Q27
1 markMCQSection B

Which of the following can be used to find the focal length of a convex lens ?

Light — Reflection and Refraction
Refraction by Spherical Lenses

Options

(A)Object at a distance of 10 cm10\text{ cm} for a lens of focal length 30 cm30\text{ cm}
(B)Light from a laboratory window
(C)Light from the sun
(D)Light from a tree next to the window
Official Answer

Light from the sun can be used to find the focal length of a convex lens because the sun is at an infinite distance, providing parallel rays that converge at the principal focus.

Suninfinityparallel raysfocus

Marking Scheme

  • 1Correct option selection (1 mark)

Hint

Look for the object that is furthest away.

Quick Oral Answer

We use the Sun because it is at infinity, and rays from infinity converge at the principal focus after passing through a convex lens.

Analysis & Explanation

To accurately find the focal length, we need incident rays that are parallel to the principal axis. Objects at a very large distance (infinity) produce such rays. The Sun is the most reliable distant source. As per the logic in Activity 9.2 (applied to lenses), these rays converge at the focus. A tree next to a window or a laboratory window might not be far enough to be considered 'at infinity' for a lens with a standard focal length.

Common Mistakes

  1. 1Choosing a nearby object like a window which will not produce a sharp image exactly at the focus.

Interesting Facts

Using the Sun to find focal length can actually burn the screen (paper) due to the concentration of heat energy, as mentioned in Activity 9.2.

Spotted a mistake or something unclear?

Tell us — we fix reported answers fast.

Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2011?

This question carries 1 mark in the CBSE Class 10 Science 2011 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 "Refraction by Spherical Lenses" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2011 paper?

This is a MCQ question from Section B in the CBSE Class 10 Science 2011 paper.