- Draw a labelled ray diagram to show the formation of image in an astronomical telescope for a distant object.
- Write three distinct advantages of a reflecting type telescope over a refracting type telescope.
- A convex lens of focal length 10 cm is placed coaxially 5 cm away from a concave lens of focal length 10 cm. If an object is placed 30 cm in front of the convex lens, find the position of the final image formed by the combined system.
CBSE OUTSIDE DELHI - SET 1 2009
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Alternate Answer
- Advantages:
- Less chromatic abberation.
- High resolving power.
- Large gathering power.
- Less spherical abberation.
The position of the image, formed by the convex lens, is given by
$\frac{1}{\text{v}'} - \frac{1}{30} = \frac{1}{10}$
$\therefore\text{v}' = 15\text{cm}$
$\therefore\text{For the concave lens, u =+(15 − 5 )cm}$
$\text{and }\text{f} = -10\text{cm}$
$\text{Hence }\frac{1}{\text{v}} -\frac{1}{10} = - \frac{1}{10}$
$\therefore\frac{1}{\text{v}} = 0 \text{ or } \text{v}\rightarrow\infty.$
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