Compound Microscope MCQ - NEET Practice Questions with Answers

Compound Microscope: 

A compound microscope is of two converging lenses called objective and eye lens. It is used to view magnified images of small objects on a glass slide. It can achieve higher levels of magnification than stereo or other low-power microscopes and reduce chromatic aberration.

$f_{\text{eyelens }}>f_{\text{objective }}$ and $(\text{ diameter }{)}_{\text{eyelens }}>(\text{ diameter }{)}_{\text{objective }}$
Intermediate image is real and enlarged. The final image is magnified, virtual and inverted.
Here in the diagram
$u_o=$ Distance of object from objective (o),
$v_o=$ Distance of image ( ${\mathrm{A}}^{\prime }{\mathrm{B}}^{\prime }$ ) formed by objective from objective,
$u_e=$ Distance of $A^{\prime }{B}^{\prime }$ from eye lens,
$v_e=$ Distance of final image from eye lens,
$f_{o=}$ Focal length of objective,
$f_{e=}$ Focal length of eye lens.

Case 1: Final image is formed at $\mathbf{D}$ : Magnification

$$m_D=\frac{v_o}{u_o}(1+\frac{D}{f_e})$$
 microscope tube (distance between two lenses) is $L_D=v_e+u_e$.
Generally, object is placed very near to the principal focus of the objective hence $u_o\equiv f_o$. The eye piece is also of small focal length and the image formed by the objective is also very near the eyepiece.

So $v_o\equiv L_D$, the length of the tube.
Hence, we can write

$$m_D=\frac{L}{f_o}(1+\frac{D}{f_e})$$
 

Case 2: Final image is formed at $\mathrm{\infty }:$ Magnification
$m_{\mathrm{\infty }}=\frac{v_0}{u_0}\cdot \frac{D}{f_e}$ and length of tube $L_{\mathrm{\infty }}=v_0+f_e$

In terms of length

$$m_{\mathrm{\infty }}=\frac{(L_{\mathrm{\infty }}-f_o-f_e)D}{f_o{f}_e}$$

- For large magnification of the compound microscope, both $f_o$ and $f_e$ should be small.
- If the length of the tube of microscope increases, then its magnifying power increases.
- The magnifying power of the compound microscope may be expressed as $M=m_o\times m_e$ where $m_0$ is the magnification of the objective and $m_e$ is magnifying the of eyepiece.