Light – Reflection and Refraction — Class 10 Science Notes & Important Questions | MP Board

Chapter Overview

The chapter Light – Reflection and Refraction is a foundational pillar of Physics in MP Board Class 10 Science. It explores how light travels in straight lines and interacts with various optical surfaces to form images through reflection and refraction. By understanding the principles governing spherical mirrors, lenses, refractive index, and optical power, students learn to explain daily phenomena ranging from mirror reflections to the bending of light in water.

Why Important for Board Exam

In the annual MP Board examination, this chapter carries significant weightage—typically around 7 to 8 marks in the Physics section. Question papers regularly include direct numerical problems on mirror and lens formulas, ray diagram constructions, multiple-choice questions (MCQs), and short-answer conceptual questions. Mastering these MP Board Light – Reflection and Refraction notes ensures students can secure full marks in ray diagrams and step-by-step numerical calculations, while also building a strong foundation for Class 11 and Class 12 Physics.

Key Concepts & Topics Covered

To prepare effectively for the board exam, students should focus on the following key concepts organized under two primary sections:

• Part 1: Reflection of Light
  1. Laws of Reflection: Angle of incidence equals angle of reflection; incident ray, reflected ray, and normal lie in the same plane.
  2. Spherical Mirrors: Difference between concave (converging) and convex (diverging) mirrors.
  3. Image Formation Rules: Ray diagrams for different object positions relative to the pole, focus, and center of curvature.
  4. Sign Conventions: New Cartesian Sign Convention for measuring object distance (u), image distance (v), and focal length (f).
  5. Mirror Formula & Magnification: Relation 1/f = 1/v + 1/u and magnification formula m = -v/u = h'/h.
• Part 2: Refraction of Light
  1. Phenomenon of Refraction: Bending of light when passing from one transparent medium to another.
  2. Laws of Refraction & Snell's Law: Constant ratio of sine of incidence angle to sine of refraction angle (sin i / sin r = constant).
  3. Refractive Index: Absolute refractive index and relative refractive index between media.
  4. Refraction through Lenses: Convex lenses (converging) and concave lenses (diverging), along with ray diagrams.
  5. Lens Formula & Power of Lens: Relation 1/f = 1/v - 1/u, magnification m = v/u, and power P = 1/f (in meters) measured in Dioptres (D).

Important Definitions

The following table contains essential terms and definitions frequently asked in MP Board Class 10 Science important questions:

Term	Definition
Reflection of Light	The phenomenon of bouncing back of light rays into the same medium when they strike a polished or reflecting surface.
Concave Mirror	A spherical mirror whose reflecting surface is curved inwards, towards the center of the sphere.
Convex Mirror	A spherical mirror whose reflecting surface is curved outwards, away from the center of the sphere.
Principal Focus (Mirror)	A point on the principal axis where rays parallel to the principal axis converge (concave mirror) or appear to diverge from (convex mirror) after reflection.
Refraction of Light	The change in direction of path of light when it passes obliquely from one transparent medium to another medium of different optical density.
Snell's Law	The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for light of a given color and given pair of media.
Absolute Refractive Index	The ratio of the speed of light in vacuum (or air) to the speed of light in a given medium (n = c / v).
Power of a Lens	The degree of convergence or divergence of light rays achieved by a lens, expressed as the reciprocal of its focal length in meters (P = 1/f).
Dioptre (D)	The SI unit of power of a lens. One dioptre is the power of a lens whose focal length is 1 meter.

Chapter Summary in Simple Language

1. Laws of Reflection and Spherical Mirrors

Light rays travel in straight lines in a uniform medium. When light hits a mirror, it follows two strict rules: the angle of incidence is always equal to the angle of reflection, and the incident ray, normal, and reflected ray all lie in the exact same plane. Spherical mirrors are parts of a hollow sphere of glass. A concave mirror curves inward and converges incoming parallel light rays. A convex mirror curves outward and diverges incoming parallel light rays.

For any spherical mirror with a small aperture, the focal length (f) is exactly half of its radius of curvature (R), giving the key relation: R = 2f. Concave mirrors form both real (inverted) and virtual (erect) images depending on where the object is placed. Because concave mirrors concentrate light at a focus, they are used in solar furnaces, torches, searchlights, and dentist inspection mirrors. Conversely, convex mirrors always form virtual, diminished, and erect images, giving drivers a broader view of traffic behind them; hence, they are universally used as rear-view mirrors in automobiles.

2. Sign Convention, Mirror Formula, and Magnification

According to the New Cartesian Sign Convention, the pole (P) of the mirror is taken as the origin. All distances measured in the direction of incident light (to the right of pole) are taken as positive, while distances measured opposite to incident light (to the left of pole) are taken as negative. Object distance (u) is always negative. The focal length of a concave mirror is negative, whereas the focal length of a convex mirror is positive.

The Mirror Formula connects object distance (u), image distance (v), and focal length (f):

1/f = 1/v + 1/u

Magnification (m) expresses how much larger or smaller the image is compared to the object. It is calculated as the ratio of image height (h') to object height (h), or in terms of distances as m = -v/u = h'/h. A negative magnification signifies a real, inverted image, whereas a positive magnification signifies a virtual, erect image.

3. Refraction of Light and Refractive Index

Refraction occurs because light travels at different speeds in different optical media. When light enters an optically denser medium from a rarer medium (e.g., air to glass), its speed decreases, causing it to bend towards the normal. When light travels from a denser medium to a rarer medium (e.g., glass to air), its speed increases, causing it to bend away from the normal. When light strikes perpendicularly, it passes through without any deviation.

The second law of refraction is known as Snell's Law: for a given pair of media, sin i / sin r = n₂₁, where n₂₁ is the refractive index of medium 2 with respect to medium 1. The absolute refractive index of a medium (n) is given by n = c / v, where c is the speed of light in vacuum (3 × 10⁸ m/s) and v is the speed of light in that specific medium. Higher refractive index indicates greater optical density and greater bending of light.

4. Lenses, Lens Formula, and Power

A spherical lens is a transparent material bounded by two spherical surfaces. A convex lens is thicker in the middle and thinner at the edges; it converges parallel light rays. A concave lens is thinner in the middle and thicker at the edges; it diverges light rays. Similar to mirrors, convex lenses can form both real and virtual images depending on object position, while concave lenses always produce virtual, erect, and diminished images.

The Lens Formula is expressed as:

1/f = 1/v - 1/u

The magnification for a lens is given by m = v/u = h'/h (note the positive sign compared to mirror magnification). The Power of a Lens (P) measures its ability to converge or diverge light rays. It is mathematically defined as the reciprocal of its focal length in meters: P = 1/f (in meters). The SI unit of power is the Dioptre (D). Convex lenses have positive power (+D), whereas concave lenses have negative power (-D).

Frequently Asked Questions

Q1. Why are convex mirrors preferred as rear-view mirrors in vehicles?

Convex mirrors are preferred as rear-view mirrors in vehicles for two major reasons: first, they always form an erect, though diminished, image of objects behind the vehicle; second, because they curve outwards, they provide a much wider field of view compared to plane or concave mirrors, enabling drivers to view a large area of traffic behind them safely.

Q2. State Snell's Law of refraction and write its mathematical expression.

Snell's Law states that the ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is constant for a given pair of media and for light of a given color. Mathematically, it is expressed as: sin i / sin r = n₂₁ (where n₂₁ represents the relative refractive index of medium 2 with respect to medium 1).

Q3. A concave lens has a focal length of 15 cm. At what distance should an object be placed from the lens so that it forms an image at 10 cm from the lens?

For a concave lens, both focal length and image distance are negative according to sign conventions: f = -15 cm, v = -10 cm. Using the Lens Formula 1/f = 1/v - 1/u:

1/(-15) = 1/(-10) - 1/u
1/u = 1/(-10) - 1/(-15) = -1/10 + 1/15 = (-3 + 2) / 30 = -1/30
Therefore, u = -30 cm. The object should be placed 30 cm in front of the concave lens.

Q4. What is meant by 1 Dioptre power of a lens?

One Dioptre (1 D) is defined as the power of a lens whose focal length is exactly 1 meter (1 m). It is the SI unit used to measure the corrective power of optical lenses in eyeglasses and optical devices.

Tips to Score Full Marks

• Master Ray Diagrams: Practice drawing ray diagrams for all 6 object positions of a concave mirror and convex lens, as well as positions for convex mirrors and concave lenses. Always use a sharp pencil and ruler, and label the principal axis, pole/optical center, focus (F), center of curvature (C), and arrows indicating light direction.
• Be Rigorous with Sign Conventions: In numerical problems, always list the given quantities with their correct positive or negative signs before applying formulas. Remember that u is always negative, focal length f is negative for concave surfaces, and positive for convex surfaces.
• Convert Focal Length to Meters for Power: When calculating power in Dioptres (D), convert focal length from centimeters to meters first using f(m) = f(cm) / 100. A common mistake in MP Board Class 10 Science exams is forgetting this unit conversion.
• Write Complete Step-by-Step Solutions: Board examiners assign step-marks. Write down the relevant formula, substitute values with correct signs, solve step-by-step, and state the final answer clearly with appropriate SI units (e.g., cm, m, or D).
• Memorize Key Applications: Pay special attention to short answer questions asking about practical uses of mirrors and lenses (such as solar cookers, searchlights, spectacles, and vehicle mirrors) as these frequently appear in short-answer sections.