Lens Ray Diagrams and Image Nature
Under the thin-lens paraxial model, a convex lens gives a real image for an object outside its focus and a virtual enlarged image inside it; a concave lens gives a virtual diminished image for a real object.
Why this shows up in the exam
Magnifiers · Projectors · Corrective lenses
Learn the idea
A converging or diverging lens forms image types determined by object position and standard ray directions. Two reliable rays reveal the image: one through the optical centre and one parallel to the axis.
🧠 Memory hook: Convex changes at F; concave stays virtual for a real object.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- 1/f = 1/v - 1/u — signed paraxial image position
- m = v/u — image orientation and scale
How to approach it
- 1Mark both focal points
- 2Trace two standard rays
- 3Classify their intersection or backward extensions
Common slip-ups that cost marks
- •Assuming every convex-lens image is real
- •Sending the central ray through a displaced optical centre
- •Confusing focal planes with 2F points
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A real object is placed 30 cm from a converging lens of focal length 10 cm. Find the real image distance.
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