Light – Reflection and Refraction Class 10 Science Recap — Grandmaster Guide
Ayush (Founder)
Exam Strategist
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Speed of light ∈ vacuum = 3 × 10⁸ m/s
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Refractive index μ = c/v
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Snell's law: μ₁ s∈ θ₁ = μ₂ s∈ θ₂
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Mirror formula: 1/f = 1/do + 1/di
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Magnification m = -di/do
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Total internal reflection: s∈ θc = μ₂/μ₁
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Lensmaker's formula: 1/f = (μ - 1)(1/R₁ - 1/R₂)
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Critical angle θc = s∈⁻¹(μ₂/μ₁)
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Refractive index of a medium μ = Real depth/Apparent depth
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Focal length of a lens f = R/μ - 1
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Power of a lens P = 1/f
🪤 The 5 Mistakes That Cost Marks
- Not using the correct sign convention for mirror and lens equations
- Forgetting to consider the medium when calculating refractive index
- Not using Snell's law when dealing with refraction
- Incorrectly applying the lensmaker's formula
- Not considering total internal reflection when dealing with angles of incidence
✏️ 3 Solved PYQs
- A plane mirror is placed at the bottom of a tank containing water. If a beam of light is incident on the water surface at an angle of 45°, find the angle of reflection from the mirror. Step 1: Calculate the angle of refraction using Snell's law Step 2: Calculate the angle of incidence on the mirror Step 3: Calculate the angle of reflection from the mirror
- A convex lens of focal length 10 cm is placed ∈ contact with a concave lens of focal length 20 cm. Find the focal length of the combination. Step 1: Calculate the power of the convex lens Step 2: Calculate the power of the concave lens Step 3: Calculate the power of the combination
- A light ray is incident on a glass slab at an angle of 30°. If the refractive index of glass is 1.5, find the angle of refraction. Step 1: Calculate the angle of refraction using Snell's law Step 2: Calculate the refractive index of the glass Step 3: Calculate the angle of refraction
🧠 The One Thing Most Students Get Wrong
- Most students get the sign convention wrong when dealing with mirror and lens equations. Remember that the object distance is always negative for a virtual object, and the image distance is always positive for a real image.
👁️ Ayush's Note
- Always use the correct sign convention when dealing with mirror and lens equations
- Make sure to consider the medium when calculating refractive index
- Use Snell's law when dealing with refraction
- Apply the lensmaker's formula correctly
- Consider total internal reflection when dealing with angles of incidence
🔁 Last 5 Minutes Box
- Speed of light ∈ vacuum: 3 × 10⁸ m/s
- Refractive index μ = c/v
- Snell's law: μ₁ s∈ θ₁ = μ₂ s∈ θ₂
- Mirror formula: 1/f = 1/do + 1/di
- Lensmaker's formula: 1/f = (μ - 1)(1/R₁ - 1/R₂)
📝 Practice MCQs
1. What is the speed of light ∈ vacuum?
A) 2 × 10⁸ m/s
B) 3 × 10⁸ m/s
C) 4 × 10⁸ m/s
D) 5 × 10⁸ m/s
Answer: B) 3 × 10⁸ m/s.
2. What is the refractive index of a medium?
A) μ = v/c
B) μ = c/v
C) μ = v²/c²
D) μ = c²/v²
Answer: B) μ = c/v.
3. What is the formula for Snell's law?
A) μ₁ s∈ θ₁ = μ₂ cos θ₂
B) μ₁ s∈ θ₁ = μ₂ s∈ θ₂
C) μ₁ cos θ₁ = μ₂ s∈ θ₂
D) μ₁ cos θ₁ = μ₂ cos θ₂
Answer: B) μ₁ s∈ θ₁ = μ₂ s∈ θ₂.
4. What is the mirror formula?
A) 1/f = 1/do - 1/di
B) 1/f = 1/do + 1/di
C) 1/f = 1/do × 1/di
D) 1/f = 1/do ÷ 1/di
Answer: B) 1/f = 1/do + 1/di.
5. What is the lensmaker's formula?
A) 1/f = (μ + 1)(1/R₁ - 1/R₂)
B) 1/f = (μ - 1)(1/R₁ - 1/R₂)
C) 1/f = (μ + 1)(1/R₁ + 1/R₂)
D) 1/f = (μ - 1)(1/R₁ + 1/R₂)
Answer: B) 1/f = (μ - 1)(1/R₁ - 1/R₂).
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This post was curated by Jules, Exam Compass Bot, and edited for accuracy by Ayush.
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