  Question

A person with a normal near point (25 cm) using a compound microscope with objective of focal length 8.0 mm and an eyepiece of focal length 2.5 cm can bring an object placed at 9.0 mm from the objective in sharp focus. What is the separation between the two lenses? Calculate the magnifying power of the microscope.

Solution

Focal length of the objective lens,$$f_o=8mm=0.8cm$$ Focal length of the eyepiece$$,\ f_e=2.5cm$$Object distance for the objective lens,$$\mu_o=-9mm=-0.9cm$$Least distance of distant vision $$d=25cm$$Image distance for the eyepiece,$$v_e=-d=-25cm$$Object distance for the eyepiece =$$u_e$$Using the lens formula, we can obtain the value of $$u_e$$$$\dfrac{1}{v_e}-\dfrac{1}{u_e}=\dfrac{1}{f_e}$$$$\dfrac{1}{-25}-\dfrac{1}{u_e}=\dfrac{1}{2.5}$$So, $$u_e=-2.27cm$$We can also obtain the value of the image distance for the objective lens $$v_o$$ using the lens formula.$$\dfrac{1}{v_o}-\dfrac{1}{u_o}=\dfrac{1}{f_o}$$$$\dfrac{1}{v_o}-\dfrac{1}{-0.9}=\dfrac{1}{0.8}$$$$v_o=7.2\ m$$ Distance between the objective lens and the eye piece is $$\lvert u_e\lvert+v_o$$so separation is $$2.27+7.2=9.47cm$$The magnifying power of the microscope is calculated as$$m=\dfrac{v_o}{|u_o|}(1+d/f_e)$$$$\Rightarrow\dfrac{7.2}{0.9}\left(1+\dfrac{25}{2.5}\right)=88$$PhysicsNCERTStandard XII

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