Light enables us to see the world around us. From the formation of images in bathroom mirrors and car rear-view mirrors to the magnification of microscopes and telescopes, the behavior of light governs optical devices. Light: Reflection and Refraction is Chapter 10 of the CBSE Class 10 Physics syllabus and carries high weightage in board examinations.
This guide covers laws of reflection, spherical mirrors (concave and convex), ray diagrams, Cartesian sign conventions, mirror formula, refraction through a glass slab, Snell’s law, refractive index, spherical lenses, lens formula, power of a lens in dioptres, and five step-by-step solved board exam numericals.
f = R / 2
P = 1 / f(m)
nᵢᵡ = vᵡ / vᵢ
- 1. Laws of Reflection & Spherical Mirrors
- 2. Ray Diagrams for Concave & Convex Mirrors
- 3. Cartesian Sign Convention & Mirror Formula
- 4. Refraction of Light, Glass Slab & Snell's Law
- 5. Absolute & Relative Refractive Index
- 6. Lenses & Ray Diagrams (Convex & Concave Lenses)
- 7. Lens Formula, Magnification & Power of Lens (Dioptres)
- 8. Solved Board Exam Numericals
- 9. Frequently Asked Questions (FAQ)
1. Laws of Reflection & Spherical Mirrors
Laws of Reflection:
- The angle of incidence (∠i) is always equal to the angle of reflection (∠r). (∠i = ∠r)
- The incident ray, the reflected ray, and the normal to the reflecting surface at the point of incidence all lie in the same plane.
• Converges parallel rays of light to a focus.
• Focal length (f): Negative (−)
• Forms real & inverted images (except when object is between F and Pole P).
• Uses: Shaving mirrors, dentist headlamps, searchlights, solar furnaces.
• Diverges parallel rays of light.
• Focal length (f): Positive (+)
• ALWAYS forms virtual, erect, and diminished images regardless of object distance.
• Uses: Rear-view mirrors in vehicles (provides wider field of view), blind spot mirrors at road intersections.
Key Terms for Spherical Mirrors:
• Pole (P): Center of the reflecting surface.
• Center of Curvature (C): Center of the sphere of which the mirror forms a part.
• Radius of Curvature (R): Radius of the sphere (Distance PC).
• Principal Focus (F): Point on principal axis where parallel rays converge (concave) or appear to diverge from (convex).
• Focal Length (f): Distance between Pole P and Focus F. Formula: R = 2f (or f = R/2).
2. Ray Diagrams for Concave & Convex Mirrors
| Object Position (Concave) | Image Position | Image Size | Image Nature |
|---|---|---|---|
| At Infinity | At Focus (F) | Highly Diminished (Point size) | Real & Inverted |
| Beyond Center of Curvature (C) | Between F and C | Diminished | Real & Inverted |
| At Center of Curvature (C) | At Center of Curvature (C) | Same Size as Object | Real & Inverted |
| Between C and F | Beyond C | Enlarged | Real & Inverted |
| At Focus (F) | At Infinity | Highly Enlarged | Real & Inverted |
| Between Focus (F) and Pole (P) | Behind the Mirror | Enlarged | Virtual & Erect |
Image Formation by Convex Mirror (2 Cases Only):
1. Object at infinity: Image formed at Focus F behind the mirror; highly diminished (point size); Virtual & Erect.
2. Object anywhere between infinity and Pole P: Image formed between P and F behind the mirror; diminished; Virtual & Erect.
3. Cartesian Sign Convention & Mirror Formula
New Cartesian Sign Convention Rules:
- The object is always placed on the left side of the mirror/lens.
- All distances are measured from the Pole P (or Optical Center O of lens).
- Distances measured in direction of incident light (+x axis) are positive (+).
- Distances measured opposite to incident light (−x axis) are negative (−).
- Distances measured perpendicular above principal axis (+y axis) are positive (+).
- Distances measured perpendicular below principal axis (−y axis) are negative (−).
• v = Image distance (− for real image in front; + for virtual image behind mirror)
• f = Focal length (− for Concave mirror; + for Convex mirror)
• m < 0 (Negative): Real and Inverted image.
• m > 0 (Positive): Virtual and Erect image.
4. Refraction of Light, Glass Slab & Snell's Law
Refraction is the bending of a light ray as it passes obliquely from one transparent medium into another due to a change in the speed of light.
- Rarer to Denser Medium: Light ray bends towards the normal (Speed decreases).
- Denser to Rarer Medium: Light ray bends away from the normal (Speed increases).
Refraction Through a Glass Slab (Lateral Displacement):
When a ray of light enters a rectangular glass slab obliquely, it refracts twice (air to glass, then glass to air). The emergent ray is parallel to the incident ray, but shifted laterally. This sideways shift is called lateral displacement.
5. Absolute & Relative Refractive Index
Absolute Refractive Index (n): Ratio of speed of light in vacuum/air (c = 3 × 10⁸ m/s) to speed of light in the given medium (v).
• Air: 1.0003 | Water: 1.33 | Kerosene: 1.44 | Glass (Crown): 1.52 | Diamond: 2.42 (Highest!)
Relative Refractive Index: Refractive index of medium 2 with respect to medium 1:
nᵢᵡ = vᵡ / vᵢ = nᵢ / nᵡ
6. Lenses & Ray Diagrams (Convex & Concave Lenses)
• Focal length (f): Positive (+)
• Forms real & inverted images (except when object is between F₁ and Optical Center O → forms enlarged, virtual & erect image behind object).
• Uses: Magnifying glass, cameras, microscopes, hypermetropia (farsightedness) correction.
• Focal length (f): Negative (−)
• ALWAYS forms virtual, erect, and diminished images between F₁ and O.
• Uses: Myopia (nearsightedness) corrective spectacles, peep-holes in doors.
| Object Position (Convex Lens) | Image Position | Image Size | Image Nature |
|---|---|---|---|
| At Infinity | At Focus F₂ | Highly Diminished (Point size) | Real & Inverted |
| Beyond 2F₁ | Between F₂ and 2F₂ | Diminished | Real & Inverted |
| At 2F₁ | At 2F₂ | Same Size as Object | Real & Inverted |
| Between F₁ and 2F₁ | Beyond 2F₂ | Enlarged | Real & Inverted |
| At Focus F₁ | At Infinity | Infinitely Large / Enlarged | Real & Inverted |
| Between Focus F₁ and Optical Center O | On the Same Side as Object | Enlarged | Virtual & Erect |
7. Lens Formula, Magnification & Power of Lens (Dioptres)
Notice magnification m = +v/u for lens vs m = -v/u for mirror.
Power of a Lens (P):
The Power of a lens is defined as the reciprocal of its focal length expressed in meters.
• Convex Lens Power: Positive (+) | Concave Lens Power: Negative (−)
• Combination of Lenses: Total Power P = P₁ + P₂ + P₃ + ...
8. Solved Board Exam Numericals
Image is real → Magnification m = −3.
Formula: m = −v / u
−3 = −v / (−10)
−3 = v / 10 → v = −30 cm.
Object distance u = −5.00 m.
Mirror Formula: 1/f = 1/v + 1/u → 1/v = 1/f − 1/u
1/v = 1/1.50 − 1/(−5.00) = 1/1.50 + 1/5.00 = (5.00 + 1.50) / 7.50 = 6.50 / 7.50
v = 7.50 / 6.50 = +1.15 m.
Magnification m = −v/u = −(1.15) / (−5.00) = +0.23.
Formula: n = c / v → v = c / n
v = (3 × 10⁸) / 1.50 = 2 × 10⁸ m/s.
Concave lens always forms virtual image on same side → v = −10 cm.
Lens Formula: 1/f = 1/v − 1/u → 1/u = 1/v − 1/f
1/u = 1/(−10) − 1/(−15) = −1/10 + 1/15 = (−3 + 2) / 30 = −1/30
u = −30 cm.
Magnification m = v / u = (−10) / (−30) = +1/3 = +0.33.
Formula: P = 1 / f(m) → f(m) = 1 / P
f = 1 / 1.5 = 2/3 m = +0.667 m = +66.7 cm.
Since power and focal length are POSITIVE, the lens is a Convex (Converging) Lens.
Explore Related CBSE Class 9 & 10 Science Guides
9. Frequently Asked Questions (FAQ)
Reflection is the bouncing back of a light ray into the same medium when it strikes a polished surface (like a mirror). The angle of incidence equals the angle of reflection (∠i = ∠r).
Refraction is the bending of a light ray when it passes obliquely from one transparent medium to another due to a change in the speed of light in different media.
Snell’s Law states that the ratio of the sine of the angle of incidence (∠i) to the sine of the angle of refraction (∠r) is a constant for a light of a given color and for a given pair of media:
sin i / sin r = constant = nᵢᵡ
where nᵢᵡ is the relative refractive index of medium 2 with respect to medium 1.
Convex mirrors are preferred as rear-view mirrors in automobiles because:
(1) They always form an erect and virtual image of traffic behind.
(2) They form a diminished image, which provides a much wider field of view compared to a plane mirror, allowing the driver to see large traffic areas behind them.
Mirror Formula: 1/f = 1/v + 1/u | Magnification m = −v/u
Lens Formula: 1/f = 1/v − 1/u | Magnification m = +v/u
Sign Convention: Object distance u is always negative (−). Focal length f is negative for concave mirror/lens and positive for convex mirror/lens. Real image distance v is negative for mirrors (in front) but positive for lenses (opposite side).
1 Dioptre (1 D) is defined as the power of a lens whose focal length is 1 meter (f = 1 m). Power P = 1 / f(m). A convex lens of focal length 1 m has a power of +1 D, whereas a concave lens of focal length 1 m has a power of −1 D.
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