NCERT Class 10 Science Chapter 9 – Light: Reflection and Refraction

NCERT Class 10 Science Chapter 9 explains the fundamental concepts of light, including reflection and refraction. In NCERT Class 10 Science Chapter 9, students study spherical mirrors, lenses, image formation and important formulae used in optics. NCERT Class 10 Science Chapter 9 is one of the most important physics chapters for CBSE board exams.

NCERT Class 10 Science Chapter 9 begins with Reflection of Light. The two Laws of Reflection state that the angle of incidence equals the angle of reflection and that the incident ray, reflected ray and normal lie in the same plane. These principles form the base of NCERT Class 10 Science Chapter 9.

A major portion of NCERT Class 10 Science Chapter 9 focuses on Spherical Mirrors. There are two types:

• Concave Mirror
• Convex Mirror

Important terms in NCERT Class 10 Science Chapter 9 include Pole (P), Centre of Curvature (C), Radius of Curvature (R), Principal Focus (F) and Principal Axis. The relation f = R/2 is important for exams.

NCERT Class 10 Science Chapter 9 explains image formation by concave and convex mirrors. The Mirror Formula is: 1/f = 1/v + 1/u

Magnification in NCERT Class 10 Science Chapter 9 is given by: m = – v/u

The second major concept in NCERT Class 10 Science Chapter 9 is Refraction of Light. Refraction occurs due to change in speed of light when it passes from one medium to another. According to Snell’s Law: n = sin i / sin r

NCERT Class 10 Science Chapter 9 also explains Refractive Index, defined as: n = c/v, where c = 3 × 10⁸ m/s.

A large section of NCERT Class 10 Science Chapter 9 discusses Spherical Lenses. There are two types:

• Convex Lens (Converging)
• Concave Lens (Diverging)

The Lens Formula in NCERT Class 10 Science Chapter 9 is: 1/f = 1/v – 1/u

Magnification for lenses: m = v/u

Another important concept in NCERT Class 10 Science Chapter 9 is Power of a Lens, defined as: P = 1/f (in metre) Unit: Dioptre (D)

NCERT Class 10 Science Chapter 9 is extremely important for CBSE board exams because numerical problems based on mirror formula, lens formula and power are frequently asked.

Students should refer to the official NCERT website at for authentic textbooks and syllabus updates.

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9.1 Reflection of light

• Reflection of Light is the phenomenon in which light rays bounce back into the same medium after striking a surface.
• When light falls on a smooth polished surface like a plane mirror, it undergoes Regular Reflection, producing a clear image.
• The incident ray, reflected ray and the Normal at the point of incidence all lie in the same plane.
• First Law of Reflection: The angle of incidence (∠i) is equal to the angle of reflection (∠r).
• Second Law of Reflection: The incident ray, reflected ray and normal lie in the same plane.
• The angle between the incident ray and the normal is called the Angle of Incidence, while the angle between reflected ray and normal is the Angle of Reflection.
• Reflection is responsible for image formation in mirrors and plays an important role in optical instruments.

9.2 Spherical mirrors

• A Spherical Mirror is a mirror whose reflecting surface forms a part of a hollow sphere.
• There are two types of spherical mirrors: Concave Mirror (reflecting surface curves inward) and Convex Mirror (reflecting surface bulges outward).
• The centre of the sphere of which the mirror is a part is called the Centre of Curvature (C).
• The radius of that sphere is called the Radius of Curvature (R).
• The midpoint of the reflecting surface is called the Pole (P).
• The straight line passing through the pole and centre of curvature is called the Principal Axis.
• The point on the principal axis where parallel rays converge (concave) or appear to diverge (convex) is called the Principal Focus (F).
• For spherical mirrors, the focal length (f) is equal to half of the radius of curvature, that is f = R/2.

9.2.1 Image Formation by Spherical Mirrors

• The image formed by a Concave Mirror depends on the position of the object with respect to the Pole (P), Focus (F) and Centre of Curvature (C).
• When the object is placed beyond C, the image is formed between F and C, and it is Real, Inverted and Diminished.
• When the object is at C, the image is formed at C, and it is Real, Inverted and Same Size.
• When the object is between C and F, the image is formed beyond C, and it is Real, Inverted and Enlarged.
• When the object is at F, the reflected rays become parallel and the image is formed at Infinity.
• When the object is between F and P, the image is formed behind the mirror and it is Virtual, Erect and Enlarged.
• A Convex Mirror always forms an image that is Virtual, Erect and Diminished, irrespective of object position.

9.2.2 Representation of Images Formed by Spherical Mirrors Using Ray Diagrams

• Image formation by spherical mirrors is represented using Ray Diagrams based on standard reflection rules.
• A ray parallel to the Principal Axis after reflection passes through the Principal Focus (F) in a concave mirror, and appears to diverge from F in a convex mirror.
• A ray passing through the Principal Focus (F) reflects parallel to the principal axis.
• A ray passing through the Centre of Curvature (C) retraces its path after reflection.
• A ray incident at the Pole (P) reflects obeying the Laws of Reflection with equal angles.
• For a concave mirror, real images are formed by intersection of reflected rays, while virtual images are formed by intersection of their extensions.
• Convex mirrors always produce images behind the mirror, which are located by extending the reflected rays backward.

9.2.3 Sign Convention for Reflection by Spherical Mirrors

• The New Cartesian Sign Convention is used for spherical mirrors.
• The Pole (P) is taken as the origin of the coordinate system.
• All distances are measured from the pole along the Principal Axis.
• Distances measured in the direction of incident light (usually from left to right) are taken as Positive.
• Distances measured opposite to the direction of incident light are taken as Negative.
• Heights measured above the principal axis are taken as Positive, and those measured below it are Negative.
• For a Concave Mirror, the focal length (f) and radius of curvature (R) are negative, while for a Convex Mirror, they are positive.

9.2.4 Mirror Formula and Magnification

• The relationship between Object Distance (u), Image Distance (v) and Focal Length (f) of a spherical mirror is given by the Mirror Formula: 1/f = 1/v + 1/u
• This formula is valid for both Concave and Convex Mirrors, provided the Sign Convention is followed correctly.
• The linear Magnification (m) produced by a mirror is defined as the ratio of height of image (hᵢ) to height of object (hₒ).
• Magnification is given by the formula: m = hᵢ / hₒ = – v/u
• For a Concave Mirror, magnification is negative when the image is Real and Inverted, and positive when the image is Virtual and Erect.
• For a Convex Mirror, magnification is always positive because the image formed is always Virtual and Erect.
• If |m| > 1, the image is Enlarged; if |m| < 1, the image is Diminished; if |m| = 1, the image is Same Size as the object.

9.3 Refraction of light

• Refraction of Light is the change in direction of light when it passes from one transparent medium to another due to change in speed.
• When light travels from a Rarer Medium (like air) to a Denser Medium (like glass), it bends Towards the Normal.
• When light travels from a denser medium to a rarer medium, it bends Away from the Normal.
• The incident ray, refracted ray and the Normal at the point of incidence lie in the same plane.
• The ratio of the sine of angle of incidence to the sine of angle of refraction is constant for a given pair of media; this is known as Snell’s Law.
• According to Snell’s Law: n = sin i / sin r, where n is the refractive index of the medium.
• Refraction occurs because the Speed of Light changes in different media.
• Refraction is responsible for phenomena such as bending of objects seen through water and image formation by lenses.

9.3.1 Refraction through a Rectangular Glass Slab

• When a ray of light passes through a Rectangular Glass Slab, it undergoes refraction at both the surfaces.
• At the first surface (air to glass), the ray bends Towards the Normal because glass is optically denser than air.
• At the second surface (glass to air), the ray bends Away from the Normal as it re-enters the rarer medium.
• The emergent ray is Parallel to the Incident Ray, but it is laterally displaced.
• The perpendicular distance between the emergent ray and the incident ray is called Lateral Displacement.
• The amount of lateral displacement depends on the Thickness of the Slab, Angle of Incidence and Refractive Index of the material.
• Since the two refracting surfaces are parallel, the overall direction of light remains unchanged.

9.3.2 The Refractive Index

• The Refractive Index (n) of a medium is defined as the ratio of the speed of light in vacuum (c) to the speed of light in that medium (v).
• Mathematically, n = c / v, where c = 3 × 10⁸ m/s (speed of light in vacuum).
• A medium with higher refractive index is said to be Optically Denser.
• The refractive index depends on the nature of the medium and the wavelength of light used.
• According to Snell’s Law, refractive index can also be expressed as n = sin i / sin r for a given pair of media.
• If n > 1, the speed of light in the medium is less than in vacuum.
• The refractive index has no unit, since it is a ratio of two speeds.

9.3.3 Refraction by Spherical Lenses

• A Spherical Lens is a transparent refracting medium bounded by two spherical surfaces or one spherical and one plane surface.
• There are two types of lenses: Convex Lens (Converging Lens) and Concave Lens (Diverging Lens).
• A Convex Lens converges parallel rays of light to a point called the Principal Focus (F).
• A Concave Lens diverges parallel rays, and they appear to come from the principal focus.
• The line passing through the centres of curvature of both surfaces is called the Principal Axis.
• The midpoint of the lens is called the Optical Centre (O), through which light passes without deviation.
• The distance between the optical centre and principal focus is called the Focal Length (f).
• For spherical lenses, the focal length depends on the Curvature of Surfaces and the Refractive Index of the material.

9.3.4 Image Formation by Lenses

• The image formed by a Convex Lens depends on the position of the object relative to the Optical Centre (O), Focus (F) and 2F.
• When the object is placed beyond 2F, the image is formed between F and 2F, and it is Real, Inverted and Diminished.
• When the object is at 2F, the image is formed at 2F, and it is Real, Inverted and Same Size.
• When the object is between F and 2F, the image is formed beyond 2F, and it is Real, Inverted and Enlarged.
• When the object is at F, the image is formed at Infinity and the refracted rays become parallel.
• When the object is between F and O, the image is formed on the same side of the lens and it is Virtual, Erect and Enlarged.
• A Concave Lens always forms an image that is Virtual, Erect and Diminished, irrespective of object position.

9.3.5 Image Formation in Lenses Using Ray Diagrams

• Image formation by lenses is represented using standard Ray Diagram Rules based on refraction principles.
• A ray parallel to the Principal Axis after refraction passes through the Principal Focus (F) in a convex lens, and appears to diverge from F in a concave lens.
• A ray passing through the Principal Focus (F) emerges parallel to the principal axis after refraction.
• A ray passing through the Optical Centre (O) emerges without deviation.
• For a convex lens, real images are formed by actual intersection of refracted rays.
• For a concave lens, images are formed by intersection of the backward extensions of refracted rays.
• Ray diagrams help determine the Nature, Position and Size of the image.

9.3.6 Sign Convention for Spherical Lenses

• The New Cartesian Sign Convention is used for spherical lenses.
• The Optical Centre (O) is taken as the origin of the coordinate system.
• All distances are measured from the optical centre along the Principal Axis.
• Distances measured in the direction of incident light are taken as Positive, and those measured opposite are Negative.
• Heights measured above the principal axis are taken as Positive, and those measured below it are Negative.
• For a Convex Lens, the focal length (f) is Positive because the principal focus lies on the positive side.
• For a Concave Lens, the focal length (f) is Negative because the principal focus lies on the negative side.

9.3.7 Lens Formula and Magnification

• The relationship between Object Distance (u), Image Distance (v) and Focal Length (f) of a spherical lens is given by the Lens Formula: 1/f = 1/v – 1/u
• This formula is valid for both Convex and Concave Lenses, provided the Sign Convention is correctly applied.
• The linear Magnification (m) produced by a lens is defined as the ratio of height of image (hᵢ) to height of object (hₒ).
• Magnification is given by: m = hᵢ / hₒ = v/u
• For a Convex Lens, magnification is negative when the image is Real and Inverted, and positive when the image is Virtual and Erect.
• For a Concave Lens, magnification is always positive because the image formed is always Virtual and Erect.
• If |m| > 1, the image is Enlarged; if |m| < 1, the image is Diminished; if |m| = 1, the image is of Same Size as the object.

9.3.8 Power of a Lens

• The Power of a Lens (P) is defined as the ability of a lens to converge or diverge light rays.
• It is mathematically expressed as the reciprocal of focal length in metres: P = 1/f
• The SI unit of power is Dioptre (D).
• A lens having focal length of 1 metre has a power of +1 Dioptre.
• A Convex Lens has Positive Power because it converges light rays.
• A Concave Lens has Negative Power because it diverges light rays.
• Shorter focal length corresponds to Higher Power, while longer focal length corresponds to lower power.

Exam Oriented Facts

• Reflection is the bouncing back of light into the same medium after striking a surface.
• Laws of Reflection:
  • ∠i = ∠r (Angle of Incidence = Angle of Reflection)
  • Incident ray, reflected ray and Normal lie in the same plane.
• Reflection from smooth surfaces produces Regular Reflection and clear images.
• Types: Concave Mirror (converging) and Convex Mirror (diverging).
• Important Terms: Pole (P), Principal Axis, Centre of Curvature (C), Radius of Curvature (R), Principal Focus (F).
• Relation: f = R/2.
• Convex mirror always forms Virtual, Erect and Diminished image.
• Concave mirror can form Real/Inverted or Virtual/Erect images depending on object position.
• Mirror Formula:1/f = 1/v + 1/u
• Magnification (m):m = hᵢ/hₒ = – v/u
• For real images → m is negative.
• For virtual images → m is positive.
• Pole is origin.
• Distances in direction of incident light → Positive.
• Opposite direction → Negative.
• Concave mirror → f negative.
• Convex mirror → f positive.
• Refraction occurs due to change in Speed of Light when it enters another medium.
• From rarer to denser medium → bends Towards Normal.
• From denser to rarer → bends Away from Normal.
• Snell’s Law: n = sin i / sin r
• n = c/v, where c = 3 × 10⁸ m/s.
• No unit (dimensionless quantity).
• Higher n → more optically dense medium.
• Emergent ray is Parallel to incident ray.
• Produces Lateral Displacement.
• Types: Convex (Converging) and Concave (Diverging).
• Important Terms: Optical Centre (O), Principal Focus (F), Focal Length (f).
• Convex lens can form Real or Virtual images.
• Concave lens always forms Virtual, Erect, Diminished image.
• Lens Formula: 1/f = 1/v – 1/u
• Magnification: m = hᵢ/hₒ = v/u
• Optical centre is origin.
• Convex lens → f positive.
• Concave lens → f negative.
• P = 1/f (in metres)
• Unit: Dioptre (D)
• Convex lens → Positive Power
• Concave lens → Negative Power

NCERT Class 10 Science Chapter 9 – Light: Reflection and Refraction builds a strong foundation in optics. Mastering NCERT Class 10 Science Chapter 9 ensures clarity in ray diagrams, sign conventions and numerical problem-solving.

NCERT Class 10 Science Chapter 9 is a high-weightage chapter in CBSE exams and also forms the base for advanced physics topics in higher classes.

Continue reading NCERT Class 10 Science Chapter 10 – The Human Eye and the Colourful World to understand applications of light in vision and atmospheric phenomena.

FAQs

Q1. What is NCERT Class 10 Science Chapter 9 about?
NCERT Class 10 Science Chapter 9 explains reflection, refraction, mirrors, lenses and optical formulae.

Q2. What is the mirror formula in NCERT Class 10 Science Chapter 9?
1/f = 1/v + 1/u

Q3. What is the lens formula in NCERT Class 10 Science Chapter 9?
1/f = 1/v – 1/u

Q4. What is the unit of power of a lens in NCERT Class 10 Science Chapter 9?
Dioptre (D).

Q5. Why is NCERT Class 10 Science Chapter 9 important for exams?
Because numerical problems from mirror and lens formula are frequently asked in CBSE board exams.

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