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?

 

 

 
 
 
 
 

Answers (1)
S safeer

\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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