(a) (i) An object of $5\mathrm{cm}$ height is placed at a distance of $20\mathrm{cm}$ from the optical centre of a concave lens of focal length $18\mathrm{cm}$. Calculate (1) image distance and (2) the magnification in this case. (ii) Compare the values of magnification obtained by a concave lens and a convex lens when both the lenses form virtual images. OR (b) A convex lens can form a (i) real, inverted and magnified image as well as (ii) virtual, erect and magnified image of an object. If the focal length of the lens is $10\mathrm{cm}$, what should be the range of the object distance in both cases? Draw ray diagrams to justify your answer.

The lens maker’s formula is $\frac{1}{f} = (\mu - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right)$ where $f$ is the focal length of the lens, $\mu$ is the refractive index of the material of the lens and $R_1$ and $R_2$ are the radii of curvature of the two surfaces of the lens. The power of a lens is the reciprocal of its focal length, i.e., $P = \frac{1}{f}$ . The focal length of a lens of power $-2.5 \mathrm{D}$ is $f = \frac{1}{P} = \frac{1}{-2.5} = -0.4 \mathrm{m} = -40 \mathrm{cm}$

Question 303 marksShort AnswerSection C

$5\mathrm{cm}$

The Human Eye and the Colourful World

Refraction of Light

Official answer

\frac{1}{f} = (\mu - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right)

lens maker’s formula

power of a lens

focal length

Marking scheme

1. $1 mark for the formula, 1 mark for the substitution of values and 1 mark for the final answer.$

Hint

$Use the lens maker’s formula and the definition of power of a lens.$

Quick oral answer

$The lens maker’s formula is used to calculate the focal length of a lens. The power of a lens is the reciprocal of its focal length.$

Common mistakes

1. $Forgetting to use the correct sign for the focal length of a concave lens.$

Analysis and explanation

-2.5 \mathrm{D}

Interesting facts

$The lens maker’s formula is used to design lenses for cameras, microscopes and telescopes.$

Question details

Q30 of 39 questions

Board: CBSE
Class: Class 10
Subject: Science
Year: 2023
Section: Section C
Marks: 3
Type: Short Answer
Chapter: The Human Eye and the Colourful World
Topic: $Refraction of Light$
Bloom's level: Analyzing
Syllabus topic: $Refraction of light through a prism$

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Frequently asked questions

$How many marks does this question carry in CBSE Class 10 Science 2023?$

$This question carries 3 marks in the CBSE Class 10 Science 2023 examination.$

$Which chapter does this question come from in Science?$

$This question is from the chapter "The Human Eye and the Colourful World" in the CBSE Class 10 Science syllabus.$

$What topic does this question cover in Science?$

$This question covers the topic "Refraction of Light" from CBSE Class 10 Science.$

$What type of question is this in the CBSE Class 10 Science 2023 paper?$

$This is a Short Answer question from Section C in the CBSE Class 10 Science 2023 paper.$