The focal lengths of the objective and the eye piece of a compound microscope are 1.0 cm and 2.5cm respectively. Find the tube length of the microscope for obtaining a magnification of 300.

Solution :
magnification of compound microscope is given by,
m = m_e x m_o
where m_e and m_o are magnifications of eyepiece and objective lens respectively,
m = m_e x m_o = ( L/f₀) x (D/f_e)  ..... 1)
given : m = 300 ; f₀ = 1 cm ; f_e = 2.5 cm and D is near point of eye = 25 cm
From 1) we get,
 L = (m x f₀ xf_e) / D
L = ( 300 x 1 x 2.5 ) / 25 
L = 30 cm