CBSE 2025, Set 1

Question

(a) (i) Draw a ray diagram of a reflecting telescope (Cassegrain) and explain the formation of image. State two important advantages that a reflecting telescope has over a refracting telescope.

(ii) In a refracting telescope, the focal length of the objective is 50 times the focal length of the eyepiece. When the final image is formed at infinity, the length of the tube is 102 cm. Find the focal lengths of the two lenses.

Edvaya · Accepted Answer

The correct answer is Subjective. **(i) Cassegrain Reflecting Telescope:** ![Cassegrain reflecting telescope ray diagram](https://www.edvaya.com/images/cbse/physics/2025/set1/q33a-cassegrain-reflecting-telescope.webp) **Formation of image:** - Parallel rays from a distant object fall on a large **concave (parabolic) primary mirror**. - These rays are reflected and converge towards the principal focus of the primary mirror. - Before reaching this focus, the rays are intercepted by a small **convex secondary mirror** placed just in front of the focus. - The secondary mirror reflects these rays through a **small hole** in the centre of the primary mirror. - The rays converge to form the **final image** which is viewed through an **eyepiece**. **Advantages of reflecting telescope over refracting telescope (any two):** 1. **No chromatic aberration** — mirrors reflect all wavelengths equally; no dispersion occurs. 2. **Less spherical aberration** — parabolic mirrors eliminate spherical aberration better than lenses. 3. **Larger aperture possible** — mirrors can be made much larger than lenses (no light absorption through glass); this gives higher **resolving power** and **brighter images**. 4. **Less mechanical support required** — mirrors can be supported from behind; heavy lenses sag. 5. **Higher magnifying power** achievable. --- **(ii) Focal lengths of refracting telescope:** Given: $f_o = 50 f_e$ and tube length $L = f_o + f_e = 102$ cm (for final image at infinity) Substituting: $$50f_e + f_e = 102 \implies 51f_e = 102$$ $$\boxed{f_e = 2 	ext{ cm}}$$ $$\boxed{f_o = 50 	imes 2 = 100 	ext{ cm}}$$

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