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Understanding Lenses in Optics

Apr 29, 2025

Lecture on Lenses

Introduction

  • Lenses are typically made from glass or clear plastics.
  • Commonly used in spectacles and optical instruments like cameras and projectors.

Types of Lenses

  1. Convex Lenses (Converging)

    • Thicker in the middle than at the edges.
    • Converges parallel beams to a focal point.
    • Examples: Biconvex, Plano-convex, Concavo-convex.
  2. Concave Lenses (Diverging)

    • Thinner in the middle than at the edges.
    • Diverges parallel beams away from a focal point.
    • Examples: Biconcave, Plano-concave, Convexo-concave.

Terms Associated with Thin Converging Lens

  • Optical Centre (C): Geometric center where light rays pass undeviated.
  • Principal Axis: Line passing symmetrically through the optical centre.
  • Focal Point (F): Point on the principal axis where parallel rays converge after passing through the lens.
  • Focal Length (f): Distance between the optical centre and the focal point.
  • Focal Plane: Plane parallel to the lens passing through the focal point.

Ray Diagrams for Lenses

  • Important Rays:
  1. Through the optical centre (undeviated).
  2. Parallel to the principal axis (converges to the focal point).
  3. Through the focal point (emerges parallel to the principal axis).
  • Steps for Drawing Ray Diagrams:

    1. Choose a scale.
    2. Represent the object with an upright arrow at the object distance from the lens.
    3. Draw rays and their emergent paths to find the image.
  • Notes:

    • Real images can be projected onto a screen and are inverted.
    • Virtual images cannot be projected onto a screen and are upright.

Linear Magnification

  • Defined as the ratio of image length to object length.

Exercises

  • Questions regarding object placement and image characteristics.

Characteristics of Images Formed by Thin Convex Lenses

  • Exercises provided to determine image characteristics and position.

Ray Diagram for Concave Lenses

  • Images are virtual, upright, and smaller.

The Human Eye and Correction of Vision

  1. Normal Eye: Can focus on both far and near objects.
  2. Short-sightedness:
    • Difficulty seeing distant objects (focus before the retina).
    • Corrected with a concave lens.
  3. Long-sightedness:
    • Difficulty seeing near objects (focus beyond the retina).
    • Corrected with a convex lens.

Exercise Example

  • Diagnosis and correction of a vision defect with appropriate lens use.