Light – Reflection and Refraction
INTRODUCTION:
Light – Reflection and Refraction is one of those Class 10 Physics chapters where understanding sign conventions, ray diagram rules, and three formulas (mirror formula, lens formula, magnification) unlocks almost every question. The chapter divides cleanly into two parts: reflection (how light bounces off mirrors - spherical mirrors, ray diagrams, mirror formula) and refraction (how light bends when passing through different media - lenses, refractive index, lens formula). Board exams consistently test ray diagrams (3 marks), numerical problems using formulas (3-5 marks), and "differentiate between" questions (concave vs convex mirrors/lenses). Master the New Cartesian Sign Convention, practice 10-15 numericals, and draw ray diagrams for all six standard object positions—this strategy alone secures 10+ marks in Board 2026.
REFLECTION OF LIGHT
Reflection of light
Reflection is the phenomenon of bouncing back of light into the same medium when it strikes a smooth surface.
Key terms:
· Incident ray: Light ray falling on the surface
· Reflected ray: Light ray bouncing back after reflection
· Normal: Perpendicular line drawn at the point of incidence
· Angle of incidence (i): Angle between incident ray and normal
· Angle of reflection (r): Angle between reflected ray and normal
Laws of reflection
1. The incident ray, reflected ray, and normal all lie in the same plane
Angle of incidence = Angle of reflection (∠i = ∠r)
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Spherical mirrors
Spherical mirror: Mirror whose reflecting surface is part of a hollow sphere.
Types
|
Mirror type |
Reflecting surface |
Effect on light |
Also called |
|
Concave mirror |
Inner curved surface (caved in) |
Converges parallel rays |
Converging mirror |
|
Convex mirror |
Outer bulging surface |
Diverges parallel rays |
Diverging mirror |
Important terms for spherical mirrors
|
Term |
Definition |
Symbol |
|
Pole (P) |
Center point of the mirror |
P |
|
Center of curvature (C) |
Center of the sphere of which mirror is part |
C |
|
Radius of curvature (R) |
Radius of the sphere of which mirror is part |
R |
|
Principal axis |
Line joining pole and center of curvature |
— |
|
Principal focus (F) |
Point on principal axis where parallel rays converge (concave) or appear to diverge from (convex) |
F |
|
Focal length (f) |
Distance between pole and focus |
f |
|
Aperture |
Diameter of the reflecting surface of mirror |
— |
Relationship: Focal length = Half of radius of
curvature
f=R/2
Ray diagram rules for spherical mirrors
For both concave and convex mirrors:
1. A ray parallel to principal axis, after reflection, passes through focus F (concave) or appears to come from F (convex)
2. A ray passing through focus F (concave) or directed towards F (convex), after reflection, becomes parallel to principal axis
3. A ray passing through center of curvature C, reflects back along the same path
4. A ray striking the pole makes equal angles with principal axis (incident and reflected)
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Image formation by concave mirror
|
Object position |
Image position |
Image nature |
Image size |
Ray diagram use |
|
At infinity |
At focus F |
Real, inverted |
Highly diminished (point) |
— |
|
Beyond C |
Between F and C |
Real, inverted |
Diminished |
— |
|
At C |
At C |
Real, inverted |
Same size |
— |
|
Between C and F |
Beyond C |
Real, inverted |
Magnified (enlarged) |
— |
|
At F |
At infinity |
Real, inverted |
Highly magnified |
Searchlights, headlights |
|
Between F and P |
Behind the mirror |
Virtual, erect |
Magnified |
Shaving mirror, makeup mirror, dentist mirror |
Image formation by convex mirror
|
Object position |
Image position |
Image nature |
Image size |
|
At infinity |
At focus F (behind mirror) |
Virtual, erect |
Highly diminished (point) |
|
Anywhere between infinity and pole |
Between P and F (behind mirror) |
Virtual, erect |
Diminished |
Uses of convex mirror: Rear-view mirrors in vehicles (gives wide field of view), shop security mirrors.
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Sign convention for mirrors (New Cartesian Sign Convention)
|
Measurement |
Sign |
Explanation |
|
Object distance (u) |
Always negative |
Object always in front of mirror |
|
Image distance (v) |
Negative if real (in front) |
Real images form in front |
|
Image distance (v) |
Positive if virtual (behind) |
Virtual images form behind mirror |
|
Focal length (f) |
Negative for concave |
Focus in front |
|
Focal length (f) |
Positive for convex |
Focus behind |
|
Height above principal axis |
Positive |
Upward direction positive |
|
Height below principal axis |
Negative |
Downward direction negative |
Mirror formula
1/f=1/v+1/u
Where:
· f = focal length of mirror
· v = image distance from pole
· u = object distance from pole
Remember: Use proper signs according to sign convention!
Magnification for mirrors
Linear magnification (m): Ratio of height of image to height of object
m=h′/h=−v/u
Where:
· h = height of object
· h' = height of image
· v = image distance
· u = object distance
Interpretation:
· If m is negative → Image is real and inverted
· If m is positive → Image is virtual and erect
· If |m| > 1 → Image is magnified (enlarged)
· If |m| < 1 → Image is diminished (smaller)
· If |m| = 1 → Image is same size as object
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REFRACTION OF LIGHT
Refraction of light
Refraction: The bending of light when it passes from one transparent medium to another.
Why does refraction occur? Light travels at different speeds in different media.
Key terms:
· Optically rarer medium: Medium where light travels faster (e.g., air)
· Optically denser medium: Medium where light travels slower (e.g., glass, water)
· Angle of incidence (i): Angle between incident ray and normal
· Angle of refraction (r): Angle between refracted ray and normal
Bending behavior:
· Light bends towards normal when entering denser medium (air → glass)
· Light bends away from normal when entering rarer medium (glass → air)
Laws of refraction
First law: Incident ray, refracted ray, and normal all lie in the same plane
Second law (Snell's Law): The ratio of sine of angle of incidence to sine of angle of refraction is constant
Sin i/sin r=constant=μ
Where μ (mu) is the refractive index.
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Refractive index
Refractive index represents the extent of bending of light in a medium.
1. Absolute refractive index: Refractive index of medium with respect to air/vacuum.
μm=c/vm
Where:
· c = speed of light in air/vacuum (3 × 10⁸ m/s)
· v_m = speed of light in medium
2. Relative refractive index: Refractive index of one medium with respect to another
μ21=v1/v2=μ2/μ1
Refractive indices of common substances:
· Air: 1.0003 (≈ 1)
· Water: 1.33
· Glass: 1.5
· Diamond: 2.42
Spherical lenses
Lens: Transparent refracting medium bounded by two surfaces, at least one of which is curved.
Types
|
Lens type |
Shape |
Effect on light |
Also called |
Focal length sign |
|
Convex lens |
Thicker at center, thinner at edges |
Converges parallel rays |
Converging lens |
Positive (+) |
|
Concave lens |
Thinner at center, thicker at edges |
Diverges parallel rays |
Diverging lens |
Negative (−) |
Important terms for lenses
|
Term |
Definition |
|
Optical center (C) |
Central point of lens; rays passing through it go undeviated |
|
Principal axis |
Line joining the two centers of curvature |
|
Principal focus (F) |
Point where parallel rays converge (convex) or appear to diverge from (concave) after refraction |
|
Focal length (f) |
Distance between optical center and principal focus |
Ray diagram rules for lenses
For both convex and concave lenses:
1. A ray parallel to principal axis, after refraction, passes through focus F (convex) or appears to come from F (concave)
2. A ray passing through optical center C goes straight without bending
3. A ray passing through focus F (convex) or directed towards F (concave), after refraction, becomes parallel to principal axis.
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Image formation by convex lens
|
Object position |
Image position |
Image nature |
Image size |
Use |
|
At infinity |
At focus F₂ |
Real, inverted |
Highly diminished |
— |
|
Beyond 2F₁ |
Between F₂ and 2F₂ |
Real, inverted |
Diminished |
Camera |
|
At 2F₁ |
At 2F₂ |
Real, inverted |
Same size |
— |
|
Between F₁ and 2F₁ |
Beyond 2F₂ |
Real, inverted |
Magnified |
Projector |
|
At F₁ |
At infinity |
Real, inverted |
Highly magnified |
— |
|
Between F₁ and optical center |
Same side as object |
Virtual, erect |
Magnified |
Magnifying glass |
Image formation by concave lens
|
Object position |
Image position |
Image nature |
Image size |
|
At infinity |
At focus F₁ |
Virtual, erect |
Highly diminished |
|
Anywhere between infinity and lens |
Between F₁ and optical center |
Virtual, erect |
Diminished |
Sign convention for lenses (New Cartesian Sign Convention)
Origin: All distances measured from optical center (C) of lens
|
Measurement |
Sign |
Explanation |
|
Object distance (u) |
Always negative |
Object always on left side (in front) |
|
Image distance (v) |
Positive if real (right side) |
Real images on opposite side |
|
Image distance (v) |
Negative if virtual (same side as object) |
Virtual images on same side |
|
Focal length (f) |
Positive for convex lens |
Converging lens |
|
Focal length (f) |
Negative for concave lens |
Diverging lens |
|
Height above principal axis |
Positive |
Upward positive |
|
Height below principal axis |
Negative |
Downward negative |
Lens formula
1/f=1/v−1/u
Where:
· f = focal length of lens
· v = image distance from optical center
· u = object distance from optical center
Note: Different from mirror formula! Minus sign instead of plus.
Magnification for lenses
m=h′/h=v/u
Where:
· h = height of object
· h' = height of image
· v = image distance
· u = object distance
Interpretation:
· If m is negative → Image is real and inverted
· If m is positive → Image is virtual and erect
· If |m| > 1 → Magnified
· If |m| < 1 → Diminished
Power of lens
Power (P): Ability of a lens to converge or diverge light rays.
P=1/f (in meters)
Unit: Dioptre (D)
Sign:
· Convex lens: Positive power
· Concave lens: Negative power
Example: If f = 50 cm = 0.5 m, then P = 1/0.5 = +2 D (convex lens).
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Mirrors vs Lenses comparison
|
Feature |
Concave mirror |
Convex mirror |
Convex lens |
Concave lens |
|
Nature |
Converging |
Diverging |
Converging |
Diverging |
|
Focal length sign |
Negative (−) |
Positive (+) |
Positive (+) |
Negative (−) |
|
Can form real image |
Yes |
No |
Yes |
No |
|
Can form virtual image |
Yes |
Yes |
Yes |
Yes |
|
Can form magnified image |
Yes |
No |
Yes |
No |
|
Formula |
1/f = 1/v + 1/u |
1/f = 1/v + 1/u |
1/f = 1/v − 1/u |
1/f = 1/v − 1/u |
|
Power |
Not defined |
Not defined |
Positive |
Negative |
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MCQs PYQ
1. The
angle of incidence equals the angle of reflection. This is:
(a) First law of reflection
(b) Second law of reflection
(c) Law of refraction
(d) Snell's law
Answer: (b) Second law of reflection. (CBSE 2020)
2. The focal length of a concave mirror is 15 cm. Its
radius of curvature is:
(a) 7.5 cm
(b) 15 cm
(c) 30 cm
(d) 45 cm
Answer: (c) R = 2f = 2 × 15 = 30 cm. (CBSE 2024)
3. A convex mirror is used as rear-view mirror because:
(a) It forms magnified image
(b) It gives wider field of view
(c) It forms real image
(d) It has long focal length
Answer: (b) Wider field of view. (CBSE 2023)
4. The SI unit of power of a lens is:
(a) Watt
(b) Dioptre
(c) Meter
(d) Candela
Answer: (b) Dioptre (D). (CBSE 2020)
5. A concave lens always forms an image which is:
(a) Real and magnified
(b) Real and diminished
(c) Virtual and diminished
(d) Virtual and magnified
Answer: (c) Virtual, erect, and diminished. (CBSE 2024)
6. The magnification produced by a plane mirror is:
(a) Zero
(b) +1
(c) −1
(d) Infinity
Answer: (b) +1 (same size, virtual, erect). (CBSE 2020)
7. If the image formed by a mirror is virtual, erect,
and magnified, the mirror is:
(a) Concave
(b) Convex
(c) Plane
(d) Either concave or convex
Answer: (a) Concave mirror (object between F and P). (CBSE 2023)
8. Light travels fastest in:
(a) Glass
(b) Water
(c) Diamond
(d) Vacuum
Answer: (d) Vacuum (or air). (CBSE 2020)
9. The power of a lens having focal length 50 cm is:
(a) +0.5 D
(b) −0.5 D
(c) +2 D
(d) −2 D
Answer: (c) P = 1/f = 1/0.5 = +2 D (convex lens). (CBSE 2024)
10. A ray of light bends towards normal when it enters
from air to glass because:
(a) Glass is denser than air
(b) Air is denser than glass
(c) Speed of light increases
(d) Refraction does not occur
Answer: (a) Glass is optically denser than air. (CBSE 2020).
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Short
Answer Questions (PYQ)
Q1. State laws of reflection.
Answer: (1) Incident
ray, reflected ray, and normal lie in the same plane. (2) Angle of incidence =
Angle of reflection (∠i
= ∠r). (CBSE
2020)
Q2. Define the principal
focus of a concave mirror.
Answer: Principal focus is the point on the principal axis where all light rays parallel to the principal axis converge after reflection from the concave mirror. (CBSE 2023)
Q3. Distinguish between real and virtual images.
Answer: Real image: formed by actual intersection of light rays, can be obtained on screen, always inverted. Virtual image: formed by apparent intersection of light rays, cannot be obtained on screen, always erect. (CBSE 2020)
Q4. What is meant by power of a lens? Write its SI unit.
Answer: Power of a
lens is its ability to converge (convex) or diverge (concave) light rays. It is
reciprocal of focal length in meters. SI unit: Dioptre (D). Formula: P = 1/f
(in meters). (CBSE 2024)
Q5. Why does a ray of light bend when it travels from one medium to another?
Answer: Light bends (refracts) because its speed changes when it travels from one medium to another. Speed depends on optical density of the medium. Light bends towards normal when entering denser medium and away from normal when entering rarer medium. (CBSE 2020)
Long Answer Questions (PYQ)
Q1. An object 5 cm
high is placed at a distance of 30 cm from a concave mirror of focal length 20
cm. Find the position, nature, and size of the image.
Answer:
Given: h = 5 cm, u = −30 cm, f = −20 cm
Using mirror formula: 1/f = 1/v + 1/u
1/(−20) = 1/v + 1/(−30)
1/v = 1/(−20) + 1/30 = (−3 + 2)/60 = −1/60
v = −60 cm (negative means real image, in front of mirror)
Magnification: m = −v/u = −(−60)/(−30)
= −2
Image height: h' = m × h = −2 × 5 = −10 cm (negative means inverted)
Answer: Image is at 60 cm in front of mirror, real, inverted, magnified (10 cm high). (CBSE 2020)
Q2. A convex lens has focal length 15 cm. An object is placed at 30
cm from the lens. Calculate image distance and magnification.
Answer:
Given: u = −30 cm, f = +15 cm
Using lens formula: 1/f = 1/v − 1/u
1/15 = 1/v − 1/(−30)
1/15 = 1/v + 1/30
1/v = 1/15 − 1/30 = (2 − 1)/30 = 1/30
v = +30 cm (positive means real image on opposite side)
Magnification: m = v/u = 30/(−30) = −1
Answer: Image distance = 30 cm (real), magnification = −1 (same size, inverted). (CBSE 2024)
Q3. Draw ray diagram for convex lens when object is placed between
F and 2F. State the nature, position, and size of image formed.
Answer: (Ray diagram description): Draw three rays: (1) Parallel to
principal axis, refracts through F₂
on other side, (2) Through optical center, goes straight, (3) Through F₁, emerges parallel to principal axis.
All three rays meet beyond 2F₂.
Image: Real, inverted, magnified, beyond 2F₂ (used in projectors). (CBSE 2020)
Q4. A concave lens has power −2 D. An object is placed at 50 cm
from the lens. Find image distance.
Answer:
Given: P = −2 D, so f = 1/P = 1/(−2) = −0.5 m = −50 cm
u = −50 cm
Using lens formula: 1/f = 1/v − 1/u
1/(−50) = 1/v − 1/(−50)
1/(−50) = 1/v + 1/50
1/v = 1/(−50) − 1/50 = (−1 − 1)/50 = −2/50 = −1/25
v = −25 cm (negative means virtual image on same side)
Answer: Image at 25 cm on same side as object, virtual, erect, diminished. (CBSE 2023)
Q5. Explain with
diagram how concave mirror is used in solar furnace.
Answer: Large concave mirror with sun at infinity focuses all parallel sun rays at its focus point. This concentrates enormous amount of solar energy at the focus, producing very high temperature. The object to be heated is kept at this focus point. This is the principle of solar furnace. (Diagram: Draw concave mirror with parallel rays from sun converging at focus). (CBSE 2020)
Conclusion
Light – Reflection and Refraction becomes manageable when you organize it into
clear sections: (1) Reflection (laws, spherical mirrors, ray diagrams
for 6 object positions with concave mirror and 1 with convex mirror, mirror
formula, magnification), and (2) Refraction (laws, refractive index,
spherical lenses, ray diagrams for 6 object positions with convex lens and 1
with concave lens, lens formula, magnification, power of lens). The key
strategy: memorize sign conventions first (u always negative, f negative for
concave mirror/lens, f positive for convex mirror/lens, v sign depends on
real/virtual), practice 15-20 numerical problems covering all formula
variations, and draw ray diagrams for all standard positions until you can do
them blindfolded. Remember that mirror formula uses + (1/f = 1/v + 1/u)
while lens formula uses − (1/f = 1/v − 1/u)—this single difference
causes most errors. This formula-based chapter rewards practice over theory and
can easily secure 10-12 marks in Board 2026 with systematic preparation
Download Class 10 Science Notes PDF
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