Light - Reflection and Refraction Revision Notes - Class 10 Science

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📌 Key Points

Speed of light in vacuum: c = 3 × 10⁸ m/s (constant in all media for calculation purposes)
Refractive index: n = c/v (ratio of speed in vacuum to speed in medium)
Laws of Reflection: (1) i = r (angle of incidence = angle of reflection), (2) Incident ray, reflected ray, and normal are coplanar
Angles in reflection are measured from the NORMAL, NOT from the surface
Plane mirror: Forms virtual, erect, same-sized image. u = v (object distance = image distance)
Mirror formula: 1/f = 1/v + 1/u. Magnification: m = -v/u
Concave mirror (f > 0): Converging. Forms real/virtual images depending on object position
Convex mirror (f < 0): Diverging. Always forms virtual, erect, diminished image
Snell's law: n₁ sin i = n₂ sin r. Light bends TOWARDS normal entering denser medium
Critical angle: sin θc = n₂/n₁ (from denser to rarer). TIR occurs when i > θc
Lens formula: 1/f = 1/v + 1/u (same as mirror). Magnification: m = v/u
Convex lens (f > 0): Converging. Real (beyond f) or virtual (within f) images
Concave lens (f < 0): Diverging. Always virtual, erect, diminished image
Power of lens: P = 1/f (f in metres). Unit: Dioptre (D)
Lens combination: P_total = P₁ + P₂ + ... or 1/f_total = 1/f₁ + 1/f₂ + ...

📘 Important Definitions

Reflection

Bouncing back of light when it strikes a surface. Follows i = r.

Refraction

Bending of light when it passes from one medium to another. Follows Snell's law.

Refractive Index

Ratio of speed of light in vacuum to speed in medium: n = c/v. Air ≈ 1, water ≈ 1.33, glass ≈ 1.5

Real Image

Formed by actual convergence of light rays. Can be projected on screen. Always inverted.

Virtual Image

Rays appear to diverge from a point but don't actually meet. Cannot be projected. Always erect.

Focal Length

Distance from mirror/lens to focal point. f = R/2 for mirrors. Positive for converging, negative for diverging.

Magnification

Ratio of image height to object height: m = h'/h = -v/u (mirrors), m = v/u (lenses). Negative = real & inverted.

Critical Angle

Angle of incidence in denser medium at which refracted ray grazes interface (r = 90°). sin θc = n₂/n₁

Total Internal Reflection

Complete reflection when light travels from denser to rarer medium at angle > critical angle. No refraction.

Power of Lens

Reciprocal of focal length: P = 1/f (f in metres). Unit: Dioptre. Positive for converging, negative for diverging.

🔢 Formulas & Laws

Speed of Light

c = 3 × 10⁸ m/s

Constant in vacuum. v = c/n in medium

Refractive Index

n = c/v

n is dimensionless. Higher n = denser medium

Mirror Formula

1/f = 1/v + 1/u

Also: 1/u + 1/v = 1/f. Valid for plane, concave, convex mirrors

Magnification (Mirror/Lens)

m = -v/u (mirror), m = v/u (lens)

m > 0: virtual/erect. m < 0: real/inverted

Focal Length

f = R/2

R = radius of curvature. Also: 1/f = 1/v + 1/u

Snell's Law

n₁ sin i = n₂ sin r

Also: sin i / sin r = n₂/n₁ = relative refractive index

Critical Angle

sin θc = n₂/n₁ = 1/n

For light going from medium (index n) to vacuum

Lens Formula

1/f = 1/v + 1/u

Same as mirror formula. f positive for convex, negative for concave

Lens Power

P = 1/f

f must be in metres. Unit: Dioptre (D)

Lens Combination

P_total = P₁ + P₂ + ... or 1/f = 1/f₁ + 1/f₂ + ...

Powers add algebraically

⚠️ Common Mistakes

✗ Wrong: Measuring angles from surface instead of from normal

✓ Correct: Always measure angle of incidence and reflection from the normal (perpendicular to surface), NOT from surface itself

✗ Wrong: Confusing which direction light bends during refraction

✓ Correct: Light bends TOWARDS normal when entering denser medium (water), AWAY from normal when entering rarer medium (air)

✗ Wrong: Using wrong sign convention for mirrors and lenses

✓ Correct: For mirrors: f positive for concave. For lenses: f positive for convex. Object distance always positive

✗ Wrong: Confusing real and virtual images

✓ Correct: Real = inverted, can be projected, formed by convergence. Virtual = erect, cannot project, formed by divergence

✗ Wrong: Thinking plane mirror can magnify

✓ Correct: Plane mirror always has m = 1 (same size image). Curved mirrors can magnify

✗ Wrong: Forgetting to convert focal length to metres for power

✓ Correct: Power P = 1/f where f MUST be in metres. If f = 50 cm = 0.5 m, then P = 2 D (NOT 50 D)

✗ Wrong: Confusing critical angle concept

✓ Correct: Critical angle occurs when light goes from DENSER to RARER medium. TIR happens when i > θc

✗ Wrong: Not understanding why concave mirror inverts at certain positions

✓ Correct: m = -v/u. When v is positive (real image), m is negative (inverted). When v is negative (virtual), m is positive (erect)

✗ Wrong: Thinking convex lens always forms virtual image

✓ Correct: Convex lens forms real image when u > f, virtual image when u < f (magnifying glass)

✗ Wrong: Confusing magnification and power

✓ Correct: Magnification = image size / object size (ratio). Power = 1/f (in dioptres). Different concepts

📝 Exam Focus

These questions are frequently asked in CBSE exams:

Mirror formula and magnification calculations with different object positions

Ray diagrams for plane and curved mirrors showing image formation

Snell's law application and critical angle problems

Lens formula with sign conventions and calculations

Power of lens and lens combination problems

Difference between real and virtual images with properties

Image formation by convex and concave lenses at different positions

Numerical problems on refraction and TIR

Applications: telescopes, microscopes, periscopes, optical fibres

Derivations: Mirror formula, Snell's law, lens formula

🎯 Last-Minute Recall