Spherical Lens Image Formation

Overview

This lecture explains image formation by convex and concave spherical lenses, covering ray diagram rules and key image properties for different object positions.

Image Formation by Convex Lenses

• Convex lenses are converging lenses that focus parallel rays to a point.
• Three ray rules:
  • Ray parallel to principal axis passes through focus after refraction.
  • Ray through optical center goes straight, undeviated.
  • Ray through focus emerges parallel to principal axis after refraction.
• To draw ray diagrams, select any two of the three rules.
• Object beyond 2F₁: Image is real, inverted, diminished, formed between F₂ and 2F₂.
• Object at 2F₁: Image is real, inverted, same size as object, at 2F₂.
• Object between F₁ and 2F₁: Image is real, inverted, magnified, formed beyond 2F₂.
• Object at F₁: Image is real, inverted, highly magnified, formed at infinity.
• Object between optical center and F₁: Image is virtual, upright, magnified, formed on the same side as the object (used in magnifying glasses).

Image Formation by Concave Lenses

• Concave lenses are diverging lenses that spread out parallel rays.
• Three ray rules:
  • Ray parallel to principal axis appears to come from focus after refraction.
  • Ray through optical center goes straight, undeviated.
  • Ray directed toward focus emerges parallel to principal axis after refraction.
• Use any two rules to draw ray diagrams.
• For any object position: Image is always virtual, upright, diminished, and formed between the object and the lens.

Comparing Convex and Concave Lenses

• Convex lens image properties vary with object position.
• Concave lens always produces a virtual, upright, diminished image regardless of object position.

Key Terms & Definitions

• Principal Axis — The straight line passing through the center of the lens.
• Optical Center (O) — The central point of the lens.
• Focus (F₁, F₂) — Points where light rays converge (convex) or diverge from (concave) after refraction.
• 2F₁, 2F₂ — Points at twice the focal length from the optical center.
• Real Image — Image formed by actual convergence of rays; can be projected onto a screen.
• Virtual Image — Image formed by apparent divergence of rays; cannot be projected.

Action Items / Next Steps

• Practice drawing ray diagrams for both types of lenses with different object positions.
• Answer the top three questions provided at the end of the lecture.
• Review the properties of images formed by spherical lenses for exam preparation.