Lenses - Convex and Concave - UNSOLVED PRACTICE SET

Q7. What is the difference between a convex lens and a concave lens in terms of shape and effect on parallel rays? Draw a diagram for each.

Q8. Trace the three standard rays used for image construction through a convex lens:
(a) ray parallel to principal axis,
(b) ray through optical centre,
(c) ray through the focus.

Q9. Describe the image formed by a convex lens when the object is placed:
(a) at infinity,
(b) at 2F,
(c) between F and optical centre.

Q10. Why does a concave lens never form a real image? Explain in terms of the direction of refracted rays. 

Q11. A magnifying glass is simply a convex lens used in a specific way. Explain how it works — what is the position of the object and what type of image is formed?

Q12. A concave lens is used to correct myopia (short-sightedness). Why is a diverging lens suitable for this correction and not a converging lens?

Q13. Describe image formation by a convex lens for all standard object positions. For each position state the location, nature, and relative size of the image:
(a) object at infinity,
(b) object beyond 2F,
(c) object at 2F,
(d) object between F and 2F,
(e) object at F,
(f) object between F and optical centre.

Q14. A student has two unmarked lenses — one convex, one concave. Describe three experiments she can perform to identify each lens without using any measuring instruments. Your answer must:
(a) describe the test with a distant object and a screen,
(b) describe what happens when she holds each lens close to printed text,
(c) explain what happens when she holds each lens up to sunlight on a piece of paper,
(d) connect each observation to the converging/diverging nature of the lens, and 

(e) state which lens can focus sunlight and which cannot. 

Q15. Compare convex and concave lenses in detail under:
(a) shape and cross-section,
(b) effect on a parallel beam of light,
(c) location of principal focus (real vs virtual),
(d) types of images formed (with examples of object positions), and
(e) two uses of each lens in real-life Indian applications, justifying why each specific lens type is chosen.

Q16. A convex lens has f = +20 cm. An object is placed at u = −60 cm.
(a) Find v using the lens formula 1/v − 1/u = 1/f.
(b) Calculate magnification m = v/u.
(c) Describe the image fully.
(d) If the object is moved to u = −30 cm, recalculate v and m and describe how the image changes.

Q17. A concave lens has f = −15 cm. An object is placed 45 cm from it (u = −45 cm).
(a) Find v.
(b) Find m.
(c) An object 6 cm tall — find the image height.
(d) Is the image real or virtual? How does the sign of v confirm this?

Q18. A convex lens of focal length 10 cm is used as a magnifying glass. An insect is placed 6 cm from the lens (u = −6 cm).
(a) Find v.
(b) Find m.
(c) Describe the image.
(d) If the insect is 2 mm long, find the image size.
(e) Now the insect moves to u = −10 cm (at the focus) — what happens to the image? Use the lens formula to show.