CBSE Class 10 Science Chapter 10 — Light Reflection and Refraction: Complete Notes, MCQs and Board Exam Guide 2027

Introduction: Why Light Chapter is Crucial for CBSE Class 10 Boards

Chapter 10 of CBSE Class 10 Science — Light: Reflection and Refraction — consistently carries 5-6 marks in board examinations. Every year you will find at least one numerical problem and one concept question from this chapter. Students who clearly understand sign conventions, mirror and lens formulas, and the concept of refractive index can score full marks reliably.

These complete CBSE Class 10 Light Reflection and Refraction notes cover all NCERT content with board exam patterns, common errors to avoid, and 10 practice MCQs for 2027.

Part 1: Reflection of Light

Laws of Reflection

When light hits a surface and bounces back, it obeys two laws:

1. The angle of incidence = angle of reflection (both measured from the normal)
2. The incident ray, reflected ray, and normal all lie in the same plane

Characteristics of Images in Plane Mirror

• Image is virtual and erect
• Image is laterally inverted
• Size of image = size of object
• Image distance = object distance (behind mirror)

Spherical Mirrors: Types and Important Terms

Spherical mirrors are portions of a hollow sphere with a reflecting inner or outer surface.

• Concave mirror: Reflecting surface curves inward (cave-like). Converging mirror. Used in torches, vehicle headlights, shaving mirrors, solar furnaces.
• Convex mirror: Reflecting surface curves outward. Diverging mirror. Used in rear-view mirrors of vehicles (wider field of view).

Important Terms:

• Pole (P): Centre of the reflecting surface
• Centre of Curvature (C): Centre of the sphere of which the mirror is a part. Distance PC = radius of curvature (R)
• Principal Axis: Line passing through pole and centre of curvature
• Principal Focus (F): Point where parallel rays converge (concave) or appear to diverge from (convex) after reflection. Distance PF = focal length (f)
• Relationship: f = R/2 (focal length = half of radius of curvature)

New Cartesian Sign Convention for Mirrors

This is the most important concept — sign errors cause the most lost marks in board exams.

• All distances are measured from the pole (P)
• Distances measured in the direction of incident light (to the left of mirror) are negative
• Distances measured opposite to incident light are positive (but for mirrors, objects are always in front, so object distance u is always negative)
• Heights above principal axis are positive
• Heights below principal axis are negative
• For concave mirror: f is negative (focus in front of mirror = negative side)
• For convex mirror: f is positive (focus behind mirror)

Mirror Formula and Magnification

The relationship between object distance (u), image distance (v), and focal length (f):

1/v + 1/u = 1/f

Magnification (m):

m = h’/h = -v/u

Where h’ = image height, h = object height

• If m is positive → image is erect and virtual
• If m is negative → image is inverted and real
• |m| > 1 → image is magnified; |m| < 1 → image is diminished

Image Formation by Concave Mirror — Summary Table:

Part 2: Refraction of Light

What is Refraction?

Refraction is the bending of light when it travels from one transparent medium to another. It occurs because light travels at different speeds in different media. Light bends towards the normal when going from rarer to denser medium, and away from the normal when going from denser to rarer medium.

Laws of Refraction (Snell’s Law):

1. The incident ray, refracted ray, and normal at the point of incidence are all in the same plane.
2. sin i / sin r = constant (the constant is the refractive index of the second medium with respect to the first medium)

Refractive Index

The refractive index (n) of a medium indicates how much light slows down in that medium compared to vacuum:

n = speed of light in vacuum (c) / speed of light in medium (v)

n = c / v

• Refractive index of vacuum = 1 (by definition)
• Refractive index of air ≈ 1 (for practical purposes)
• Refractive index of water = 1.33, glass ≈ 1.5, diamond = 2.42
• Higher refractive index = denser optical medium = light slows down more

Refraction Through a Glass Slab: Lateral Displacement

When light passes through a glass slab (parallel faces), it emerges parallel to the original direction but shifted sideways. This sideways shift is called lateral displacement. The amount of lateral displacement depends on the thickness of the slab, the angle of incidence, and the refractive index.

Lenses: Types and Terms

• Convex (converging) lens: Thicker at centre. Converges parallel rays to a focus. Used in magnifying glasses, cameras, microscopes, spectacles for hypermetropia.
• Concave (diverging) lens: Thinner at centre. Diverges light. Used in spectacles for myopia.

Lens Formula:

1/v – 1/u = 1/f

Note: This is different from the mirror formula! Mirrors use 1/v + 1/u = 1/f. Lenses use 1/v – 1/u = 1/f.

Magnification for Lenses:

m = h’/h = v/u

Note: No negative sign in lens magnification formula (unlike mirrors).

Power of a Lens

Power (P) of a lens is the reciprocal of its focal length measured in metres:

P = 1/f (where f is in metres)

Unit: Dioptre (D)

• Convex lens has positive power (+D)
• Concave lens has negative power (−D)
• Example: Focal length = 50 cm = 0.5 m → P = 1/0.5 = +2 D (convex)
• Example: Focal length = −50 cm = −0.5 m → P = 1/(−0.5) = −2 D (concave)

Board Exam Weightage and Common Errors

This chapter carries 5-6 marks in CBSE Class 10 boards typically as:

• 1-mark: Define refractive index OR state laws of reflection
• 2-mark: Draw ray diagram for concave/convex mirror OR given image distance calculate magnification
• 3-mark: Numerical on mirror/lens formula OR compare image characteristics

Most Common Errors in Board Exams:

• Sign convention mistakes: Forgetting that object distance u is always negative for mirrors (object is in front)
• Wrong formula for lenses: Using 1/v + 1/u = 1/f instead of 1/v – 1/u = 1/f
• Power units: Not converting focal length from cm to metres before calculating power
• Magnification sign interpretation: Confusing virtual/real based on sign of m
• Focal length sign: Forgetting that concave mirror has negative f and convex mirror has positive f

10 Practice MCQs: CBSE Class 10 Light

Quiz data error: Syntax error

Frequently Asked Questions (FAQs)

1. What is the difference between the mirror formula and lens formula?

The mirror formula is 1/v + 1/u = 1/f, while the lens formula is 1/v – 1/u = 1/f. The sign convention also differs slightly: for mirrors, both focal length and distances are measured from the pole. For lenses, distances are measured from the optical centre. The key difference to remember for board exams is the + vs − sign: mirrors add, lenses subtract (on the left side of the equation).

2. Why is the object distance u always negative in the mirror formula?

In the New Cartesian Sign Convention, we place the mirror at the origin and measure all distances from the pole. Light travels from left to right (incident light direction). The object is always placed in front of the mirror — on the left side. Since the left side is opposite to the direction of incident light, all distances measured to the left are negative. Therefore, object distance u is always negative for real objects in mirror problems. This is the single most important sign convention rule for mirror numericals.

3. What is the difference between refractive index and critical angle?

The refractive index (n) of a medium is the ratio of speed of light in vacuum to speed of light in that medium (n = c/v). It describes how much a medium slows down light. The critical angle is the specific angle of incidence (when light travels from denser to rarer medium) at which the refracted ray travels along the boundary (refraction angle = 90°). Beyond the critical angle, no refraction occurs — all light reflects back (Total Internal Reflection). The two are related: sin(critical angle) = 1/n. Note: Critical angle is not in the CBSE Class 10 NCERT syllabus but may appear as a bonus concept in some schools.

4. How does the power of a combination of lenses work?

When two or more lenses are placed in contact, the total power of the combination equals the sum of individual powers: P = P₁ + P₂ + P₃ … This is why an optician prescribes corrective lenses in dioptres (e.g., −2.5 D for myopia). Since powers simply add, combinations are easy to calculate. For example, a +3 D and a −1 D lens together give +2 D total power. This concept, while briefly mentioned in Class 10, is more thoroughly examined in Class 12 physics optics and NEET/JEE preparation.