Overview
This lecture explains how images are formed by convex and concave spherical lenses, including the rules for drawing ray diagrams and the properties of images in different cases.
Convex Lens: Construction & Rules
- The convex lens is a converging lens.
- Draw the principal axis, mark the optical center (O), principal foci (F1 and F2), and twice the focal points (2F1, 2F2).
- Rule 1: A ray parallel to the principal axis passes through the focus after refraction.
- Rule 2: A ray passing through the optical center goes straight without deviation.
- Rule 3: A ray passing through the focus emerges parallel to the principal axis.
- Use any two rules when drawing ray diagrams.
Image Formation with Convex Lens (Cases)
- Case 1: Object beyond 2F1 — Image is real, inverted, diminished, formed between F2 and 2F2.
- Case 2: Object at 2F1 — Image is real, inverted, same size, formed at 2F2.
- Case 3: Object between F1 and 2F1 — Image is real, inverted, magnified, formed beyond 2F2.
- Case 4: Object at F1 — Image is real, inverted, highly magnified, formed at infinity.
- Case 5: Object between optical center and F1 — Image is virtual, erect, magnified, formed on same side as object (behind lens); used as a magnifying glass.
Concave Lens: Construction & Rules
- The concave lens is a diverging lens.
- Principal axis, optical center, foci (F1, F2), and their doubles (2F1, 2F2) are marked similarly but with foci on opposite sides compared to convex.
- Rule 1: A ray parallel to the principal axis appears to diverge from the focus after refraction.
- Rule 2: Ray through optical center goes undeviated.
- Rule 3: A ray aimed at the focus emerges parallel to the principal axis.
- Use any two rules to draw ray diagrams.
Image Formation with Concave Lens
- Image is always virtual, upright, diminished, and located between the object and the lens.
- These properties remain the same regardless of the object’s position.
Key Terms & Definitions
- Principal Axis — The straight line passing through the center of the lens.
- Optical Center (O) — The center point of the lens.
- Principal Focus (F1, F2) — Points where rays parallel to the principal axis converge (convex) or appear to diverge (concave).
- Real Image — Formed by actual intersection of rays; can be projected on a screen.
- Virtual Image — Formed by the apparent intersection of rays; cannot be projected.
- Magnified/Diminished Image — Image larger or smaller than the object.
Action Items / Next Steps
- Draw ray diagrams for each lens and case in your notebook.
- Practice the top three questions from the lecture.
- Review properties of images for both convex and concave lenses.