# Explain the working of a refracting telescope with the help of a labelled diagram. What are the main limitations of this type of telescope and how are these overcome in a reflecting telescope?

$\Rightarrow$ The telescope is used to provide angular magnification of distant objects. Hence, the objective has a large focal length and a much larger aperture than eyepiece.

$\Rightarrow$ Light from a distant object enters the objective and a real image is formed in the tube at its second focal point. The eyepiece magnifies this image producing a final magnified image.

$\Rightarrow$ The magnifying power,

$m\cong \frac{\beta }{\alpha }\cong \frac{h}{f_{e}}\cdot \frac{f_{o}}{h}=\frac{f_{o}}{f_{e}}$

$\beta \Rightarrow$ The angle subtended at the eye by the final image.

$\alpha \Rightarrow$ Angle the object subtends at the lens or the eye.

$\Rightarrow$ In this case, the length of the telescope tube is $f_{o}+f_{e}$.

Limitations of Refracting telescope

(i) The image formed is not free from any kind of chromatic aberrations and spherical aberrations.

(ii) The aperture of the objective lens should be large for high resolving power.

$\Rightarrow$ In Reflecting type telescopes, since the mirror is used there is no chance of chromatic aberration.

$\Rightarrow$ Since parabolic reflecting surface is chosen, spherical aberration is also removed.

$\Rightarrow$ Mechanical support is much less of a problem since a mirror weighs much less than a lens of the equivalent optical quality

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