MP Board Class 10 Science Chapter 9: Light – Reflec…
MP Board Class 10 Science Chapter 9: Light – Reflection and Refraction (प्रकाश – परावर्तन एवं अपवर्तन) — This chapter is one of the most important topics in your Class 10 Science board exam. Carrying 12–16 marks in the MP Board annual exam, Light covers the laws of reflection, image formation by spherical mirrors, sign conventions, mirror formula, refraction through lenses, lens formula, and power of lenses. Mastering this chapter is essential for scoring high in physics and building a strong foundation for Class 12 Optics.
📑 Table of Contents
💡 1. Reflection of Light – प्रकाश का परावर्तन
Reflection of light is the phenomenon of bouncing back of light when it strikes a smooth, polished surface. The ray that strikes the surface is called the incident ray, the ray that bounces back is the reflected ray, and the perpendicular line at the point of incidence is the normal.
Laws of Reflection
- First Law: The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.
- Second Law: The angle of incidence (∠i) is always equal to the angle of reflection (∠r).
📘 Key Fact: Reflection can be regular (from smooth surfaces like plane mirrors) or diffused (from rough surfaces like paper). In both cases, the laws of reflection apply individually to each ray.
🪞 2. Spherical Mirrors – गोलीय दर्पण
Spherical mirrors are mirrors whose reflecting surface is part of a hollow sphere. There are two types — concave mirror (curving inward like a cave) and convex mirror (curving outward). Concave mirrors converge light while convex mirrors diverge it.
Key Terms for Spherical Mirrors
Image Formation by Concave Mirror
📐 3. Mirror Formula & Sign Convention
The Mirror Formula
Magnification: m = -v/u = hi/ho
Where f = focal length, u = object distance from pole, v = image distance from pole, hi = image height, ho = object height.
Solved Example — Mirror Formula
Question: An object is placed at a distance of 20 cm from a concave mirror of focal length 15 cm. Find the position, nature, and magnification of the image.
Solution:
Given: u = −20 cm (object is in front of mirror), f = −15 cm (concave mirror)
Using mirror formula: 1/f = 1/u + 1/v
1/v = 1/f − 1/u = 1/(−15) − 1/(−20) = −1/15 + 1/20
1/v = (−4 + 3)/60 = −1/60
v = −60 cm (negative sign means image is in front of mirror, so it is REAL)
Magnification: m = −v/u = −(−60)/(−20) = 60/(−20) = −3
Negative m means the image is INVERTED. |m| = 3 means the image is 3 times ENLARGED.
Answer: Image is formed at 60 cm in front of the mirror, real, inverted, and 3× enlarged.
Uses of Spherical Mirrors
- Concave mirrors are used in: torches and headlights (convergent beam), shaving mirrors (enlarged virtual image), solar cookers and solar furnaces (converging sunlight at focus), reflecting telescopes, and ophthalmoscopes for examining eyes.
- Convex mirrors are used in: rear-view mirrors in vehicles (wide field of view, erect image), security mirrors in shops and parking lots, and sunglasses for side-viewing.
New Cartesian Sign Convention
- The pole (P) of the mirror is taken as the origin.
- The principal axis is the X-axis.
- Distances measured in the direction of incident light are positive (+).
- Distances measured opposite to incident light are negative (−).
- Heights above the principal axis are positive; heights below are negative.
- For concave mirrors: f is always negative (−).
- For convex mirrors: f is always positive (+).
🔦 4. Refraction of Light – प्रकाश का अपवर्तन
Refraction is the bending of light when it travels from one transparent medium to another. This bending occurs because the speed of light changes as it moves from one medium to another. Light travels fastest in a vacuum at 3 × 10⁸ m/s, slower in water (2.25 × 10⁸ m/s), and even slower in glass (about 2 × 10⁸ m/s).
Everyday Examples of Refraction
- Pencil in water appearing bent — Light from the submerged part bends away from the normal at the water-air interface, making the pencil look broken at the water surface.
- Pool appearing shallower — The apparent depth of a swimming pool is less than its real depth because of refraction. A pool that is 3 m deep may appear only 2.25 m deep.
- Twinkling of stars — Starlight undergoes continuous refraction as it passes through Earth’s different atmospheric layers with varying densities, causing the apparent position to change continuously.
- Rainbow formation — Sunlight refracts, disperses, and reflects inside water droplets in the atmosphere, separating white light into its seven constituent colours (VIBGYOR).
Laws of Refraction
- The incident ray, the refracted ray, and the normal at the interface all lie in the same plane.
- Snell’s Law: The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media. n₁ sin i = n₂ sin r
Refractive Index
Where c = speed of light in vacuum (3 × 10⁸ m/s), v = speed of light in the medium
📘 Key Concepts: (1) When light travels from rarer to denser medium → bends TOWARDS the normal. (2) From denser to rarer → bends AWAY from normal. (3) If i = 0° (normal incidence), r = 0° — no bending occurs.
Refraction Through a Rectangular Glass Slab
When a ray of light enters a rectangular glass slab, it first bends towards the normal (entering denser medium), travels through the slab, and then bends away from the normal (exiting to rarer medium). The emergent ray is parallel to the incident ray but laterally displaced. The lateral displacement depends on the thickness of the slab, the refractive index, and the angle of incidence.
Comparison: Reflection vs Refraction
Real and Apparent Depth
When an object is placed in water, it appears shallower than it actually is because of refraction. The apparent depth is less than the real depth.
Example: A pond 2 m deep with nwater = 1.33 appears only 1.5 m deep.
🔍 5. Lenses – लेंस: Convex & Concave
A lens is a transparent medium bounded by two spherical surfaces. A convex lens (converging lens) is thicker at the centre and converges parallel light rays. A concave lens (diverging lens) is thinner at the centre and diverges parallel rays.
Image Formation by Convex Lens
Uses of Lenses in Daily Life
⚡ 6. Lens Formula & Power of Lens
Lens Formula
Magnification: m = v/u = hi/ho
Where f = focal length, u = object distance, v = image distance
Solved Example — Lens Formula
Question: An object 4 cm high is placed at a distance of 30 cm from a convex lens of focal length 20 cm. Find the position, nature, and size of the image.
Solution:
Given: hₒ = +4 cm, u = −30 cm (object in front of lens), f = +20 cm (convex lens)
Using lens formula: 1/f = 1/v − 1/u
1/v = 1/f + 1/u = 1/20 + 1/(−30) = 1/20 − 1/30
1/v = (3 − 2)/60 = 1/60
v = +60 cm (positive sign means image is on the opposite side of the lens — REAL)
Magnification: m = v/u = 60/(−30) = −2
hᵢ = m × hₒ = −2 × 4 = −8 cm
Answer: Image is formed 60 cm on the other side of the lens, real, inverted, 8 cm high (2× enlarged).
Sign Convention for Lenses
- Optical centre (O) of lens is the origin.
- Distances measured in the direction of incident light are positive.
- Convex lens: f is always positive (+).
- Concave lens: f is always negative (−).
- Object distance u is always negative for both lenses.
- Virtual image: v is negative; Real image: v is positive.
Power of a Lens
Unit: Dioptre (D)
Convex lens: P is positive (+)
Concave lens: P is negative (−)
Combination of Lenses
Net Focal Length: 1/f = 1/f₁ + 1/f₂ + 1/f₃ + …
📋 Previous Year Questions (2017–2026)
❓ Frequently Asked Questions
Q1: What is the difference between reflection and refraction?
Reflection is the bouncing back of light from a surface, while refraction is the bending of light when it passes from one medium to another. In reflection, the light stays in the same medium; in refraction, it enters a different medium and changes speed.
Q2: Why is a concave mirror used in solar cookers?
Concave mirrors converge parallel rays of sunlight to a single point (focus), generating intense heat at that point. This concentrated solar energy is used for cooking. The large aperture collects more sunlight, and the parabolic shape focuses all rays to the focal point.
Q3: What is the sign convention for spherical mirrors?
According to the New Cartesian Sign Convention, the pole (P) is the origin, all distances measured in the direction of incident light are positive, and opposite are negative. For concave mirrors, focal length (f) is negative; for convex mirrors, f is positive. Object distance (u) is always negative for real objects placed in front of the mirror.
Q4: What is Snell’s Law?
Snell’s Law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media: n₁ sin i = n₂ sin r. The constant is called the relative refractive index of the second medium with respect to the first.
Q5: How does a convex lens form different types of images?
Depending on the object’s position, a convex lens can form real, inverted images (when object is beyond F) or virtual, erect images (when object is between F and O). Real images can be diminished, same size, or enlarged. The closer the object is to the lens (but beyond F), the larger the image.
Q6: What is the power of a lens? How is it calculated?
Power of a lens is defined as the reciprocal of its focal length expressed in metres: P = 1/f. Its SI unit is dioptre (D). A convex lens has positive power, while a concave lens has negative power. For example, a lens with f = +20 cm has power P = 1/0.20 = +5 D.
Q7: Why does a pencil appear bent when placed in a glass of water?
Due to the refraction of light. Light from the submerged part of the pencil travels from water (denser) to air (rarer) and bends away from the normal. This causes the apparent position of the submerged portion to be higher than its actual position, making the pencil appear bent at the water-air interface.
Q8: What is a real image? How is it different from a virtual image?
A real image is formed when light rays actually meet at a point and can be projected onto a screen. A virtual image is formed when light rays appear to meet at a point behind the mirror or lens and cannot be projected. Real images are always inverted; virtual images are always erect.
Q9: What is the relationship between focal length and radius of curvature?
For spherical mirrors, the focal length (f) is equal to half the radius of curvature (R): f = R/2. For example, if the radius of curvature of a concave mirror is 30 cm, its focal length is 15 cm.
Q10: Why are convex mirrors used as rear-view mirrors in vehicles?
Convex mirrors always form virtual, erect, and diminished images, giving a wider field of view compared to plane or concave mirrors. The smaller image size allows a larger area to be visible, helping drivers see more of the traffic behind them. This improves safety by reducing blind spots.
Q11: What is the lens formula and when is it used?
The lens formula is 1/f = 1/v − 1/u, where f is focal length, v is image distance, and u is object distance. It is used to calculate the position of the image formed by a lens. Along with magnification formula m = v/u = hᵢ/hₒ, it helps determine the size and nature of the image.
Q12: What is total internal reflection? Is it in the Class 10 syllabus?
Total internal reflection occurs when light travels from a denser to a rarer medium and the angle of incidence exceeds the critical angle — all light is reflected back into the denser medium. While this concept is briefly introduced in the context of refraction in Class 10, it is covered in detail in Class 12 Physics. For Class 10, focus on basic refraction, lenses, and mirrors.
Q13: Compare concave and convex mirrors in a table.
Comparison: Concave mirrors have a reflecting surface that curves inward and converge light; they form both real and virtual images. Convex mirrors have a reflecting surface that curves outward and diverge light; they always form virtual, erect, and diminished images. Concave mirrors are used in torches, headlights, and shaving mirrors. Convex mirrors are used as rear-view mirrors in vehicles and for security surveillance.
Q14: How is magnification different for mirrors and lenses?
For mirrors: m = −v/u (negative m means inverted image, positive m means erect image). For lenses: m = v/u (same sign convention). For mirrors, if |m| > 1, image is enlarged; |m| < 1, image is diminished. The same rule applies to lenses. Remember: for mirrors, there's an extra negative sign in the formula that is not present in the lens magnification formula.