Q33
1 markMCQSection B

If you are to determine the focal length of a convex lens, you should have

Light — Reflection and Refraction
Focal Length of a Convex Lens

Options

(A)a convex lens and a screen
(B)a convex lens and a lens holder
(C)a lens holder, a screen holder and a scale
(D)a convex lens, a screen, holders for them and a scale
Official Answer

To determine the focal length of a convex lens, a student must have a convex lens, a screen, holders for both of them, and a measuring scale.

convex lensscreenholdersscalefocal length

Marking Scheme

  • 1Identification of all essential apparatus (lens, screen, holders, and scale) (1 mark)

Hint

Think about how you will keep the lens and screen stable during the experiment and how you will measure the distance between them.

Quick Oral Answer

We use a distant object because the rays coming from infinity are parallel to the principal axis and, after refraction through the convex lens, converge at its focus, allowing us to measure the focal length directly as the distance between the lens and the screen.

Analysis & Explanation

To experimentally determine the focal length of a convex lens (for example, by focusing a distant object to form a sharp image on a screen), the following apparatus is required:

  1. Convex Lens: The refracting optical element under study.
  2. Screen: To receive the real and inverted image of the distant object.
  3. Holders: To keep both the lens and the screen stable, upright, and aligned along the same principal axis.
  4. Scale: To measure the distance between the center of the lens and the screen, which corresponds to the focal length ff.

  • Option A is incorrect because without holders and a scale, the components cannot be aligned stably or measured.
  • Option B is incorrect because it lacks a screen to capture the image and a scale to measure the distance.
  • Option C is incorrect because the lens and screen themselves are missing.

Common Mistakes

  1. 1Choosing options that lack a measuring scale or holders, which are essential for accurate experimental measurement.

Interesting Facts

The method of focusing a distant object on a screen gives an approximate focal length because the light rays coming from a very distant object are assumed to be parallel to the principal axis, converging exactly at the principal focus.

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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 "Focal Length of a Convex Lens" 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.