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.