Lenses

• Convex (converging) and concave (diverging) lenses.
• How lenses bend light and form images (introductory).
• Uses: magnifying glass, spectacles, cameras, telescopes.
• Difference between lens and mirror image formation.

1. Convex lens — thicker at middle; converges parallel rays to real focus.
2. Concave lens — thinner at middle; diverges rays; focus is virtual.
3. Optical centre — ray through centre passes undeviated (approx.).
4. Magnifying glass — convex lens for object within focal length → erect magnified virtual image.
5. Diagram (text) — parallel rays through convex lens meet at F on principal axis.
6. Real world — reading glasses (convex for hypermetropia), peepholes sometimes use concave lens.

Using convex lens as magnifying glass

Step 1 — Hold lens close to small print (object within focal length).
Step 2 — Look through lens — erect, enlarged virtual image.
Step 3 — Move lens away — image changes size and may invert when object beyond F.
Step 4 — NCERT: convex lens converges light; concave diverges.

• Calling all lenses magnifying (concave often reduces image size).
• Misconception: convex lens always forms real image (close object → virtual).
• Confusing focus of lens with centre of lens.
• Mixing lens and mirror ray diagrams.

• Difference between convex and concave lens.
• Which lens is used as magnifying glass?
• What happens to parallel rays after convex lens?

Open the Practice tab for graded questions on Lenses.