Define, 'refractive index of a transparent medium.' What is its unit? Which has a higher refractive index, glass or water?
Define, 'refractive index of a transparent medium.' What is its unit? Which has a higher refractive index, glass or water?
Official answer
The refractive index of a transparent medium is defined as the ratio of the speed of light in vacuum (or air) to the speed of light in that medium.
Mathematically: n = c / v
where c is the speed of light in vacuum and v is the speed of light in the given transparent medium.
Unit: The refractive index is a ratio of two speeds and therefore has no unit. It is a dimensionless quantity.
Higher refractive index: Glass has a higher refractive index than water.
(For reference: n(glass) ≈ 1.5, n(water) ≈ 1.33.)
Marking scheme
- 1.1 mark for the correct definition of refractive index (ratio of speed of light in vacuum to speed of light in the medium).
- 2.1 mark for stating the unit correctly (no unit / dimensionless).
- 3.1 mark for stating that glass has a higher refractive index than water.
Hint
Think of refractive index as a comparison between the speed of light in air/vacuum and in the medium. The unit comes from the definition — ratios of like quantities have no unit.
Quick oral answer
Refractive index of a transparent medium is the ratio of the speed of light in vacuum to the speed of light in that medium. It has no unit, and glass has a higher refractive index than water.
Common mistakes
- 1.Writing the definition as the ratio of speed of light in the medium to that in vacuum (inverted ratio).
- 2.Stating the unit as m/s or giving no unit by mistake.
- 3.Saying water has a higher refractive index than glass (incorrect).
Analysis and explanation
When light enters a denser transparent medium from vacuum, it slows down. The refractive index quantifies this slowing down. A higher refractive index means light travels more slowly in that medium and bends more towards the normal while entering it. Glass is optically denser than water, so its refractive index is higher. The fact that refractive index is a pure ratio makes it a dimensionless number with no unit.
Interesting facts
Diamond has one of the highest refractive indices among common transparent materials (around 2.42), which is why it sparkles.
The refractive index of air is very close to 1, which is why we often use air in place of vacuum for simple calculations.
Question details
Q17 of 36 questions
- Board
- CBSE
- Class
- Class 10
- Subject
- Science
- Year
- 2011
- Section
- Section A
- Marks
- 3
- Type
- Short Answer
- Chapter
- Light — Reflection and Refraction
- Topic
- Refractive index of a transparent medium
- Bloom's level
- remember
- Syllabus topic
- Refraction of light — refractive index
Spotted a mistake or something unclear?
Tell us and we will fix it.
Frequently asked questions
How many marks does this question carry in CBSE Class 10 Science 2011?
This question carries 3 marks 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 "Refractive index of a transparent medium" from CBSE Class 10 Science.
What type of question is this in the CBSE Class 10 Science 2011 paper?
This is a Short Answer question from Section A in the CBSE Class 10 Science 2011 paper.