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S5 Physics: Lenses Practice Paper 3
S5 Physics: Lenses Practice Paper 3
Paper three opens with a distant object giving a diminished real image, then u = 2f and an object inside the focal length for a virtual image, then magnified real images with object height, then finding f from u and v and reversing a required magnification, ending with the u minus f error.
Quick answer 60 cm gives 20 cm and 0.33, 30 cm gives 15 cm and 0.5, 16 cm gives 16 cm and 1, 12 cm gives −30 cm and 2.5, 27 cm gives 54 cm and 2, 30 cm gives 120 cm and 4, 18 cm gives 36 cm with a 12 cm image, u20 v30 gives f12 and 1.5, m3 gives u20 v60, and 50 cm gives 33.3 cm not 30 cm.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 questions |
| Suggested time | 25 minutes |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S5 Physics: Lenses – Tutor Lesson Plan |
| Level | S5 |
02 Questions (10 items)
- FoundationA convex lens of focal length 15 cm has an object 60 cm in front of it. Find the image distance and magnification.
- FoundationA convex lens of focal length 10 cm has an object 30 cm away. Find the image distance and magnification.
- FoundationA convex lens of focal length 8 cm has an object at 2f, that is 16 cm in front. Find the image distance and magnification.
- FoundationA convex lens of focal length 20 cm has an object only 12 cm in front. Find the image distance and magnification, and state the nature of the image.
- IntermediateA convex lens with focal length 18 cm has an object 27 cm away. Find the image distance and magnification.
- IntermediateA convex lens of focal length 24 cm has an object 30 cm away. Find the image distance and magnification.
- IntermediateA convex lens of focal length 12 cm has a 6 cm tall object 18 cm in front. Find the image distance, magnification and image height.
- ChallengeIn an experiment an object 20 cm from a lens forms a sharp real image on a screen 30 cm away. Find the focal length and the magnification.
- ChallengeA convex lens of focal length 15 cm must produce a real image of magnification 3. Where should the object be, and where does the image form?
- Error redoA convex lens has focal length 20 cm and the object is 50 cm in front. A student answers “v = 50 − 20 = 30 cm”. What is correct?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Image distance 20 cm, magnification about 0.33 (one third). | Beyond 2f the image is inverted and diminished, forming between f and 2f. |
| 2 | Image distance 15 cm, magnification 0.5. | Put 1/f − 1/u over a common denominator, here 30, before taking the reciprocal. |
| 3 | Image distance 16 cm, magnification 1. | At u = 2f the image forms at v = 2f and is the same size, but inverted. |
| 4 | Image distance −30 cm (virtual image), magnification 2.5, upright and magnified. | Inside the focal length v comes out negative: a virtual, upright, magnified image on the same side. |
| 5 | Image distance 54 cm, magnification 2. | At u = 1.5f the magnification is exactly 2, a quick way to check the answer. |
| 6 | Image distance 120 cm, magnification 4. | Just beyond f the image distance stretches sharply; that is expected, not an error. |
| 7 | Image distance 36 cm, magnification 2, image height 12 cm. | Find v first, then multiply the object height by m; a fixed order stops v and m getting mixed up. |
| 8 | Focal length 12 cm, magnification 1.5. | When u and v are both given, use 1/f = 1/u + 1/v, the reverse of the usual direction. |
| 9 | Object distance 20 cm, image distance 60 cm. | For a real image v = 3u, so 1/f = 1/u + 1/3u gives u = 4f/3. |
| 10 | The image distance is about 33.3 cm (100/3 cm) with magnification about 0.67. | The image distance is never u minus f; it must come from 1/v = 1/f − 1/u. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
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