CBSE Class 12 Physics: Astronomical Telescope Marking Scheme
(i) The images formed by the objective lens and the eyepiece are respectively:
Step 1 (Governing Principle): [0.5 Mark]
In an astronomical telescope, the objective lens forms a real, inverted, and diminished image of a distant object at its focal plane. This image acts as a real object for the eyepiece, which then acts as a simple magnifier to form a virtual, magnified final image.
Step 2 (Deduction): [0.5 Mark]
Objective lens: Real image. Eyepiece: Virtual image.
Step 3 (Final Answer): [0 Mark]
Correct Option: (B) real, virtual
(ii) The magnification produced by the telescope does not depend upon the:
Step 1 (Governing Formula): [0.5 Mark]
The magnifying power ($m$) of an astronomical telescope in normal adjustment is given by:
$$m = -\frac{f_o}{f_e}$$
where $f_o$ is the focal length of the objective and $f_e$ is the focal length of the eyepiece.
Step 2 (Analysis): [0.5 Mark]
The formula shows that $m$ depends on $f_o$ and $f_e$. The aperture determines the light-gathering power (brightness) and resolving power, but does not affect the angular magnification ($m$).
Step 3 (Final Answer): [0 Mark]
Correct Option: (D) apertures of objective lens and eyepiece
(iii) Which of the following statements is not correct for this telescope?
Step 1 (Governing Principle): [0.5 Mark]
For a telescope in normal adjustment, the length of the tube ($L$) is:
$$L = f_o + f_e$$
Step 2 (Evaluation of Statements): [0.5 Mark]
- (A) Correct: $f_o > f_e$ is required for magnification.
- (B) Correct: Since $m = f_o/f_e$, increasing $f_o$ increases $m$.
- (C) Incorrect: The distance between lenses is exactly $(f_o + f_e)$ in normal adjustment, not "more than".
- (D) Correct: Increasing $f_e$ decreases the ratio $f_o/f_e$.
Step 3 (Final Answer): [0 Mark]
Correct Option: (C) The distance between two lenses is more than $(f_o + f_e)$.
💡 EL10 Concept Insight:
Think of the objective lens as a "light bucket" that captures a tiny, real picture of a far-away star, and the eyepiece as a magnifying glass that you use to look closely at that tiny picture to make it look huge to your eye.