CBSE Class 10 Science · Chapter: Light
Reflection & Refraction — 15 Most Important Questions
Board exam–style questions with full explanations. Original questions aligned to NCERT & CBSE pattern.
📘 CBSE Board 2025
✅ 1, 2, 4 & 5 Mark Questions
💡 Click for Explanation
1 Mark Each
Section A — Very Short Answer
Answer: 12 cm
The relationship between focal length (f) and radius of curvature (R) is:
f = R / 2
Given R = 24 cm,
f = 24 / 2 = 12 cm
The focal length of a concave mirror is always half its radius of curvature. Since concave mirrors are converging mirrors, this focal length is taken as negative (−12 cm) in the sign convention.
Answer: Convex Mirror
Reason: A convex mirror always produces a virtual, erect, and diminished image, regardless of the position of the object. This gives the driver a wider field of view of traffic behind the vehicle, making it safer to drive.
Answer: Away from the normal
When light travels from a denser medium (e.g., glass or water) to a rarer medium (e.g., air), it slows down less — actually it speeds up — causing it to bend away from the normal at the point of incidence.
This is why a straw appears bent when placed in a glass of water, as the light coming from the straw bends away from the normal as it exits water and enters air.
Answer:
The angle of incidence is the angle formed between the incident ray of light and the normal drawn at the point of incidence on the reflecting surface.
Normal is an imaginary line perpendicular to the mirror surface at the point where the light ray strikes. The angle of incidence is always measured from the normal, not from the surface itself.
Answer: The speed of light decreases.
When light travels from air (optically rarer medium) to glass (optically denser medium), it encounters more resistance due to the higher refractive index of glass. As a result, its speed decreases. This change in speed causes the light ray to bend towards the normal — a phenomenon called refraction.
2 Marks Each
Section B — Short Answer
Object is placed at the Centre of Curvature (C).
When an object is placed at the centre of curvature (C) of a concave mirror, the image is formed at C itself — it is real, inverted, and of the same size as the object.
Ray Diagram (Text Description):
- Ray 1: A ray parallel to the principal axis reflects through the focus (F).
- Ray 2: A ray passing through the centre of curvature (C) reflects back along the same path.
- Both rays meet at C on the same side as the object → forms a real, inverted, same-size image at C.
Draw a concave mirror with centre C and focus F on the principal axis. Place the object arrow at C and the image arrow (inverted) at C.
Snell's Law (Laws of Refraction):
Statement: The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a pair of media and for a given colour of light. This constant is called the refractive index.
Mathematical Expression:
sin i / sin r = n₂₁ = constant
Where:
- i = angle of incidence
- r = angle of refraction
- n₂₁ = refractive index of medium 2 with respect to medium 1
Also stated as: The incident ray, the refracted ray, and the normal at the point of incidence — all lie in the same plane.
Given: Object distance u = −15 cm (negative, as per sign convention), Focal length f = −10 cm (concave mirror).
Mirror Formula:
1/f = 1/v + 1/u
1/(−10) = 1/v + 1/(−15)
1/v = −1/10 + 1/15
1/v = (−3 + 2)/30 = −1/30
∴ v = −30 cm
The image is formed 30 cm in front of the mirror (real and inverted), on the same side as the object. The negative sign confirms it is a real image.
Definition: The refractive index of a medium is the ratio of the speed of light in vacuum (or air) to the speed of light in that medium.
n = Speed of light in vacuum / Speed of light in medium = c / v
Calculation:
n = (3 × 10⁸) / (2.25 × 10⁸)
n = 1.33
The refractive index of water is approximately 1.33, which means light travels about 1.33 times slower in water than in vacuum.
4 Marks Each
Section C — Long Answer
Refraction through a Rectangular Glass Slab:
When a ray of light enters a rectangular glass slab at one face, it bends
towards the normal (because it goes from a rarer medium — air — to a denser medium — glass). When it exits from the opposite face, it bends
away from the normal (going from denser to rarer medium).
Why the emergent ray is parallel to the incident ray:
- The two refracting surfaces (top and bottom faces) of a rectangular slab are parallel to each other.
- When light travels through two parallel surfaces, the angle of refraction at the first surface equals the angle of incidence at the second surface.
- As a result, the bending at the two surfaces is equal and opposite, and the emergent ray is parallel to the incident ray.
- However, the emergent ray is laterally displaced (shifted sideways) from the original incident ray.
Diagram (Description): Draw a rectangular glass slab. Show the incident ray hitting the top surface at angle i₁, bending towards normal inside the glass (angle r), then hitting the bottom surface at angle r, and emerging at angle i₂ = i₁, parallel to the incident ray but laterally displaced.
Image Formation by a Concave Mirror:
- Object at infinity: Image at Focus (F) — Real, Inverted, Highly Diminished (point-sized). Used in solar furnaces.
- Object beyond C: Image between F and C — Real, Inverted, Diminished.
- Object at C: Image at C — Real, Inverted, Same size as object.
- Object between C and F: Image beyond C — Real, Inverted, Magnified (enlarged).
- Object at F: Image at infinity — Real, Inverted, Highly Magnified. Used in torch/headlights.
- Object between F and P (Pole): Image behind the mirror — Virtual, Erect, Magnified. Used as shaving/makeup mirror.
Key Point: A concave mirror forms a virtual image only when the object is placed between the focus and the pole. In all other positions, the image is real and inverted.
Laws of Reflection of Light:
First Law: The angle of incidence (∠i) is always equal to the angle of reflection (∠r).
∠i = ∠r
Second Law: The incident ray, the reflected ray, and the normal to the mirror at the point of incidence — all three lie in the same plane.
Diagram Description: Draw a flat mirror. A ray hits the surface. The normal is drawn perpendicular at the point of contact. Angle between incident ray and normal = angle between reflected ray and normal. All three (incident ray, normal, reflected ray) are in the same plane.
Validity for curved mirrors: For curved mirrors (concave or convex), at any point on the surface, a tangent can be drawn, and the normal is perpendicular to that tangent. The laws of reflection hold at each such point individually, treating the surface locally as a flat mirror at that tiny point.
Given: Object height (h) = +5 cm, Focal length f = +20 cm (convex lens), Object distance u = −30 cm (sign convention)
Lens 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 (image on the other side of lens → real image)
Magnification:
m = v/u = 60/(−30) = −2
Image height = m × h = −2 × 5 = −10 cm
Nature: Real, Inverted, Magnified (2× the object size), formed 60 cm on the other side of the lens. The negative sign of image height confirms it is inverted.
5 Marks Each
Section D — Higher Order Thinking Skills (HOTS)
Concave Lens (Diverging Lens):
A concave lens always forms a virtual, erect, and diminished image on the same side as the object, regardless of where the object is placed. The image is always located between the optical centre and the focus on the same side as the object.
Application: Used to correct short-sightedness (myopia) — spectacles for people who cannot see distant objects clearly.
Convex Lens (Converging Lens) — 5 Positions:
- At infinity: Image at F₂ — Real, Inverted, Highly Diminished.
- Beyond 2F₁: Image between F₂ and 2F₂ — Real, Inverted, Diminished.
- At 2F₁: Image at 2F₂ — Real, Inverted, Same size.
- Between F₁ and 2F₁: Image beyond 2F₂ — Real, Inverted, Magnified.
- Between F₁ and Optical Centre: Image on same side as object — Virtual, Erect, Magnified. (Acts as magnifying glass.)
Application of convex lens: Used to correct long-sightedness (hypermetropia), and in cameras, microscopes, and magnifying glasses.
Total Internal Reflection (TIR):
When light travels from a
denser medium to a
rarer medium, it bends away from the normal. As the angle of incidence increases, the angle of refraction also increases. At a particular angle of incidence called the
Critical Angle (C), the refracted ray grazes along the surface (angle of refraction = 90°). If the angle of incidence exceeds the critical angle, no refraction occurs and all the light is reflected back into the denser medium. This phenomenon is called
Total Internal Reflection.
Conditions for TIR:
- Light must travel from a denser medium to a rarer medium.
- The angle of incidence must be greater than the critical angle for that pair of media.
Application 1 — Optical Fibres:
Optical fibres are thin glass or plastic strands that transmit light using total internal reflection. Light entering one end of the fibre is repeatedly totally internally reflected along the length of the fibre, without escaping through the sides. Used in internet cables (broadband), medical endoscopes, and decorative lighting.
Application 2 — Mirage:
On a hot day, the air near the ground is hotter and less dense than the air above. When light from the sky travels downward through these layers, it undergoes successive refractions and eventually total internal reflection near the ground. The observer sees an inverted image of the sky, which looks like water on the road — this is a mirage.
⚠️ Disclaimer: These questions are original and have been created for educational practice purposes, aligned with the CBSE Class 10 Science syllabus. While every effort has been made to ensure accuracy, there may be inadvertent typographical errors. Students are advised to verify all answers, formulas, and values from their NCERT textbook or official CBSE resources before the board examination. EduBrightPages is not responsible for any discrepancy. When in doubt, always cross-check with a qualified teacher or a trusted source.