Overview
This lecture explains how convex and concave lenses use refraction to focus or disperse light, discusses real versus virtual images, and explores lens power.
Types of Lenses
- Convex (converging) lenses bulge out and focus parallel rays of light inward to a single point— the principal focus.
- Concave (diverging) lenses cave in and spread (diverge) parallel light rays outward.
- Both lens types have a principal focus on each side, equidistant from the center.
Refraction and Focal Point
- Refraction is the bending of light as it passes through a lens, altering its direction.
- The principal focus (or focal point) is where refracted rays either meet (convex) or appear to diverge from (concave).
- The axis is a straight line passing through the center of the lens.
Focal Length and Lens Power
- Focal length is the distance from the center of the lens to the principal focus.
- Shorter focal length means a more powerful lens, which refracts light more strongly.
- More curvature or a material with higher refractive index increases lens power.
Real vs. Virtual Images
- Real images form where actual light rays converge and can be captured on a screen (e.g. image formed on the retina).
- Virtual images form where light rays appear to diverge from but don't actually meet; these can't be captured (e.g. image in a mirror).
- Real images are often inverted; virtual images are usually upright.
Describing Images
- When describing an image, note if it is real or virtual, upright or inverted, and whether it is larger or smaller than the object.
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 light rays meet or appear to come from after passing through the lens.
- Focal Length — Distance from lens center to principal focus.
- Refraction — Bending of light as it passes through materials of different densities.
- Real Image — Image formed where light rays actually meet; can be projected.
- Virtual Image — Image formed where rays appear to meet but do not actually do so; cannot be projected.
Action Items / Next Steps
- Review the differences between real and virtual images.
- Practice tracing ray diagrams for both convex and concave lenses.
- Prepare to identify and describe image properties in sample exam questions.