Class 10 Notes

Class 10 Science Notes Chapter 10 (Chapter 10) – Examplar Problems (English) Book

Philoid

Philoid

— 8 min read

Examplar Problems (English)
Detailed Notes with MCQs of Chapter 10: Light – Reflection and Refraction from your NCERT Exemplar book. This is a crucial chapter, not just for your board exams but also for various government exams where basic science concepts are tested. Pay close attention to the definitions, laws, formulas, and applications.

Chapter 10: Light – Reflection and Refraction: Detailed Notes

1. Nature of Light

  • Light is a form of electromagnetic radiation that enables us to see objects.
  • It exhibits dual nature: behaves as both a wave (shows diffraction, interference) and a particle (photons, explains photoelectric effect). For most phenomena in this chapter (reflection, refraction), we consider its ray or wave nature.
  • Light travels in straight lines (rectilinear propagation) in a uniform medium.
  • Speed of light in vacuum (c) ≈ 3 × 10⁸ m/s. Speed decreases in denser mediums.

2. Reflection of Light

  • The bouncing back of light rays into the same medium after striking a surface.
  • Laws of Reflection:
    • First Law: The angle of incidence (∠i) is equal to the angle of reflection (∠r). (∠i = ∠r)
    • Second Law: The incident ray, the reflected ray, and the normal to the reflecting surface at the point of incidence all lie in the same plane.
  • These laws apply to all types of reflecting surfaces (plane and spherical).

3. Plane Mirrors

  • Image Formation: Forms a virtual, erect image.
  • Characteristics of Image:
    • Same size as the object (Magnification, m = +1).
    • Located as far behind the mirror as the object is in front of it (Image distance = Object distance).
    • Laterally inverted (left appears right and vice versa).
  • Uses: Looking glass, periscopes, kaleidoscopes.

4. Spherical Mirrors

  • Mirrors whose reflecting surface is a part of a sphere.

  • Types:

    • Concave Mirror: Reflecting surface curves inwards (converging mirror).
    • Convex Mirror: Reflecting surface curves outwards (diverging mirror).

  • Terminology:

    • Pole (P): Centre of the reflecting surface of the mirror.
    • Centre of Curvature (C): Centre of the sphere of which the mirror is a part.
    • Radius of Curvature (R): Radius of the sphere of which the mirror is a part (Distance PC).
    • Principal Axis: Straight line passing through the Pole (P) and Centre of Curvature (C).
    • Principal Focus (F):
      • Concave Mirror: Point on the principal axis where parallel rays converge after reflection.
      • Convex Mirror: Point on the principal axis from which parallel rays appear to diverge after reflection.
    • Focal Length (f): Distance between the Pole (P) and the Principal Focus (F). (f = R/2)
    • Aperture: Diameter of the reflecting surface.

  • New Cartesian Sign Convention (for Mirrors):

    • Object is always placed to the left of the mirror (incident light travels from left to right).
    • All distances are measured from the Pole (P).
    • Distances measured in the direction of incident light are positive (+ve).
    • Distances measured against the direction of incident light are negative (-ve).
    • Heights measured upwards and perpendicular to the principal axis are positive (+ve).
    • Heights measured downwards and perpendicular to the principal axis are negative (-ve).
    • Consequence: u is always -ve. f is -ve for concave, +ve for convex. R is -ve for concave, +ve for convex. Real images have -ve v, virtual images have +ve v.

  • Image Formation by Concave Mirror (Summary):

    • Object at Infinity → Image at F (Real, Inverted, Point-sized)
    • Object beyond C → Image between F and C (Real, Inverted, Diminished)
    • Object at C → Image at C (Real, Inverted, Same size)
    • Object between F and C → Image beyond C (Real, Inverted, Magnified)
    • Object at F → Image at Infinity (Real, Inverted, Highly Magnified)
    • Object between P and F → Image behind mirror (Virtual, Erect, Magnified) - Special Case

  • Image Formation by Convex Mirror (Summary):

    • Object at Infinity → Image at F (behind mirror) (Virtual, Erect, Point-sized)
    • Object anywhere between Infinity and P → Image between P and F (behind mirror) (Virtual, Erect, Diminished)
    • Note: Convex mirrors always form virtual, erect, and diminished images, irrespective of object position (except infinity).

  • Uses:

    • Concave: Shaving mirrors, dentists' mirrors, headlights/searchlights (bulb at F), solar furnaces (concentrate sunlight).
    • Convex: Rear-view mirrors in vehicles (wider field of view), security mirrors in shops.

  • Mirror Formula: Relates object distance (u), image distance (v), and focal length (f).
    1/v + 1/u = 1/f (Remember to use sign convention)

  • Magnification (m): Ratio of the height of the image (h') to the height of the object (h).
    m = h'/h = -v/u

    • If m is negative (-ve), image is Real & Inverted.
    • If m is positive (+ve), image is Virtual & Erect.
    • If |m| > 1, image is Magnified.
    • If |m| < 1, image is Diminished.
    • If |m| = 1, image is Same size.

5. Refraction of Light

  • The bending of light as it passes obliquely from one transparent medium to another.

  • Cause: Change in the speed of light as it enters a different medium.

    • Light bends towards the normal when going from a rarer medium (e.g., air) to a denser medium (e.g., glass). Speed decreases.
    • Light bends away from the normal when going from a denser medium (e.g., glass) to a rarer medium (e.g., air). Speed increases.

  • Laws of Refraction:

    • First Law: The incident ray, the refracted ray, and the normal to the interface of the two media at the point of incidence all lie in the same plane.
    • Second Law (Snell's Law): The ratio of the sine of the angle of incidence (sin i) to the sine of the angle of refraction (sin r) is constant for a given pair of media and for light of a given colour. This constant is the refractive index of the second medium with respect to the first (n₂₁).
      sin i / sin r = n₂₁ = n₂ / n₁ = v₁ / v₂
      (where n₁ and n₂ are absolute refractive indices, v₁ and v₂ are speeds of light in medium 1 and 2 respectively).

  • Refractive Index (n): Measure of how much a medium bends light. It's the ratio of the speed of light in vacuum (c) to the speed of light in the medium (v).
    n = c / v

    • Absolute Refractive Index: Refractive index of a medium with respect to vacuum (or air, approximately). e.g., n_glass ≈ 1.5, n_water ≈ 1.33. Higher 'n' means optically denser medium.
    • Relative Refractive Index (n₂₁): Refractive index of medium 2 with respect to medium 1. n₂₁ = n₂ / n₁. Also, n₁₂ = 1 / n₂₁.

  • Refraction through a Rectangular Glass Slab:

    • Emergent ray is parallel to the incident ray.
    • The emergent ray is laterally displaced relative to the original path of the incident ray. Lateral displacement depends on the angle of incidence, thickness of the slab, and refractive index of the glass.

6. Spherical Lenses

  • A transparent medium bounded by two surfaces, at least one of which is spherical.

  • Types:

    • Convex Lens: Thicker at the centre, thinner at the edges (converging lens).
    • Concave Lens: Thinner at the centre, thicker at the edges (diverging lens).

  • Terminology:

    • Optical Centre (O): Central point of the lens. A ray passing through O goes undeviated.
    • Centres of Curvature (C₁, C₂): Centres of the spheres of which the lens surfaces are parts. A lens has two centres of curvature.
    • Principal Axis: Line joining the two centres of curvature (passes through O).
    • Principal Focus (F):
      • Convex Lens: Point on the principal axis where parallel rays converge after refraction (Real focus). Has two foci (F₁, F₂), equidistant from O. F₂ is usually considered the principal focus.
      • Concave Lens: Point on the principal axis from which parallel rays appear to diverge after refraction (Virtual focus). Has two foci (F₁, F₂), equidistant from O. F₁ is usually considered the principal focus.
    • Focal Length (f): Distance between the Optical Centre (O) and the Principal Focus (F).

  • New Cartesian Sign Convention (for Lenses):

    • Similar to mirrors, but distances are measured from the Optical Centre (O).
    • Object is to the left (u is -ve).
    • Distances in direction of incident light are +ve; against are -ve.
    • Heights upwards are +ve; downwards are -ve.
    • Consequence: f is +ve for convex lens, -ve for concave lens. Real images have +ve v, virtual images have -ve v.

  • Image Formation by Convex Lens (Summary):

    • Object at Infinity → Image at F₂ (Real, Inverted, Point-sized)
    • Object beyond 2F₁ → Image between F₂ and 2F₂ (Real, Inverted, Diminished)
    • Object at 2F₁ → Image at 2F₂ (Real, Inverted, Same size)
    • Object between F₁ and 2F₁ → Image beyond 2F₂ (Real, Inverted, Magnified)
    • Object at F₁ → Image at Infinity (Real, Inverted, Highly Magnified)
    • Object between O and F₁ → Image on same side as object (Virtual, Erect, Magnified) - Special Case (Magnifying Glass)

  • Image Formation by Concave Lens (Summary):

    • Object at Infinity → Image at F₁ (Virtual, Erect, Point-sized)
    • Object anywhere between Infinity and O → Image between O and F₁ (Virtual, Erect, Diminished)
    • Note: Concave lenses always form virtual, erect, and diminished images, irrespective of object position (except infinity).

  • Uses:

    • Convex: Magnifying glass, cameras, microscopes, telescopes, spectacles (for hypermetropia).
    • Concave: Spectacles (for myopia), Galilean telescopes, peepholes in doors.

  • Lens Formula: Relates object distance (u), image distance (v), and focal length (f).
    1/v - 1/u = 1/f (Remember to use sign convention)

  • Magnification (m): Ratio of the height of the image (h') to the height of the object (h).
    m = h'/h = v/u (Note: No negative sign here, unlike mirrors)

    • Interpretation of sign and magnitude of m is the same as for mirrors.

7. Power of a Lens (P)

  • Degree of convergence or divergence of light rays achieved by a lens.
  • Defined as the reciprocal of the focal length (f) expressed in metres.
    P = 1 / f (in metres)
  • Unit: Dioptre (D). 1 D = 1 m⁻¹.
  • Sign: Power is positive (+ve) for a convex lens (converging) and negative (-ve) for a concave lens (diverging).
  • Combination of Lenses: If several thin lenses are placed in contact, the power of the combination (P) is the algebraic sum of the powers of individual lenses (P₁, P₂, P₃...).
    P = P₁ + P₂ + P₃ + ...
    The equivalent focal length (f) of the combination is given by:
    1/f = 1/f₁ + 1/f₂ + 1/f₃ + ...

Multiple Choice Questions (MCQs)

  1. A ray of light travelling from a denser medium to a rarer medium bends:
    a) Towards the normal
    b) Away from the normal
    c) Along the normal
    d) Parallel to the interface

  2. Which type of mirror is used as a rear-view mirror in vehicles and why?
    a) Concave mirror, as it forms magnified images.
    b) Convex mirror, as it provides a wider field of view.
    c) Plane mirror, as it forms images of the same size.
    d) Convex mirror, as it always forms real images.

  3. An object is placed at the centre of curvature (C) of a concave mirror. The image formed is:
    a) Real, inverted, and diminished
    b) Virtual, erect, and magnified
    c) Real, inverted, and same size
    d) Real, inverted, and magnified

  4. The refractive index of diamond is 2.42. What is the meaning of this statement in relation to the speed of light?
    a) Speed of light in diamond is 2.42 times the speed of light in vacuum.
    b) Speed of light in vacuum is 2.42 times the speed of light in diamond.
    c) Speed of light in diamond is equal to the speed of light in vacuum.
    d) Diamond cannot refract light.

  5. A convex lens has a focal length of 20 cm. Its power in dioptres is:
    a) +0.05 D
    b) -0.05 D
    c) +5 D
    d) -5 D

  6. According to the New Cartesian Sign Convention for spherical mirrors, which of the following is always negative?
    a) Object distance (u)
    b) Image distance (v) for real images
    c) Focal length (f) of a convex mirror
    d) Height of an erect image (h')

  7. Magnification produced by a plane mirror is always:
    a) +1
    b) -1
    c) 0
    d) Less than 1

  8. Where should an object be placed in front of a convex lens to get a real image of the size of the object?
    a) At the principal focus (F₁)
    b) At twice the focal length (2F₁)
    c) At infinity
    d) Between the optical centre (O) and principal focus (F₁)

  9. Snell's law is given by: (n₁ and n₂ are absolute refractive indices of medium 1 and 2)
    a) n₁ sin i = n₂ sin r
    b) n₂ sin i = n₁ sin r
    c) sin i / sin r = n₁ / n₂
    d) n₁ n₂ = sin i / sin r

  10. A concave lens always forms an image which is:
    a) Real, erect, diminished
    b) Virtual, inverted, diminished
    c) Virtual, erect, magnified
    d) Virtual, erect, diminished

Answer Key for MCQs:

  1. b) Away from the normal
  2. b) Convex mirror, as it provides a wider field of view.
  3. c) Real, inverted, and same size
  4. b) Speed of light in vacuum is 2.42 times the speed of light in diamond.
  5. c) +5 D (P = 1/f(m) = 1/0.20m = +5 D)
  6. a) Object distance (u)
  7. a) +1
  8. b) At twice the focal length (2F₁)
  9. a) n₁ sin i = n₂ sin r (derived from sin i / sin r = n₂ / n₁)
  10. d) Virtual, erect, diminished

Study these notes thoroughly. Focus on understanding the concepts behind the formulas and sign conventions, as application-based questions are common in competitive exams. Good luck!

Read more