Question: Q. 2. (i) Draw a ray diagram depicting the formation of the image by an astronomical telescope in normal adjustment.
(ii) 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) |
|---|---|---|
| $\mathrm{L}_{1}$ | 3 | 8 |
| $\mathrm{L}_{2}$ | 6 | 1 |
|---|---|---|
| $\mathrm{L}_{3}$ | 10 | 1 |
A [OD 2017]
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Solution:
Ans. (i) Try yourself similar Q. 1 SAT Question-II
(ii) Objective Lens : Lens $L_{1}$ Eyepiece Lens : Lens $L_{2}$ Reason :
The objective should have large aperture and large focal length while the eyepiece should have small aperture and small focal length.
[CBSE Marking Scheme, 2017]
Commonly Made Error
- Most of the students could not draw correct labelled ray diagram of the astronomical telescope.
[AI Q. 3. Draw a schematic ray diagram of reflecting telescope showing how rays coming from a distant object are received at the eye-piece. Write its two important advantages over a refracting telescope.
U[Delhi 2016]
Ans.
Advantages :
(i) Large gathering power
(ii) Large magnifying power
(iii) No chromatic aberration (iv) Spherical aberration is also removed
(v) Easy mechanical support
(vi) Large resolving power
(Any two) $1 / 2+1 / 2$
[CBSE Marking Scheme 2016]
