Refractive Index MCQs Quiz | Class 10

Understanding Refractive Index: Key Concepts for Class 10 Science

The concept of refractive index is fundamental to understanding how light behaves when it travels from one transparent medium to another. It explains phenomena like the bending of light, the apparent depth of objects in water, and the working principles of lenses and prisms. This topic is crucial for Class 10 Science students studying the chapter on Light – Reflection and Refraction.

1. What is Refractive Index?

Refractive index is a measure of how much the speed of light is reduced when it passes through a medium compared to its speed in a vacuum. When light enters a new medium at an angle, its speed changes, causing it to bend. This bending is known as refraction.

• Definition: The refractive index (often denoted by ‘n’) of a medium is the ratio of the speed of light in a vacuum (c) to the speed of light in that medium (v).
• Formula: \(n = c / v\)
• Dimensionless: Since it’s a ratio of two speeds, the refractive index has no units.
• Optical Density: A higher refractive index indicates an optically denser medium, meaning light travels slower in it.

2. Absolute Refractive Index

The absolute refractive index of a medium is its refractive index with respect to a vacuum. Since the speed of light in a vacuum is the maximum possible speed, the absolute refractive index of any medium is always greater than or equal to 1.

From the table, it is clear that light travels fastest in vacuum and slowest in diamond among the listed materials.

3. Relative Refractive Index

Sometimes, light travels from one transparent medium to another, neither of which is a vacuum. In such cases, we talk about the relative refractive index of one medium with respect to another.

• Definition: The relative refractive index of medium 2 with respect to medium 1 (\(n_{21}\)) is the ratio of the speed of light in medium 1 (\(v_1\)) to the speed of light in medium 2 (\(v_2\)). It can also be expressed as the ratio of their absolute refractive indices.
• Formula: \(n_{21} = v_1 / v_2 = n_2 / n_1\)
• Bending Rules:
  • If light goes from an optically rarer medium (lower n) to an optically denser medium (higher n), it bends towards the normal.
  • If light goes from an optically denser medium (higher n) to an optically rarer medium (lower n), it bends away from the normal.

4. Factors Affecting Refractive Index

• Nature of the medium: Different materials have different optical densities.
• Wavelength of light: The refractive index varies slightly with the color (wavelength) of light. This phenomenon is called dispersion.
• Temperature: Generally, as temperature increases, the optical density of a medium decreases slightly, leading to a decrease in its refractive index.

Quick Revision Points

• Refraction is the bending of light when it passes from one medium to another.
• Absolute refractive index \(n = c/v\).
• Relative refractive index \(n_{21} = v_1/v_2 = n_2/n_1\).
• Optically denser medium has a higher refractive index and light travels slower.
• Light bends towards the normal when going from rarer to denser, and away from the normal when going from denser to rarer.
• Refractive index is a dimensionless quantity, typically >= 1.

Extra Practice Questions

1. Why does a coin placed at the bottom of a bucket of water appear to be raised?
2. Explain why the speed of light changes when it enters a different medium.
3. If light passes from a medium with refractive index ‘X’ to a medium with refractive index ‘Y’, and it bends away from the normal, what can you infer about X and Y?
4. What would be the relative refractive index of air with respect to water if the refractive index of water is 1.33?
5. Describe an experiment to demonstrate the refraction of light through a glass slab.