Question

In a compound microscope, the focal lengths of two lenses are 1.5 cm and 6.25 cm an object is placed at 2 cm from objective and the final image is formed at 25 cm from eye lens. The distance between the two lenses is –

• A. 6.00 cm
• B. 7.75 cm
• C. 9.25 cm
• D. 11.00 cm

Answer 1

Answer: (D)

11.00 cm

Answer 2

f_o = 1.5 cm, f_e = 6.25 cm, u_o = –2 cm, v_e = – D = – 25cm

By objective lens

or v_o = 6 cm

By eye piece

$\frac { 1 } { 6.25 } = \frac { 1 } { - 25 } - \frac { 1 } { - u _ { e } } \Rightarrow \frac { 1 } { u _ { e } } = \frac { 1 } { 6.25 } + \frac { 1 } { 25 } = \frac { 4 } { 25 } + \frac { 1 } { 25 } = \frac { 1 } { 5 }$

u_e = 5 cm, Length of tube = L = v_o + u_e
= 6.0 cm + 5.0 cm, L = 11 cm