What is meant by power of a lens? Write its SI unit. A student uses a lens of focal length 40 cm and another of -20 cm. Write the nature and power of each lens.
What is meant by power of a lens? Write its SI unit. A student uses a lens of focal length 40 cm and another of -20 cm. Write the nature and power of each lens.
Official answer
The power of a lens is defined as the reciprocal of its focal length. It is expressed in dioptre (D). 1 D = 1 m^(-1). The lens having a focal length of 40 cm has a power of 100/40 = 2.5 D and is a convex lens as it has a positive power. The lens having a focal length of -20 cm has a power of 100/-20 = -5 D and is a concave lens as it has a negative power.
Marking scheme
- 1.3 marks
Hint
Recall the definition of power of a lens and its SI unit. Determine the nature of the lens based on its focal length.
Quick oral answer
The power of a lens is a measure of its ability to converge or diverge light rays. It is defined as the reciprocal of the focal length of the lens. The SI unit of power is dioptre (D), which is equal to 1 m^(-1). A lens with a positive power is a convex lens, which converges light rays, while a lens with a negative power is a concave lens, which diverges light rays.
Common mistakes
- 1.Forgetting to consider the sign of the focal length when determining the power of the lens.
Analysis and explanation
The power of a lens is a measure of its ability to converge or diverge light rays. It is defined as the reciprocal of the focal length of the lens. The SI unit of power is dioptre (D), which is equal to 1 m^(-1). A lens with a positive power is a convex lens, which converges light rays, while a lens with a negative power is a concave lens, which diverges light rays.
Interesting facts
The power of a lens can be used to determine its focal length, and vice versa.
Question details
Q12 of 27 questions
- Board
- CBSE
- Class
- Class 10
- Subject
- Science
- Year
- 2018
- Section
- Section A
- Marks
- 3
- Type
- Short Answer
- Chapter
- Light — Reflection and Refraction
- Topic
- Refraction at Spherical Surfaces and by Lenses
- Bloom's level
- Remembering
- Syllabus topic
- Refraction at Spherical Surfaces and by Lenses
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Frequently asked questions
How many marks does this question carry in CBSE Class 10 Science 2018?
This question carries 3 marks in the CBSE Class 10 Science 2018 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 at Spherical Surfaces and by Lenses" from CBSE Class 10 Science.
What type of question is this in the CBSE Class 10 Science 2018 paper?
This is a Short Answer question from Section A in the CBSE Class 10 Science 2018 paper.