| Date | May 2015 | Marks available | 3 | Reference code | 15M.3.SL.TZ2.20 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Calculate and Determine | Question number | 20 | Adapted from | N/A |
Question
This question is about a thin converging (convex) lens.
The diagram shows an object placed in front of a thin converging lens.
The focal points of the lens are labelled F.
(i) Using the diagram, determine the power of the lens.
(ii) On the diagram, construct lines to show how the image of the object is formed by the lens.
(iii) State and explain whether the image is a real image or a virtual image.
Argus uses an astronomical telescope to observe a telecommunications tower. The height of the tower is 82 m and the distance from Argus to the tower is 4.0 km. The image formed by the telescope has an angular diameter of 0.10 rad and is formed at infinity.
(i) Determine the angular magnification of the telescope.
(ii) The focal length of the eyepiece is 15 cm. Calculate the focal length of the objective lens.
Markscheme
(i) identifying focal length from diagram or f=5.0cm;
\(\left( {P = \frac{1}{f} = \frac{1}{{5.0}}} \right) = 0.20\left( {{\rm{c}}{{\rm{m}}^{ - 1}}} \right)\) or 20 (D) or 20 \(({m^{ - 1}})\);
Award [2] for a bald correct answer.
(ii) first ray from tip of object correctly refracted by lens;
a second ray from tip of object correctly refracted;
correct extrapolation back to tip of image;
Accept rays without arrows and solid construction lines back to the image.
(iii) image is virtual;
image cannot be formed on a screen / rays do not cross;
\(M = \left( {\frac{{0.1}}{{2.05 \times {{10}^{ - 2}}}} = } \right)4.9\);
Allow ECF in second marking point for using incorrect angle.
Award [2] for a bald correct answer.
Allow 75 (cm) due to rounding.
Examiners report
The magnifying glass ray diagram was almost always correct in (a). All candidates knew that the image was virtual, but often gave vague statements about what this means.
Part (b) was also done well with only a few candidates making POT errors when finding the angular magnification of the telescope.
