Reflection of Light

Reflection of Light

  • Definition of Light

    • Light is a form of energy that enables us to see objects around us.
    • Essential for visibility; we need light sources to illuminate objects.

  • Luminous vs. Non-Luminous Objects

    • Luminous Objects: Emit light (e.g., sun, electric bulbs).
    • Examples: Sun, stars, electric bulbs, candles.
    • Non-Luminous Objects: Do not emit their own light; visible due to reflected light.
    • Examples: Flowers, tables, and other everyday objects.
    • The moon reflects sunlight and is considered non-luminous.

  • Reflection of Light

    • Defined as the bouncing back of light rays when they hit a surface.
    • Similar to the bouncing of a tennis ball against a wall.
    • Example: A beam of light falls on a mirror and reflects back.

  • Characteristics of Reflection

    • Objects with shiny surfaces (like mirrors) reflect more light compared to dull surfaces.
    • Polished surfaces create clearer reflections.

  • Plane Mirror

    • A plane mirror has a smooth, flat surface; usually coated with silver or aluminum.
    • It reflects light to form clear images, following the laws of reflection.

Nature of Light

  • Wave Theory of Light

    • Light comprises electromagnetic waves that don't require a medium for propagation.
    • Wavelength: approximately 4 × 10^-7 to 8 × 10^-7 m.
    • Speed in a vacuum: about 3 × 10^8 m/s.

  • Particle Theory of Light

    • Composed of particles (photons) traveling in straight lines at high speed.

  • Dual Nature of Light

    • Light exhibits both wave and particle characteristics, leading to the Quantum Theory of Light.
    • Some phenomena (e.g., diffraction, interference) support wave theory; others (e.g., reflection, refraction) support particle theory.

Laws of Reflection of Light

  • First Law: The incident ray, reflected ray, and normal line to the surface at the point of incidence lie in the same plane.
  • Second Law: The angle of reflection is equal to the angle of incidence.
    • If angle of incidence (i) is 35°, then angle of reflection (r) is also 35°.

Regular vs. Diffuse Reflection

  • Regular Reflection

    • Occurs on smooth surfaces (e.g., mirrors) where parallel light rays are reflected in a single direction.
    • Produces clear images.

  • Diffuse Reflection

    • Occurs on rough surfaces; parallel light rays scatter in different directions.
    • No clear image is formed (e.g., reflections off paper surfaces).

Image Formation by Plane Mirrors

  • Image Characteristics

    • Virtual (not projectable on a screen), erect, same size as the object, laterally inverted.
    • Distance from the mirror equals the distance of the object in front of it.

  • Lateral Inversion

    • The right side of an object appears as the left side in its mirror image and vice versa.
    • Example: The word "RED" in front of a mirror appears reversed.

Uses of Plane Mirrors

  • Everyday mirrors for personal use.
  • Enlarged views in jewellery shops.
  • Safety mirrors at road turns.
  • Facilitating reflection in periscopes.

Sample Problems and Solutions

  1. Angle of Reflection:

    • If the incident ray makes an angle of 35° with the surface, the angle of reflection = 90° - 35° = 55°.

  2. David's Distance Problem:

    • If David is 4 m from the mirror and moves 1 m closer, new distance = 3 m; distance to the image = 3 m. Total distance = 6 m.