Parallel light rays from a distant object incident on a device 'X' form a point size image on a screen. The device 'X' must be
Parallel light rays from a distant object incident on a device 'X' form a point size image on a screen. The device 'X' must be
Options
The correct answer is (D): either a concave mirror or a convex lens.
When parallel rays (from a distant object) are incident on:
• a CONCAVE mirror all rays converge and meet at the principal focus F, forming a real, point-sized image on a screen placed at F (as shown by the burning-paper activity in the NCERT text).
• a CONVEX lens all rays converge at the second principal focus F, again forming a real, point-sized image on a screen.
In both cases, a sharp, point-sized image is obtained on a screen. Therefore, device 'X' could be either a concave mirror or a convex lens.
Marking Scheme
- 11 mark for selecting the correct option (D).
Hint
Think about which mirrors/lenses converge parallel rays to a single point (focus) where a real image can be caught on a screen.
Quick Oral Answer
A convex lens of short focal length (like a magnifying glass held far from a wall in sunlight) can also burn paper — proving it converges parallel rays to a point just like a concave mirror.
Analysis & Explanation
Option (A) is partially correct because a concave mirror does form a point image of a distant object at F. Option (B) is also partially correct because a convex lens does the same at its focus. Since both devices can perform the task, the question's wording 'must be' forces us to choose the option that includes BOTH — which is (D). Option (C) is wrong because a concave lens diverges parallel rays; they never converge to form a real point image on a screen.
Common Mistakes
- 1Choosing only 'concave mirror' (A) and missing that a convex lens also converges parallel light.
- 2Choosing 'concave lens' (C) — it diverges light and cannot form a real image on a screen.
Interesting Facts
A concave mirror can actually burn a paper using sunlight (as in NCERT Activity 9.2) by concentrating the parallel solar rays at its focus.
The largest practical use of a convex lens converging parallel rays is in the objective lens of a telescope.
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Frequently Asked Questions
How many marks does this question carry in CBSE Class 10 Science 2012?
This question carries 1 mark in the CBSE Class 10 Science 2012 examination.
Which chapter does this question come from in Science?
This question is from the chapter "Light — Reflection and Refraction" in the CBSE Class 10 Science syllabus.
What topic does this question cover in Science?
This question covers the topic "Image formation by spherical mirrors and lenses for parallel rays" from CBSE Class 10 Science.
What type of question is this in the CBSE Class 10 Science 2012 paper?
This is a MCQ question from Section B in the CBSE Class 10 Science 2012 paper.