Class 10 Notes

Class 10 Science Notes Chapter 10 (Light reflection and refraction) – Science Book

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Philoid

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Detailed Notes with MCQs of Chapter 10: Light - Reflection and Refraction. This is a fundamental chapter, not just for your Class 10 understanding but also forms the basis for optics in higher studies and is frequently tested in government exams. Pay close attention to the definitions, laws, formulas, and sign conventions.

Chapter 10: Light - Reflection and Refraction: Detailed Notes

1. Introduction to Light

  • Light is a form of energy that enables us to see objects.
  • It travels in straight lines (Rectilinear propagation of light).
  • Light exhibits dual nature: behaves as both a wave and a particle (photon). In this chapter, we primarily consider its ray nature.
  • Speed of light in vacuum (or air) is approximately c = 3 x 10⁸ m/s.

2. Reflection of Light

  • Definition: 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 surface at the point of incidence all lie in the same plane.
    • Note: These laws apply to all reflecting surfaces, whether plane or curved.
  • Types of Reflection:
    • Regular Reflection: Occurs when parallel incident rays strike a smooth surface (like a mirror) and reflect as parallel rays. Forms clear images.
    • Diffused Reflection (Irregular Reflection): Occurs when parallel incident rays strike a rough surface and reflect in various directions. Does not form clear images but makes non-luminous objects visible.

3. Mirrors

  • Plane Mirror:

    • A flat, smooth reflecting surface.
    • Characteristics of Image formed by a Plane Mirror:
      • Virtual (cannot be obtained on a screen)
      • Erect (upright)
      • Same size as the object (Magnification = +1)
      • Laterally inverted (left appears right and vice versa)
      • Image distance (v) is equal to the object distance (u) from the mirror. (v = u, but behind the mirror)

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

    • Concave Mirror: Reflecting surface curves inwards. Also called a converging mirror.
    • Convex Mirror: Reflecting surface curves outwards. Also called a diverging mirror.

  • Important Terms for Spherical Mirrors:

    • Pole (P): The centre of the reflecting surface of the mirror.
    • Centre of Curvature (C): The centre of the sphere of which the mirror is a part.
    • Radius of Curvature (R): The radius of the sphere of which the mirror is a part (Distance PC).
    • Principal Axis: The straight line passing through the Pole (P) and the Centre of Curvature (C).
    • Principal Focus (F):
      • Concave Mirror: A point on the principal axis where parallel rays converge after reflection. It's real.
      • Convex Mirror: A point on the principal axis from which parallel rays appear to diverge after reflection. It's virtual.
    • Focal Length (f): The distance between the Pole (P) and the Principal Focus (F). (Distance PF).
    • Relationship between R and f: For spherical mirrors of small aperture, R = 2f or f = R/2.

  • Image Formation by Spherical Mirrors (Ray Diagrams):

    • Rules for drawing rays:
      1. A ray parallel to the principal axis passes/appears to pass through the focus (F) after reflection.
      2. A ray passing/appearing to pass through the focus (F) becomes parallel to the principal axis after reflection.
      3. A ray passing/directed towards the centre of curvature (C) reflects back along the same path.
      4. A ray incident obliquely towards the pole (P) reflects obliquely, following the laws of reflection (∠i = ∠r).
    • Image Formation by Concave Mirror:
      Object Position Image Position Size of Image Nature of Image
      At Infinity At Focus (F) Highly diminished Real & Inverted
      Beyond C Between F and C Diminished Real & Inverted
      At C At C Same size Real & Inverted
      Between C and F Beyond C Enlarged Real & Inverted
      At Focus (F) At Infinity Highly enlarged Real & Inverted
      Between P and F Behind the mirror Enlarged Virtual & Erect
    • Image Formation by Convex Mirror:
      Object Position Image Position Size of Image Nature of Image
      At Infinity At Focus (F) (behind) Highly diminished Virtual & Erect
      Anywhere else Between P and F (behind) Diminished Virtual & Erect
    • Uses:
      • Concave: Shaving mirrors, dentist mirrors, torches, searchlights, solar furnaces, reflecting telescopes.
      • Convex: Rear-view mirrors in vehicles (provide a wider field of view), security mirrors in shops.

  • Mirror Formula and Magnification:

    • New Cartesian Sign Convention:
      1. The pole (P) is the origin.
      2. The principal axis is the X-axis.
      3. The object is always placed to the left of the mirror (incident light travels from left to right).
      4. Distances measured in the direction of incident light are positive; against it are negative. (Generally, distances left of P are negative, right are positive).
      5. Distances measured upward perpendicular to the principal axis are positive; downward are negative.
      • Key implications:
        • u (object distance) is always negative.
        • f is negative for concave mirrors, positive for convex mirrors.
        • Real images (usually formed in front of the mirror) have negative v. Virtual images (formed behind the mirror) have positive v.
        • Height of erect image (h') is positive. Height of inverted image (h') is negative. Object height (h) is usually positive.
    • Mirror Formula: Relates object distance (u), image distance (v), and focal length (f).
      1/v + 1/u = 1/f
    • 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, the image is real and inverted.
      • If m is positive, the image is virtual and erect.
      • If |m| > 1, the image is enlarged.
      • If |m| < 1, the image is diminished.
      • If |m| = 1, the image is the same size.

4. Refraction of Light

  • Definition: The phenomenon of 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.
  • 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 (∠i) to the sine of the angle of refraction (∠r) is constant for a given pair of media and a given colour of light.
      sin i / sin r = constant = n₂₁
      where n₂₁ is the relative refractive index of medium 2 with respect to medium 1.
  • Refractive Index (n):
    • A measure of how much light bends when entering a medium. It's related to the speed of light in the medium.
    • Absolute Refractive Index (n): Refractive index of a medium with respect to vacuum (or air).
      n = c / v (where c = speed of light in vacuum, v = speed of light in the medium).
    • Relative Refractive Index (n₂₁): Refractive index of medium 2 with respect to medium 1.
      n₂₁ = v₁ / v₂ = n₂ / n₁
    • Optically Denser Medium: Medium with a higher refractive index (light travels slower).
    • Optically Rarer Medium: Medium with a lower refractive index (light travels faster).
    • Note: When light goes from rarer to denser medium, it bends towards the normal (∠r < ∠i). When it goes from denser to rarer, it bends away from the normal (∠r > ∠i).
  • Refraction through a Rectangular Glass Slab:
    • The emergent ray is parallel to the incident ray but is laterally displaced.
    • Lateral displacement depends on the angle of incidence, thickness of the slab, and refractive index of the glass.

5. Lenses

  • Definition: A transparent material bound by two surfaces, at least one of which is curved.

  • Types:

    • Convex Lens (Converging Lens): Thicker at the centre, thinner at the edges. Converges parallel rays.
    • Concave Lens (Diverging Lens): Thinner at the centre, thicker at the edges. Diverges parallel rays.

  • Important Terms for Lenses:

    • Optical Centre (O): The 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: The line joining the two centres of curvature and passing through the optical centre.
    • Principal Focus (F):
      • Convex Lens: Has two foci (F₁, F₂). F₂ is the principal focus where parallel rays converge after refraction. F₁ is the point from which rays become parallel after refraction. Real focus.
      • Concave Lens: Has two foci (F₁, F₂). F₂ is the principal focus from which parallel rays appear to diverge after refraction. F₁ is the point towards which rays must be directed to become parallel after refraction. Virtual focus.
    • Focal Length (f): The distance between the Optical Centre (O) and the Principal Focus (F₂).

  • Image Formation by Lenses (Ray Diagrams):

    • Rules for drawing rays:
      1. A ray parallel to the principal axis passes/appears to pass through the second principal focus (F₂) after refraction.
      2. A ray passing/appearing to pass through the first principal focus (F₁) becomes parallel to the principal axis after refraction.
      3. A ray passing through the optical centre (O) goes undeviated.
    • Image Formation by Convex Lens:
      Object Position Image Position Size of Image Nature of Image
      At Infinity At Focus F₂ Highly diminished Real & Inverted
      Beyond 2F₁ Between F₂ and 2F₂ Diminished Real & Inverted
      At 2F₁ At 2F₂ Same size Real & Inverted
      Between F₁ and 2F₁ Beyond 2F₂ Enlarged Real & Inverted
      At Focus F₁ At Infinity Highly enlarged Real & Inverted
      Between O and F₁ On same side as object Enlarged Virtual & Erect
    • Image Formation by Concave Lens:
      Object Position Image Position Size of Image Nature of Image
      At Infinity At Focus F₂ (same side) Highly diminished Virtual & Erect
      Anywhere else Between O and F₂ (same side) Diminished Virtual & Erect
    • Uses:
      • Convex: Magnifying glass, cameras, microscopes, telescopes, eyeglasses for hypermetropia (long-sightedness).
      • Concave: Eyeglasses for myopia (short-sightedness), Galilean telescopes, wide-angle spyholes in doors.

  • Lens Formula and Magnification:

    • Sign Convention (Similar to mirrors, but origin is Optical Centre O):
      1. Optical Centre (O) is the origin.
      2. Principal axis is the X-axis.
      3. Object is always placed to the left.
      4. Distances left of O are negative, right are positive.
      5. Distances upward are positive, downward are negative.
      • Key implications:
        • u is always negative.
        • f is positive for convex lenses, negative for concave lenses.
        • Real images (usually formed on the right) have positive v. Virtual images (usually formed on the left) have negative v.
    • Lens Formula:
      1/v - 1/u = 1/f (Note the minus sign, different from mirror formula)
    • Magnification (m):
      m = h'/h = v/u (Note the positive sign, different from mirror magnification)
      • Interpretation of sign and magnitude of m is the same as for mirrors.

  • Power of a Lens (P):

    • Definition: The ability of a lens to converge or diverge light rays. It is the reciprocal of the focal length in meters.
    • P = 1 / f (where f is in meters)
    • Unit: Dioptre (D). 1 D = 1 m⁻¹.
    • Sign: Power is positive for convex lenses (converging) and negative for concave lenses (diverging).
    • Power of Combination: For lenses in contact, the total power is the algebraic sum of individual powers: P = P₁ + P₂ + ...

Multiple Choice Questions (MCQs)

  1. The laws of reflection hold true for:
    (a) Plane mirrors only
    (b) Concave mirrors only
    (c) Convex mirrors only
    (d) All reflecting surfaces

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

  3. Which type of mirror is used as a rear-view mirror in vehicles and why?
    (a) Concave mirror, as it forms enlarged images.
    (b) Plane mirror, as it forms images of the same size.
    (c) Convex mirror, as it provides a wider field of view.
    (d) Parabolic mirror, for better focus.

  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) Light bends 2.42 degrees when entering diamond.
    (d) Diamond reflects 2.42% of the light.

  5. According to Snell's law, the ratio sin i / sin r is constant. This constant is known as:
    (a) Magnification
    (b) Power of the lens
    (c) Refractive index of medium 2 w.r.t medium 1
    (d) Focal length

  6. A convex lens has a focal length of +20 cm. According to the sign convention, the power of this lens is:
    (a) +0.05 D
    (b) -0.05 D
    (c) +5 D
    (d) -5 D

  7. When light travels from air into a glass slab, it bends:
    (a) Towards the normal
    (b) Away from the normal
    (c) Does not bend
    (d) Parallel to the interface

  8. A virtual, erect, and magnified image can be formed by:
    (a) Convex mirror only
    (b) Concave mirror only
    (c) Plane mirror only
    (d) Both convex and concave mirrors

  9. The lens formula is given by:
    (a) 1/v + 1/u = 1/f
    (b) 1/v - 1/u = 1/f
    (c) m = -v/u
    (d) P = 1/f (f in cm)

  10. A ray of light passing through the optical centre of a thin lens will:
    (a) Converge at the focus
    (b) Diverge from the focus
    (c) Emerge parallel to the principal axis
    (d) Emerge without any deviation

Answers to MCQs:

  1. (d) All reflecting surfaces
  2. (b) Real, inverted, and same size
  3. (c) Convex mirror, as it provides a wider field of view.
  4. (b) Speed of light in vacuum is 2.42 times the speed of light in diamond.
  5. (c) Refractive index of medium 2 w.r.t medium 1
  6. (c) +5 D (Power = 1/f(m) = 1/0.20m = +5 D)
  7. (a) Towards the normal (Air is rarer, glass is denser)
  8. (b) Concave mirror only (When object is between P and F)
  9. (b) 1/v - 1/u = 1/f
  10. (d) Emerge without any deviation

Study these notes thoroughly, focusing on understanding the concepts behind the formulas and diagrams. Practice applying the sign conventions correctly. Good luck with your preparation!

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