Overview
This lecture explains the differences between convex and concave lenses, explores image formation using ray diagrams, and details the key properties and rules governing both lens types.
Introduction to Spherical Lenses
- Spherical lenses are transparent materials with at least one curved surface.
- Two main types are convex (converging) and concave (diverging) lenses.
Convex (Converging) Lens
- Convex lenses are thicker in the center and thinner at the edges.
- They converge parallel incoming light rays to a focal point on the other side.
- The principal focus (F) is the point where rays parallel to the principal axis meet after refraction.
Rules for Convex Lens Image Formation
- A ray parallel to the principal axis passes through the focus after refraction.
- A ray passing through the center of the lens goes straight without deviation.
- A ray passing through the focus emerges parallel to the principal axis.
Image Properties with Convex Lens
- The position and size of the image depend on the object's distance from the lens.
- Image can be real or virtual, inverted or erect, magnified or diminished.
- Real images are formed on the opposite side, while virtual images are formed on the same side as the object.
Ray Diagrams for Convex Lenses
- Ray diagrams use the above rules to locate the image based on object placement (beyond 2F, at 2F, between F and 2F, etc.).
Concave (Diverging) Lens
- Concave lenses are thinner in the center and thicker at the edges.
- They diverge parallel incoming light rays as if they originated from the focal point on the same side as the object.
Rules for Concave Lens Image Formation
- A ray parallel to the principal axis appears to diverge from the focus after passing through the lens.
- A ray heading towards the center of the lens passes undeviated.
- A ray heading towards the focus emerges parallel to the principal axis.
Image Properties with Concave Lens
- Concave lenses always form images that are virtual, erect, and diminished in size.
- Images appear on the same side of the lens as the object.
Practice and Examples
- Practice questions involve determining image position and nature for various object placements using the rules and ray diagrams.
Key Terms & Definitions
- Convex lens — A lens that is thicker at the center and converges light rays.
- Concave lens — A lens that is thinner at the center and diverges light rays.
- Principal axis — The straight line passing through the center of the lens.
- Principal focus (F) — The point where rays parallel to the principal axis converge (convex) or appear to diverge from (concave).
- Real image — An image formed by converging rays that can be projected on a screen.
- Virtual image — An image formed by diverging rays that cannot be projected on a screen.
Action Items / Next Steps
- Draw ray diagrams for objects at various positions relative to both convex and concave lenses.
- Answer practice questions on image nature and placement for both lens types.
- Review the key rules for image formation by both lenses.