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Principles of Light Reflection and Refraction

May 9, 2025

Light: Reflection and Refraction

Overview

  • Light: A form of energy that travels in a straight line.
  • Key Concepts: Reflection and refraction of light.

Reflection of Light

Definition

  • Occurs when a light ray strikes a smooth, polished surface (e.g., a mirror) and bounces back.

Mirrors

  • Plain Mirrors:
    • Create virtual images located behind the mirror at the same distance as objects in front.
    • Images are laterally reversed (left-right reversed).
    • No magnification; image size equals object size.
  • Spherical Mirrors:
    • Two types: Convex (outward curved) and Concave (inward curved).

Key Terms

  • Pole (P): Center point of the reflecting surface.
  • Center of Curvature (C): Center of the sphere from which the mirror is derived.
  • Radius of Curvature (R): Radius of the imaginary sphere.
  • Principal Axis: Line joining the pole and center of curvature.
  • Principal Focus (F): Point where parallel light rays meet or appear to meet after reflection.
  • Focal Length (f): Distance between the pole and the principal focus.

Image Formation by Spherical Mirrors

  • Concave Mirrors:
    • Can form real and inverted or virtual and erect images.
    • Uses include shaving mirrors, dentist mirrors, and reflectors.
  • Convex Mirrors:
    • Always form virtual and erect images.
    • Used as rear-view mirrors in vehicles.

Mirror Formula and Magnification

  • Mirror Formula: ( \frac{1}{v} + \frac{1}{u} = \frac{1}{f} )
    • ( v ) = image distance, ( u ) = object distance, ( f ) = focal length.
  • Magnification: Ratio of image height to object height: ( M = \frac{h'}{h} ).

Refraction of Light

Definition

  • Change in direction of light as it passes from one medium to another due to a change in speed.

Effects of Refraction

  • Objects (e.g., straws in water) appear bent due to refraction.

Laws of Refraction

  1. Incident ray, refracted ray, and normal lie in the same plane.
  2. Snell's Law: ( \frac{\sin i}{\sin r} = \text{constant} )

Refractive Index

  • Measure of the reduction of light speed in a medium compared to vacuum.
  • Absolute Refractive Index: ( n = \frac{c}{v} )

Refraction by Lenses

Types of Lenses

  • Convex Lens: Converges light rays; thicker at center.
  • Concave Lens: Diverges light rays; thicker at edges.

Image Formation by Lenses

  • Convex Lens:
    • Forms real, inverted images or virtual, erect images depending on object position.
  • Concave Lens:
    • Always forms virtual, erect, diminished images.

Lens Formula and Magnification

  • Lens Formula: ( \frac{1}{v} - \frac{1}{u} = \frac{1}{f} )
  • Magnification: ( M = \frac{h'}{h} = \frac{v}{u} )

Power of a Lens

  • Power ( P = \frac{1}{f} ) (in diopters).
    • Positive for converging lenses, negative for diverging lenses.

These notes cover the fundamental principles of light reflection and refraction, discussing various properties and applications of mirrors and lenses.

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