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S5 Physics: Lenses Practice Paper 4
S5 Physics: Lenses Practice Paper 4
Paper four opens with four direct substitutions into the lens equation, then reverses the question to find f from u and v and u from the magnification, then handles the virtual case inside the focus and image height, closing with the mistake of dropping the minus sign.
Quick answer 16.7 cm at 0.667, 12 cm at 0.5, 30 cm at 5, 120 cm at 3, 60 cm at 5, f back-calculated as 10 with 0.5, magnification 2 giving u 20 and f 13.3, minus 15 cm virtual at 2.5, 30 cm with a 4.5 cm image, and minus 12 cm virtual at 2.
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 has focal length 10 cm and the object stands 25 cm in front. Find the image distance and say whether the image is larger or smaller.
- FoundationA convex lens of focal length 8 cm has an object 24 cm away. Find the image distance and the magnification.
- FoundationFocal length 5 cm, object distance 6 cm. Find the image distance and the magnification.
- FoundationFocal length 30 cm with the object 40 cm away. Find the image distance and the magnification.
- IntermediateA convex lens of focal length 10 cm has the object 12 cm in front. Find the image distance, the magnification and the nature of the image.
- IntermediateA convex lens gives an image at 15 cm for an object at 30 cm. Find the focal length and the magnification.
- IntermediateA real inverted image has magnification 2 and an image distance of 40 cm. Find the object distance and the focal length.
- ChallengeA convex lens of focal length 10 cm has the object 6 cm in front. Find the image distance and state whether the image is real or virtual and upright or inverted.
- ChallengeA convex lens of focal length 20 cm forms an image of a 9 cm tall object placed 60 cm away. Find the image distance, magnification and image height.
- Error redoFocal length 12 cm with the object 6 cm away. A student answers “image distance 12 cm, real image”. What is correct?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Image distance about 16.7 cm; the image is real, inverted and smaller. | Use 1/f = 1/u + 1/v, so 1/10 = 1/25 + 1/v gives 1/v = 3/50. |
| 2 | Image distance 12 cm and magnification 0.5. | From 1/8 = 1/24 + 1/v we get 1/v = 1/12, and the magnification is v over u. |
| 3 | Image distance 30 cm and magnification 5. | Just beyond the focal point the image distance stretches far: 1/5 minus 1/6 is 1/30. |
| 4 | Image distance 120 cm and magnification 3. | 1/30 minus 1/40 is 1/120, and 120 over 40 gives a magnification of 3. |
| 5 | Image distance 60 cm, magnification 5, a real inverted and magnified image. | With the object only slightly beyond the focus, 1/10 minus 1/12 gives 1/60 so the image is five times as far. |
| 6 | Focal length 10 cm and magnification 0.5. | 1/f = 1/30 + 1/15 = 3/30, so f is 10 cm. |
| 7 | Object distance 20 cm and focal length about 13.3 cm. | From m = v over u we get u = 20 cm, then 1/f = 1/20 + 1/40 = 3/40. |
| 8 | The image distance is minus 15 cm, that is 15 cm on the same side as the object, upright, virtual and magnified. | When the object is inside the focus 1/v comes out negative, the minus sign means a virtual image, and 15 over 6 gives 2.5. |
| 9 | Image distance 30 cm, magnification 0.5 and image height 4.5 cm. | 1/20 minus 1/60 is 1/30, and the image height is 9 times 0.5. |
| 10 | The image distance is minus 12 cm: an upright virtual image magnified twice. | Inside the focal length the image is always virtual, and the minus sign from the equation carries that meaning. |
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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