Light – Reflection and Refraction Class 10 Science is one of the most scoring chapters in Class 10 Science (Physics). This chapter covers how light behaves when it strikes smooth and transparent surfaces. You’ll learn about laws of reflection, refraction through lenses, mirror formula, and real-life applications such as lenses in cameras, microscopes, and the human eye. These important questions are designed to help you prepare for your CBSE 2025–26 Board Exam.
Light – Reflection and Refraction Class 10 Science Important Questions
Light is a form of energy that enables us to see objects around us. It travels in a straight line in a homogeneous medium. When light hits a polished surface like a mirror, it bounces back—this is called Reflection. When light passes through transparent materials like glass or water and changes its direction, it is called Refraction.
- Reflection helps us understand mirrors and images.
- Refraction explains lenses, optical devices, and vision correction.
- This chapter is fundamental for future topics in Optics in Class 11 and 12 Physics.
Key Concepts and Laws
| Concept | Explanation | Key Point |
|---|---|---|
| Reflection of Light | Bouncing back of light from a polished surface like a mirror. | Angle of incidence = Angle of reflection |
| Refraction of Light | Bending of light when it travels from one medium to another. | Occurs due to change in speed of light. |
| Mirror Formula | \( \frac{1}{f} = \frac{1}{v} + \frac{1}{u} \) | f: focal length, v: image distance, u: object distance |
| Lens Formula | \( \frac{1}{f} = \frac{1}{v} – \frac{1}{u} \) | Used for convex and concave lenses. |
| Refractive Index | \( n = \frac{\sin i}{\sin r} \) | Higher the refractive index, slower the light speed in that medium. |
Example or Analogy
Imagine a ray of light as a disciplined runner. When the runner (light) hits a wall, it bounces back—this is reflection. But if it runs into water, its speed and direction change—this is refraction. Everyday examples include seeing your face in a mirror or a straw appearing bent in a glass of water.
Important Formulas / Facts
\( \frac{1}{f} = \frac{1}{v} + \frac{1}{u} \) (Mirror Formula)
\( \frac{1}{f} = \frac{1}{v} – \frac{1}{u} \) (Lens Formula)
\( m = \frac{h_2}{h_1} = \frac{-v}{u} \) (Magnification Formula)
\( n = \frac{\text{Speed of light \in air}}{\text{Speed of light in medium}} \) (Refractive Index)
Chapter-wise Important Questions
| Topic | Example Questions | Concept Tested |
|---|---|---|
| Laws of Reflection | State and prove the laws of reflection with a neat diagram. | Understanding angles and incident/reflected rays. |
| Spherical Mirrors | Differentiate between concave and convex mirrors with examples. | Mirror types and image formation. |
| Mirror Formula | An object is placed 10 cm in front of a concave mirror of focal length 20 cm. Find the image position. | Mirror equation and sign conventions. |
| Refraction through Glass Slab | Explain why a pencil appears bent in a glass of water. | Apparent depth and refraction principle. |
| Lenses and Focal Length | Find the nature and position of the image for an object placed 15 cm from a convex lens of focal length 10 cm. | Lens formula and ray diagram application. |
| Refractive Index | Define refractive index and derive its relation with light speed. | Understanding optical density. |
Topic-wise Questions and Answers
| Topic | Example Question | Concept Tested | Answer (Student-Friendly Explanation) | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Laws of Reflection | State and prove the laws of reflection with a neat diagram. | Understanding angles and incident/reflected rays. |
Answer: The laws of reflection state that:
When a ray of light strikes a smooth surface like a mirror, it bounces back obeying these rules. \( \text{\Angle of incidence (i)} = \text{\Angle of reflection (r)} \) Diagram: A ray hitting a mirror at an angle to the normal and reflecting symmetrically demonstrates this equality. |
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| Spherical Mirrors | Differentiate between concave and convex mirrors with examples. | Mirror types and image formation. |
Answer: A concave mirror is curved inward, while a convex mirror is curved outward.
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| Mirror Formula | An object is placed 10 cm in front of a concave mirror of focal length 20 cm. Find the image position. | Mirror equation and sign conventions. |
Given: Object distance, \(u = -10\,\text{cm}\); focal length, \(f = -20\,\text{cm}\) Mirror formula: \( \frac{1}{f} = \frac{1}{v} + \frac{1}{u} \) \( \frac{1}{-20} = \frac{1}{v} + \frac{1}{-10} \) \( \frac{1}{v} = \frac{1}{-20} + \frac{1}{10} = \frac{1}{20} \) \( v = +20\,\text{cm} \) Therefore, the image forms at 20 cm in front of the mirror — real and inverted. |
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| Refraction through Glass Slab | Explain why a pencil appears bent in a glass of water. | Apparent depth and refraction principle. |
Answer: Light bends or refracts when it passes from one medium to another due to a change in speed. When light travels from water to air, it bends away from the normal, making the pencil appear bent at the surface. \( \text{Apparent depth} = \frac{\text{Real depth}}{\text{Refractive index}} \) |
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| Lenses and Focal Length | Find the nature and position of the image for an object placed 15 cm from a convex lens of focal length 10 cm. | Lens formula and ray diagram application. |
Given: \(u = -15\,\text{cm}, f = +10\,\text{cm}\) Using lens formula: \( \frac{1}{f} = \frac{1}{v} – \frac{1}{u} \) \( \frac{1}{10} = \frac{1}{v} + \frac{1}{15} \) \( \frac{1}{v} = \frac{1}{10} – \frac{1}{15} = \frac{1}{30} \) \( v = 30\,\text{cm} \) Result: The image forms at 30 cm on the other side of the lens, real and inverted. |
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| Refractive Index | Define refractive index and derive its relation with light speed. | Understanding optical density. |
Answer: The refractive index of a medium is the ratio of the speed of light in vacuum (\(c\)) to the speed of light in that medium (\(v\)). \( n = \frac{c}{v} \) It indicates how much the medium slows down light. A higher value means greater optical density and more bending of light. |
Solved Numerical Example
Question: A ray of light passes from air (n₁ = 1) into glass (n₂ = 1.5) at an angle of incidence 30°. Find the angle of refraction.
Using Snell’s law: \( n_1 \sin i = n_2 \sin r \)
\( 1 \times \sin 30° = 1.5 \times \sin r \)
\( \sin r = \frac{1}{1.5} \times 0.5 = 0.333 \)
\( r = 19.47° \)
Hence, the ray bends towards the normal as it enters the denser medium (glass).
Conclusion
Quick Quiz (2 Questions)- If an object is placed at 30 cm in front of a concave mirror of focal length 15 cm, find the image distance.
- Light travels from air (speed = 3×108 m/s) into water (speed = 2.25×108 m/s). Find the refractive index of water.
Answer Key
- Using \( \frac{1}{f} = \frac{1}{v} + \frac{1}{u} \), we get \( v = -30 \) cm (real image formed at the same distance).
- \( n = \frac{3×10^8}{2.25×10^8} = 1.33 \)