Physics : Chapter 01 : Refraction of Light

📘 Title: Refraction of Light

Chapter: 3
Subject: Physics
Grade: 9 (Kerala Syllabus)

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🔍 Summary (150–250 words)

The chapter Refraction of Light explains the bending of light as it travels from one medium to another due to a change in speed. It introduces the concept of refraction using real-life examples like the apparent bending of a stick in water or the apparent shift of objects seen through glass. The direction and behavior of light when moving between optically different media (denser or rarer) are analyzed.

Key principles include the laws of refraction, refractive index, and Snell’s law. The chapter distinguishes between the angle of incidence and angle of refraction, and discusses how these angles vary depending on the media involved. Important concepts such as real and apparent depth, total internal reflection, and critical angle are explained with diagrams and applications.

The chapter also covers how lenses use refraction to form images and the behavior of light rays through convex and concave lenses. Applications in everyday life, such as the working of optical instruments and mirages, are briefly discussed to show how refraction is part of many observable phenomena.

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📌 Capsule Notes

🌈 Basic Concept of Refraction:

• Refraction: Bending of light when it passes from one transparent medium to another.

• Caused by change in speed of light.

• Light bends towards normal when moving to a denser medium, away from normal in rarer medium.

🔁 Laws of Refraction:

1. The incident ray, refracted ray, and the normal all lie on the same plane.

2. Snell’s Law:
   $\dfrac{\sin i}{\sin r} = \text{constant} = n$ (refractive index)

📏 Refractive Index:

• Ratio of speed of light in vacuum to that in the medium.

• No unit; higher value means medium is denser optically.

• Refractive index of glass > water > air.

📐 Angles:

• Angle of Incidence (i): Angle between incident ray and normal.

• Angle of Refraction (r): Angle between refracted ray and normal.

🌀 Real and Apparent Depth:

• Apparent depth is less than actual depth in denser medium.

• Used in pools, aquariums.

🌟 Total Internal Reflection:

• Happens when light travels from denser to rarer medium.

• Conditions:

  • Light moves from denser to rarer.

  • Angle of incidence > critical angle.

• Applications: Optical fibres, diamond sparkle, mirage.

🔍 Lenses and Refraction:

• Convex Lens: Converges rays; forms real or virtual image.

• Concave Lens: Diverges rays; always forms virtual image.

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❓ Q&A – Questions with Answers

1. What is refraction of light?
   → Bending of light when it travels from one medium to another due to change in speed.

2. State the two laws of refraction.
   → (i) Incident ray, refracted ray, and normal lie in the same plane.
   (ii) $\dfrac{\sin i}{\sin r} = \text{constant}$ (Snell’s Law).

3. What is meant by refractive index?
   → Ratio of speed of light in vacuum to that in a medium.

4. When does light bend towards the normal?
   → When it moves from rarer to denser medium.

5. Give one example of refraction in daily life.
   → A pencil appears bent in water.

6. What is apparent depth?
   → The depth at which an object appears to be when viewed through a denser medium.

7. What is total internal reflection?
   → A phenomenon where light is completely reflected back into the denser medium instead of refracting.

8. What are the conditions for total internal reflection?
   → (i) Light travels from denser to rarer medium.
   (ii) Angle of incidence > critical angle.

9. Define critical angle.
   → The angle of incidence in a denser medium for which angle of refraction in rarer medium becomes 90°.

10. Name two applications of total internal reflection.
    → Optical fibres, diamond sparkle.

11. What is the value of refractive index for air?
    → Approximately 1.

12. Why does a coin in water appear raised?
    → Due to refraction, light rays bend, making the coin appear at a shallower depth.

13. Which lens converges light rays?
    → Convex lens.

14. Which lens always forms a virtual image?
    → Concave lens.

15. What happens to speed of light in denser medium?
    → It decreases.

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📚 Definitions & Key Terms

• Refraction: Bending of light at the boundary between two media.

• Refractive Index: A measure of how much light slows down in a medium.

• Angle of Incidence (i): Angle between incident ray and normal.

• Angle of Refraction (r): Angle between refracted ray and normal.

• Snell’s Law: $\dfrac{\sin i}{\sin r} = n$

• Total Internal Reflection: Complete reflection of light within a denser medium.

• Critical Angle: Minimum angle of incidence at which TIR occurs.

• Apparent Depth: The visual depth of an object viewed from outside the medium.

• Convex Lens: A lens that converges light rays.

• Concave Lens: A lens that diverges light rays.

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🧠 Main Points for Revision

• Refraction = Bending of light due to change in speed.

• Snell’s Law: $\dfrac{\sin i}{\sin r} = n$

• Light bends: Toward normal (denser), away from normal (rarer).

• TIR occurs: From denser to rarer, angle > critical angle.

• Refractive index: No unit; glass > water > air.

• Convex lens: Converging.

• Concave lens: Diverging.

• Apparent depth < real depth.

• TIR applications: Fibre optics, diamonds, mirages.

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📂 Topic-Wise Breakdown

I. Introduction to Refraction

• Light changes speed → changes direction.

• Examples from daily life.

II. Laws of Refraction

• Stated and explained with ray diagrams.

• Snell’s law introduced.

III. Refractive Index

• Defined and compared for common media.

• Table of values (air, water, glass).

IV. Apparent Depth

• Refraction effects on depth perception.

V. Total Internal Reflection

• Explained with diagrams.

• Critical angle defined.

• Real-life uses: mirage, optical fibre.

VI. Refraction by Lenses

• Convex and concave lens behavior.

• Ray diagrams and image formation.