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In an astronomical telescope in normal adjustment, a straight black line of length L is drawn on the objective lens. The eyepiece forms a real image of this line whose length is ЁЭСЩ. What is the angular magnification of the telescope?
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Let fo and fe be the focal length of the objective and eyepiece respectively.
For normal adjustment, the distance from the objective to the eyepiece is fo + fe.
Taking the line on the objective as an object and the eyepiece as a lens.
u = -(fo + fe) and f = fe
`1/v = 1/([-{f_o + f_e}]) = 1/f_e` ⇒ v = `((f_o + f_e)/f_o)f_e`
Linear magnification (eyepiece) = `v/u = "Image size"/"Object size" = f_e/f_o = l/L`
∴ Angular magnification of the telescope
M = `f_o/f_e = L/l`
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Why should the objective of a telescope have large focal length and large aperture? Justify your answer.
Draw a ray diagram depicting the formation of the image by an astronomical telescope in normal adjustment.
You are given the following three lenses. Which two lenses will you use as an eyepiece and as an objective to construct an astronomical telescope ? Give reason
| Lenses | Power (D) | Aperture (cm) |
| L1 | 3 | 8 |
| L2 | 6 | 1 |
| L3 | 10 | 1 |
With regard to an astronomical telescope of refracting type~ state how you will increase its:
1) magnifying power
2) resolving power
State the condition under which a large magnification can be achieved in an astronomical telescope.
Write two important limitations of a refracting telescope over a reflecting-type telescope.
A lady cannot see objects closer than 40 cm from the left eye and closer than 100 cm from the right eye. While on a mountaineering trip, she is lost from her team. She tries to make an astronomical telescope from her reading glasses to look for her teammates. (a) Which glass should she use as the eyepiece? (b) What magnification can she get with relaxed eye?
(i) What is meant by resolving power of a telescope?
(ii) State any one method of increasing the resolving power of an astronomical telescope.
Draw a ray diagram of a refracting astronomical telescope when final image is formed at infinity. Also write the expression for its angular magnification (magnifying power).
Read the following paragraph and answer the questions.
| A number of optical devices and instruments have been designed and developed such as periscope, binoculars, microscopes and telescopes utilising the reflecting and refracting properties of mirrors, lenses and prisms. Most of them are in common use. Our knowledge about the formation of images by the mirrors and lenses is the basic requirement for understanding the working of these devices. |
- Why the image formed at infinity is often considered most suitable for viewing. Explain
- In modern microscopes, multicomponent lenses are used for both the objective and the eyepiece. Why?
- Write two points of difference between a compound microscope and an astronomical telescope
OR
Write two distinct advantages of a reflecting type telescope over a refracting type telescope.
