Question

Your task in physics lab is to make a microscope from two lenses. One lens has a focal length of 12 cm , the other a focal length of 5.0 cm . You plan to use the more powerful lens as the objective, and you want its image to be 16 cm from the lens, as in a standard biological microscope.

A) How far (in cm) should the objective lens be from the object to produce a real image 16 cm from the objective?

B) What will be the magnification of your microscope? (I have answered 7.7 but it was incorrect.)

Answer 1

(A) Here for the microscope –

Focal length of the objective, fo = 5.0 cm

Focal length of eyepiece, fe = 12 cm.

Now, for the objective if 'u' is the object distance then from the lens formula –

1/fo = 1/u + 1/v ,

=> 1/5 = 1/u + 1/16

=> 1/u = 0.1375

=> u = 7.27 cm

(B) Again, regarding magnification, at near point –

M = v / u ( 1 + D / fe) , where D = 25 cm ( distance of distinct vision)
= 16 / 7.27 ( 1 + 25 / 12 ) = 6.78

So, magnification of the microscope = 6.78