{3}}$
$\Rightarrow \frac{1}{F} = \frac{1}{3} + \frac{1}{3} + \frac{1}{3}$
$\Rightarrow F = 1 c m$
Magnification of the lens combination in normal adjustment is
$M = 1 + \frac{D}{F} = 1 + \frac{25}{1} = 26$