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Light Reflection & Refraction: Complete One Shot Class 10 Physics

Light: The Basics

Light is a form of energy that is not visible itself but enables us to see objects. It travels in a straight line (rectilinear propagation) at a speed of 3 × 10⁸ m/s and can travel through a vacuum.

Reflection of Light

Reflection is the bouncing back of light rays when they strike a surface. For a deeper dive into the principles behind this, see our guide on Understanding the Reflection of Electromagnetic Waves: A Comprehensive Guide.

Key Terms in Reflection (Ray Diagram)

  • Incident Ray: The ray that strikes the surface.
  • Reflected Ray: The ray that bounces off.
  • Normal: An imaginary line perpendicular to the surface at the point of incidence.

Laws of Reflection

  1. Angle of incidence (∠i) = Angle of reflection (∠r)
  2. The incident ray, reflected ray, and normal all lie in the same plane.

Lateral Inversion

This is the phenomenon where the left and right sides of an object appear reversed in a mirror. Example: The word "AMBULANCE" is written backwards on vehicles so it appears correctly in rear-view mirrors.

Plane Mirror Characteristics

  • Forms a virtual and erect image.
  • Image size = Object size.
  • Image distance = Object distance from the mirror.
  • Focal length = Infinite.

Spherical Mirrors

Spherical mirrors are of two types:

  • Concave Mirror: Converging mirror (caves in like a cave). Used for magnifying images.
  • Convex Mirror: Diverging mirror (bulges out). Always forms small, virtual images.

To better understand the geometry and principles that govern these mirrors, refer to Understanding Geometrical Optics: Principles, Mirrors, and Lenses.

Important Terms for Spherical Mirrors

  • Pole (P): The midpoint of the reflecting surface.
  • Center of Curvature (C): The center of the sphere of which the mirror is a part.
  • Radius of Curvature (R): Distance from P to C.
  • Principal Axis: The line passing through P and C.
  • Principal Focus (F): The midpoint of C and P.
  • Focal Length (f): Distance from P to F.
  • Aperture: The diameter of the reflecting surface.

Key Relation: R = 2f (Radius of Curvature = 2 × Focal Length)

Rules for Ray Diagrams (Concave & Convex)

  1. A ray parallel to the principal axis passes through (or appears to pass through) the focus (F).
  2. A ray passing through the focus becomes parallel to the principal axis.
  3. A ray passing through the center of curvature (C) reflects back along the same path.
  4. A ray striking the pole (P) reflects at the same angle to the principal axis.

Image Formation by Concave Mirror (with the "Candle" Trick)

Use the counting trick with positions marked 1 to 6 from Infinity to the Pole:

| Object Position | Image Position | Nature of Image | Size | | :--- | :--- | :--- | :--- | | 1. At Infinity | At F | Real & Inverted | Highly Diminished | | 2. Beyond C (Between ∞ and C) | Between C and F | Real & Inverted | Diminished | | 3. At C | At C | Real & Inverted | Same Size | | 4. Between C and F | Beyond C | Real & Inverted | Enlarged | | 5. At F | At Infinity | Image not formed (as per updated NCERT) | - | | 6. Between P and F | Behind the mirror | Virtual & Erect | Enlarged |

Key Insight for Answering Questions:

  • Real images are inverted and formed on the left (in front) of the mirror.
  • Virtual images are erect and formed on the right (behind) the mirror.

Image Formation by Convex Mirror

Convex mirrors have only two main cases. They always form a virtual, erect, and diminished image.

  • Object at Infinity: Image at F (point-sized, virtual).
  • Object Anywhere Else: Image is formed between P and F (behind the mirror).

Uses of Mirrors

  • Concave: Shaving mirrors, dentist mirrors, headlights, solar furnaces.
  • Convex: Rear-view mirrors (vehicles), security mirrors (shops/hotels) for a wider field of view.

Sign Convention & Mirror Formula

Sign Convention

  • All distances measured from the pole (P).
  • Left side (in front of mirror): Negative
  • Right side (behind mirror): Positive
  • Upward direction: Positive
  • Downward direction: Negative

Standard Terms

  • u: Object distance (always negative for real objects)
  • v: Image distance
  • f: Focal length (concave = negative, convex = positive)
  • h₀: Object height (positive if above axis)
  • hᵢ: Image height (positive if above axis)

Mirror Formula

1/v + 1/u = 1/f

Magnification (m)

m = hᵢ / h₀ = -v / u

  • m < 0: Image is real and inverted.
  • m > 0: Image is virtual and erect.
  • |m| > 1: Image is enlarged.
  • |m| < 1: Image is diminished.

Numerical Examples & Practice Questions

Example 1: Magnification

An object of height 15 cm placed in front of a concave mirror. The image is 45 cm tall and real.

  • h₀ = +15 cm (upright)
  • hᵢ = -45 cm (inverted, so negative)
  • m = hᵢ / h₀ = -45 / 15 = -3

Example 2: Erect Image in Concave Mirror

To obtain an erect (virtual) image using a concave mirror (f = 10 cm), where should the object be placed?

Answer: The object must be placed between the pole (P) and the focus (F), i.e., at a distance less than 10 cm from the mirror.

For a complete board exam crash course covering this and related topics, visit Light Reflection & Refraction Class 10: Complete Board Exam Crash Course.

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