Light, Reflection, and Refraction Chapter 9 Class 10 Revision Notes

Light is one of the most important topics in Class 10 Science, and it helps us understand how we see objects around us. The chapter Light Reflection and Refraction Class 10 explains how light travels, how it changes direction, and how images are formed by mirrors and lenses. In these light reflection and refraction Chapter 9 class 10 notes, students learn the basic ideas of reflection of light, refraction of light, spherical mirrors, lenses, and the human eye in a simple and clear way.

These light reflection and refraction class 10 notes NCERT solutions are designed to help students understand the concepts step-by-step. When light falls on a surface, it may bounce back, which is called reflection, or it may bend when it passes from one medium to another, which is called refraction. These concepts are very important for understanding devices like mirrors, spectacles, microscopes, and cameras.

Students often look for class 10 light reflection and refraction notes PDF to revise the chapter quickly before exams. These notes also include key formulas, ray diagrams, and important definitions that make learning easier. Along with theory, light reflection and refraction class 10 questions and answers help students practice and test their understanding of the chapter.

Nature of Light 

Light is a fascinating physical phenomenon that has been studied and debated for centuries. Modern physics recognises its dual nature it behaves as both a wave and a particle depending on the experimental context.

Newton's Corpuscular Theory (Particle Nature)

Isaac Newton proposed that light travels as a stream of extremely small, fast-moving particles called corpuscles. This theory successfully explained reflection and refraction but failed to account for interference and diffraction.

Huygens' Wave Theory

Christian Huygens proposed that light propagates as mechanical waves requiring a medium (hypothetical "ether"). This was later refined: light waves are electromagnetic in nature and require no medium to travel. They can propagate through vacuum at a maximum speed of 3 × 10⁸ m/s.

Wave theory explained interference and diffraction but failed to explain the photoelectric effect.

Planck's Quantum Theory

Max Planck introduced the concept of photons discrete energy packets (quanta) of light. Each photon carries energy:

E = hν

Where:

• h = Planck's constant = 6.6 × 10⁻³⁴ J·s
• ν = frequency of light

Einstein used quantum theory to explain the photoelectric effect, earning the Nobel Prize.

De Broglie's Dual Nature

Louis de Broglie reconciled the two views: light exhibits both wave nature (interference, diffraction, polarisation) and particle nature (photoelectric effect, Compton effect). This is known as the wave-particle duality of light.

Theory	Explains	Fails to explain
Newton's Corpuscular	Reflection, Refraction	Interference, Diffraction
Huygens' Wave Theory	Interference, Diffraction	Photoelectric effect
Planck's Quantum Theory	Photoelectric effect	Interference, Diffraction
De Broglie's Dual Nature	All phenomena	—

Properties of Light

• Light travels in straight lines (rectilinear propagation).
• Its speed in vacuum/air is maximum: 3 × 10⁸ m/s (more precisely, 297,489,978 m/s).
• Light does not require a material medium it travels through vacuum.
• It exhibits reflection, refraction, interference, diffraction, polarisation, and double refraction.
• Luminous objects emit their own light (e.g., sun, stars, lamps).
• Non-luminous objects are visible only through scattered/reflected light (e.g., moon, books, trees).
• Human eyes are most sensitive to yellow light and least sensitive to violet and red. This is why commercial vehicles are painted yellow and sodium vapour lamps are used in street lighting.

Types of Medium

Medium	Description	Examples
Transparent	Light travels freely over large distances	Glass, water, air
Opaque	Light cannot pass through	Metals, wood, paper
Translucent	Light passes partially; intensity reduces quickly	Fog, butter paper

Propagation of Light

Ray of Light

A straight line showing the direction of travel of light is called a ray of light.

Beam of Light

A collection (bundle) of light rays is called a beam of light. Beams can be:

• Divergent — rays spread outward from a point
• Convergent — rays meet at a point
• Parallel — rays travel side by side

Reflection of Light

When light rays fall on a polished surface and bounce back into the same medium, the phenomenon is called reflection of light.

Terms

• Incident ray — the ray that strikes the reflecting surface
• Reflected ray — the ray that bounces back from the surface
• Normal — a line perpendicular to the reflecting surface 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 the normal at the point of incidence all lie in the same plane.
2. The angle of incidence equals the angle of reflection: ∠i = ∠r

These laws apply to all types of reflecting surfaces plane and spherical.

Types of Reflection

Regular (Specular) Reflection	Irregular (Diffuse) Reflection
Occurs on smooth, polished surfaces (mirrors)	Occurs on rough surfaces
Reflected rays are parallel	Rays scatter in all directions
Forms clear images	No distinct image formed

Plane Mirrors

A plane mirror is a highly polished flat (plane) surface. One side of a flat glass plate is coated with silver or nickel, making it reflective.

Properties of Image Formed by a Plane Mirror

1. Virtual — cannot be obtained on a screen
2. Erect — upright (not inverted)
3. Same size as the object
4. Formed behind the mirror, at the same distance as the object is in front
5. Laterally inverted — left and right are swapped

Lateral Inversion vs. Inversion

• Inversion: image turns 180° about a horizontal axis (upside down)
• Lateral inversion: image turns 180° about a vertical axis (left-right swap)

Multiple Images in Inclined Mirrors

When two plane mirrors are inclined at angle θ to each other, the number of images n formed is:

• If 360°/θ is even: n = (360/θ) − 1
• If 360°/θ is odd and object is symmetrically placed: n = (360/θ) − 1
• If 360°/θ is odd and object is asymmetrically placed: n = 360/θ
• If mirrors are parallel (θ = 0°): n = ∞

Example: θ = 60° → n = (360/60) − 1 = 5 images

Deviation by Plane Mirror

A plane mirror deviates light by: δ = 180° − 2i

Maximum deviation = 180° (at normal incidence, i = 0°)

Important Points

• Focal length of plane mirror = infinity
• Power of plane mirror = zero
• If mirror rotates by angle θ, reflected ray rotates by 2θ
• After reflection: speed, wavelength, and frequency remain unchanged, amplitude/intensity changes

Spherical Mirrors: Concave and Convex

A spherical mirror is a mirror whose reflecting surface forms part of a hollow sphere.

Types

Feature	Concave Mirror	Convex Mirror
Reflecting surface	Inner hollow surface	Outer bulging surface
Nature	Converging	Diverging
Image (generally)	Real and inverted (for real objects beyond F)	Virtual and erect
Use	Shaving mirror, doctor's torch, headlights	Rear-view mirror, street light reflectors

Important Terminology

Term	Definition
Pole (P)	Centre of the spherical mirror
Centre of curvature (C)	Centre of the hollow sphere of which the mirror is a part
Radius of curvature (R)	Radius of the hollow sphere
Principal axis	Line passing through C and P
Principal focus (F)	Point where parallel rays converge (concave) or appear to diverge from (convex) after reflection
Focal length (f)	Distance between pole P and focus F, f = R/2
Aperture	Effective width of the mirror

Rules for Drawing Ray Diagrams (Concave Mirror)

1. A ray parallel to the principal axis → reflects through focus F
2. A ray through focus F → reflects parallel to principal axis
3. A ray through centre of curvature C → retraces its own path
4. A ray at pole → reflects symmetrically about the principal axis (∠i = ∠r)

Image Formation by Concave Mirror

Object Position	Image Position	Nature of Image
At infinity	At F	Real, inverted, highly diminished
Between ∞ and C	Between F and C	Real, inverted, smaller
At C	At C	Real, inverted, same size
Between C and F	Between C and ∞	Real, inverted, enlarged
At F	At infinity	Real, inverted, infinitely large
Between F and P	Behind the mirror	Virtual, erect, enlarged

Image Formation by Convex Mirror

Object Position	Image Position	Nature
At infinity	At F (behind mirror)	Virtual, erect, highly diminished
Anywhere in front	Between P and F (behind mirror)	Virtual, erect, diminished

Convex mirror always forms virtual, erect, and diminished images making it ideal as a rear-view mirror (wider field of view than plane mirror).

Real vs Virtual Image

Real Image	Virtual Image
Formed by actual intersection of rays	Formed where rays appear to diverge from
Can be obtained on screen	Cannot be obtained on screen
Always inverted	Always erect
In front of mirror	Behind the mirror

Mirror Formula and Magnification

Mirror Formula

1/f = 1/u + 1/v

Where:

• u = object distance from pole (negative for real objects)
• v = image distance from pole
• f = focal length

Also: R = 2f and f = R/2

Sign Convention

• Distances measured from the pole
• Direction of incident light → positive
• Opposite to incident light → negative
• Heights above principal axis → positive
• Heights below principal axis → negative

Linear Magnification

m = -v/u = h_i/h_o = f/f-u = f-v/f

• m is negative → real, inverted image
• m is positive → virtual, erect image
• |m| > 1 → enlarged; |m| < 1 → diminished, |m| = 1 → same size

Power of a Mirror

P = -1/f(m) =-100/f(cm)

Power of a plane mirror = 0 dioptres

Refraction of Light 

Refraction is the bending of light as it passes from one medium to another, caused by a change in its velocity.

• Speed of light is maximum in vacuum (3 × 10⁸ m/s)
• Speed is less in denser media

Laws of Refraction (Snell's Law)

1. The incident ray, refracted ray, and normal at the point of incidence all lie in the same plane.
2. Snell's Law: μ₁ sin i = μ₂ sin r = constant

Where μ is the refractive index of the medium.

Relative Refractive Index

1μ₂ = μ₂/μ₁ = ν₁/v₂

When medium 1 is air: it becomes the absolute refractive index.

Bending of Light

Transition	Behaviour
Rarer → Denser	Ray bends towards the normal (∠i > ∠r)
Denser → Rarer	Ray bends away from the normal (∠i < ∠r)

Key fact: Refractive index is maximum for violet light and minimum for red light.

Apparent Depth and Normal Shift

When an object in a denser medium is viewed from a rarer medium:

d_apparent = d_actual/μ 

Normal shift: x =- d_ac (1-1/μ)

This is why objects submerged in water appear shallower than they really are.

Lateral Shift through a Glass Slab

d = t/cos r sin(i - r)

Where t = thickness of the slab.

Total Internal Reflection (TIR)

When light travels from a denser to a rarer medium and the angle of incidence exceeds the critical angle (θc), light does not refract it is entirely reflected back into the denser medium.

Conditions for TIR

1. Light must travel from denser → rarer medium
2. Angle of incidence > critical angle (i > θc)

Critical Angle Formula

sin θ_c = μ_R/μ_D= 1/μ

Transition	Critical Angle
Glass → Air	~42°
Water → Air	~49°
Glass → Water	~63°

Applications of TIR

Application	Explanation
Sparkling of diamonds	Refractive index of diamond = 2.5; θc = 24°. Cutting ensures i > θc, causing repeated TIR and brilliant sparkle
Optical fibres	Light undergoes multiple TIR along the core (higher μ) surrounded by cladding (lower μ), transmitting data with zero loss
Mirage	In deserts, air near hot ground has lower μ. Light from distant objects undergoes TIR, creating an illusion of water
Looming	In polar regions, μ decreases with height, causing TIR and apparent elevation of objects

TIR produces 100% reflection no intensity loss. This is why TIR-based images are brighter than those from mirrors or lenses.

Lenses - Convex and Concave

A lens is a transparent optical element bounded by two curved surfaces (or one curved and one plane).

Types of Lenses

Convex (Converging) Lens	Concave (Diverging) Lens
Thicker at centre	Thinner at centre
Converges parallel rays	Diverges parallel rays
Positive focal length	Negative focal length

Subtypes include: bi-convex, plano-convex, concavo-convex (convex), bi-concave, plano-concave, convexo-concave (concave)

Terms

• Optical Centre (O) — point through which any ray passes undeviated
• Principal Axis — line through the optical centre perpendicular to the lens
• First Principal Focus (F₁) — object point for which the image forms at infinity
• Second Principal Focus (F₂) — image point when object is at infinity (used for focal length convention)
• Focal Length (f) — distance from optical centre to F₂
• Aperture — effective diameter; image intensity ∝ (aperture)²

Rules for Image Formation (Thin Lens)

1. Ray through optical centre → passes undeviated
2. Ray through/towards first focus → emerges parallel to principal axis
3. Ray parallel to principal axis → passes through (or appears to pass through) second focus F₂

Image Formation by Convex Lens

Object Position	Image Position	Nature
At infinity	At F₂	Real, inverted, highly diminished
Between ∞ and 2F	Between F and 2F	Real, inverted, diminished
At 2F	At 2F	Real, inverted, same size
Between 2F and F	Between 2F and ∞	Real, inverted, enlarged
At F	At infinity	Real, inverted, infinitely large
Between F and O	On same side as object	Virtual, erect, enlarged

Image Formation by Concave Lens

Object Position	Image Position	Nature
At infinity	At F (same side as object)	Virtual, erect, highly diminished
Anywhere in front	Between F and O	Virtual, erect, diminished

A concave lens always forms a virtual, erect, diminished image regardless of object position.

Lens Formula, Power, and Magnification

Lens Formula

1/f = 1/v - 1/μ

Lensmaker's Formula

1/f = (μ_lens / μ_medium - 1) (1/R₁ - 1/R₂)

Focal length depends on: refractive index, radii of curvature R₁ and R₂, and wavelength (colour) of light.

Magnification of a Lens

M = h₁/h_o = v/u

Power of a Lens

P = 1/f(in metres)

Unit: Dioptre (D)

Combination of Lenses

In contact: 

1/F = 1/f₁ + 1/f₂, P = P₁ + P₂

Separated by distance d: 

1/F = 1/f₁ + 1/f₂ - d/f₁f₂, P = P₁+ P₂ - dP₁P₂

Sign Summary for Lenses

Quantity	Real Image	Virtual Image
u	−	−
v	+	−
h₁ (object height)	+	+
h₂ (image height)	−	+
m	−	+

Class 10 Chapter 9 Light, Reflection, and Refraction Solved Examples

Q: An object is placed in front of a plane mirror. If the mirror is moved away from the object by distance x, by how much does the image move?

Solution:

• Initial object distance = d; image is also at distance d behind mirror
• After mirror shifts by x: new object distance = d + x; new image distance = d + x behind mirror
• Original image position from object = 2d
• New image position from object = 2(d + x)
• Image moves by 2x

Q: A man 2 m tall with eye level at 1.84 m wants to see his full image. What is the minimum mirror length?

Solution:

• To see head (H): mirror segment AC = ½ × HE = ½ × (2 − 1.84) = 0.08 m
• To see feet (F): mirror segment BC = ½ × EF = ½ × 1.84 = 0.92 m
• Minimum mirror length = 0.08 + 0.92 = 1 m

Q: Two plane mirrors are inclined so a ray parallel to the second mirror reflects from it parallel to the first. Find the angle between them.

Solution:

• Let angle between mirrors = θ
• Using geometry in triangle OBC: 3θ = 180°
• θ = 60°

Q: Find image position, nature, and magnification when object is 10 cm in front of a concave mirror of R = 15 cm.

Solution:

• f = −R/2 = −7.5 cm, u = −10 cm
• 1/v = 1/f − 1/u = 1/(−7.5) − 1/(−10) = −2.5/30 = −1/30 × 2.5
• v = −30 cm (real image, 30 cm in front)
• m = −v/u = −(−30)/(−10) = −3 (real, inverted, 3× magnified)

Q: Object is 5 cm in front of same concave mirror (f = −7.5 cm).

Solution:

• 1/v = 1/(−7.5) − 1/(−5) = 1/15
• v = +15 cm (virtual image, 15 cm behind mirror)
• m = −(+15)/(−5) = +3 (virtual, erect, 3× magnified)

Q: Jogger approaches at 5 m/s. How fast does the image move when jogger is 39 m away?

Solution:

• f = +1 m, u = −39 m → v = fu/(u−f) = 1 × (−39)/(−39−1) = 39/40 m
• After 1 s: u = −34 m → v′ = 34/35 m
• Image displacement = 39/40 − 34/35 = (1365−1360)/1400 = 5/1400
• Image speed = 1/280 m/s

Q: A 5 cm needle placed 10 cm from a convex mirror of f = 40 cm. Find image.

Solution:

• 1/v = 1/40 + 1/10 = 5/40 = 1/8 → v = +8 cm (virtual, 8 cm behind mirror)
• m = −v/u = −8/(−10) = +0.8
• Image size = 0.8 × 5 = 4 cm (virtual, erect, diminished)

Q: A concave mirror of f = 10 cm is 35 cm from a wall. How far from the wall should the object be placed for its image to fall on the wall?

Solution:

• f = −10 cm, v = −35 cm
• 1/u = 1/f − 1/v = −1/10 + 1/35 = −7/70 + 2/70 = −1/14
• u = −14 cm
• Distance from wall = 35 − 14 = 21 cm

Q: Find object position for image 3× the size (f = 30 cm).

Solution (Case a — Real image, m = −3):

• m = f/(f−u) → −3 = −30/(−30−u) → u = −40 cm

Solution (Case b — Virtual image, m = +3):

• 3 = −30/(−30−u) → u = −20 cm

Q: A 3 cm square wire 25 cm from a concave mirror (f = 10 cm). Find area of image.

Solution:

• u = −25 cm, f = −10 cm
• 1/v = −1/10 + 1/25 = −3/50 → v = −50/3 cm
• m = −v/u = −(50/3)/(25) = −2/3
• Image side = (2/3) × 3 = 2 cm
• Area = (2)² = 4 cm²

Q: Light of frequency 5 × 10¹⁴ Hz travels through a liquid with measured wavelength 450 × 10⁻⁹ m. Find μ.

Solution:

• μ = c/(νλ) = (3 × 10⁸)/(5 × 10¹⁴ × 450 × 10⁻⁹)
• μ = 1.33

Q: Object 7.0 cm tall placed 27 cm in front of concave mirror of f = 18 cm. Find image.

Solution:

• 1/v = 1/(−18) − 1/(−27) = −3/54 + 2/54 = −1/54
• v = −54 cm (real, 54 cm in front)
• m = −(−54)/(−27) = −2
• Image height = −2 × 7 = −14 cm (real, inverted, enlarged)

Q: Object 5.0 cm placed 20 cm in front of a convex mirror of R = 30 cm.

Solution:

• f = +15 cm, u = −20 cm
• 1/v = 1/15 + 1/20 = 7/60 → v = +8.57 cm (virtual, behind mirror)
• m = −v/u = −(8.57)/(−20) = +0.43
• Image height ≈ +2.14 cm (virtual, erect, diminished)

Q: Object 3 cm tall is placed 10 cm from a convex lens of f = 15 cm. Find image.

Solution:

• 1/v = 1/f + 1/u = 1/15 + 1/(−10) = 2/30 − 3/30 = −1/30
• v = −30 cm (virtual, same side as object)
• m = v/u = (−30)/(−10) = +3
• Image height = 3 × 3 = 9 cm (virtual, erect, magnified)

Q: Object placed 20 cm in front of a concave lens of f = −15 cm. Find image.

Solution:

• 1/v = 1/(−15) + 1/(−20) = −4/60 − 3/60 = −7/60
• v = −60/7 ≈ −8.57 cm (virtual, same side as object)
• m = v/u = (−8.57)/(−20) = +0.43 (virtual, erect, diminished)

Q: A beam converges to point O, 10 cm behind a convex mirror of f = 20 cm. Find image.

Solution:

• u = +10 cm (virtual object), f = +20 cm
• 1/v = 1/20 − 1/10 = −1/20 → v = −20 cm (real, in front)
• m = −(−20)/(+10) = +2 (real, erect, enlarged)

Q: Two lenses of power +3D and −1D are placed in contact. Find combined focal length.

Solution:

• P = P₁ + P₂ = 3 + (−1) = +2 D
• F = 1/P = 0.5 m = 50 cm (converging lens system)

Q: A point light source is at the bottom of a tank of water (μ = 4/3) filled to height h = 12 cm. Find radius of bright circle on surface.

Solution:

• sin θc = 1/μ = 3/4
• r = h/√(μ² − 1) = 12/√((16/9) − 1) = 12/√(7/9) = 12 × 3/√7
• r = 36/√7 ≈ 13.6 cm

Q: A fish is 12 cm below the surface of water (μ = 4/3). How deep does it appear to an observer above?

Solution:

• d_apparent = d_actual/μ = 12/(4/3) = 12 × 3/4
• Apparent depth = 9 cm

Q: Two mirrors are inclined at 45°. How many images of an object are formed?

Solution:

• 360/45 = 8 (even integer)
• n = 8 − 1 = 7 images

Class 10 Chapter 9 Light, Reflection, and Refraction Quick Revision Summary

Concept	Formula / Fact
Speed of light in vacuum	3 × 10⁸ m/s
Laws of reflection	∠i = ∠r; all rays in same plane
Mirror formula	1/f = 1/u + 1/v
Focal length (spherical mirror)	f = R/2
Magnification (mirror)	m = −v/u
Snell's Law	μ₁ sin i = μ₂ sin r
Apparent depth	d_ap = d_ac / μ
Critical angle	sin θc = 1/μ
Lens formula	1/f = 1/v − 1/u
Power of lens	P = 1/f(m), unit: Dioptre
Combined lens power	P = P₁ + P₂
Planck's equation	E = hν