Overview
This lecture explains how convex and concave lenses use refraction to focus or disperse light, covers real and virtual images, and describes how lens curvature affects power.
Types of Lenses
- Convex (converging) lenses bulge outward and refract parallel rays inward to meet at the principal focus.
- Concave (diverging) lenses curve inward and refract parallel rays outward, dispersing light away from the lens.
- Both lens types have a principal focus (focal point) on each side, equidistant from the lens center.
Lens Function and Symmetry
- Lenses are symmetrical so light can travel through either direction, with focus flipping accordingly.
- The principal focus (or focal point) always lies on the optical axis passing through the center of the lens.
- Focal length is the distance from the lens center to the principal focus.
Lens Power and Curvature
- A shorter focal length means a more powerful lens, refracting light more strongly.
- Increasing a lens’s curvature or using a material with a higher refractive index makes the lens more powerful.
Real vs. Virtual Images
- Images form where light rays from an object point appear to come together after passing through a lens.
- Real images form when rays actually meet; these can be projected onto a screen (e.g., image on retina).
- Real images are often inverted, but our brain corrects their orientation.
- Virtual images form when rays only appear to meet after being traced back; they cannot be projected (e.g., image in a mirror).
- Virtual images are seen where the rays seem to originate but do not actually converge.
Describing Images
- Images are described as real or virtual, upright or inverted, and larger or smaller than the object.
- Example: real image—smaller, inverted; virtual image—smaller, upright.
Key Terms & Definitions
- Convex lens — lens that bulges outward and converges light rays.
- Concave lens — lens that curves inward and diverges light rays.
- Principal focus (focal point) — point where parallel light rays converge or appear to diverge after passing through the lens.
- Focal length — distance from lens center to the principal focus.
- Real image — image formed by actual convergence of light rays; can be projected onto a screen.
- Virtual image — image formed where light rays only appear to meet; cannot be projected.
Action Items / Next Steps
- Review diagrams of convex and concave lenses showing real and virtual image formation.
- Practice describing images formed by lenses using terms: real/virtual, upright/inverted, and size relative to the object.