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Date	November 2018	Marks available	3	Reference code	18N.3.SL.TZ0.9
Level	Standard level	Paper	Paper 3	Time zone	0 - no time zone
Command term	Calculate	Question number	9	Adapted from	N/A

Question

The diagram shows two light rays that form an intermediate image by the objective lens of an optical compound microscope. These rays are incident on the eyepiece lens. The focal points of the two lenses are marked.

The object O is placed at a distance of 24.0 mm from the objective lens and the final image is formed at a distance 240 mm from the eyepiece lens. The focal length of the objective lens is 20.0 mm and that of the eyepiece lens is 60.0 mm. The near point of the observer is at a distance of 240 mm from the eyepiece lens.

Draw rays on the diagram to show the formation of the final image.

[2]

Calculate the distance between the lenses.

[3]

b.i.

Determine the magnification of the microscope.

[2]

b.ii.

Markscheme

proper construction lines ✔

image at intersection of proper construction lines ✔

distance of intermediate image from objective is

$\frac{1}{v} = \frac{1}{{20}} - \frac{1}{{24}}$ ie: v = 120 «mm» ✔

distance of intermediate image from eyepiece is

$\frac{1}{u} = \frac{1}{{60}} - \left( { - \frac{1}{{240}}} \right)$ ie: u = 48 «mm» ✔

lens separation 168 «mm» ✔

b.i.

ALTERNATIVE 1:

eyepiece: m = $\frac{{ - v}}{u} = \frac{{240}}{{28}}$ = 5 ✔

AND

objective m = $\frac{{ - v}}{u} = \frac{{ - 120}}{{24}}$ = −5 ✔

Total m = −5 × 5 = −25 ✔

ALTERNATIVE 2:

m = $\left( {\frac{{240}}{{60}} + 1} \right) \times \left( { - \frac{{120}}{{24}}} \right)$ ✔

m = −25 ✔

Accept positive or negative values throughout.

b.ii.

Examiners report

[N/A]

b.i.

[N/A]

b.ii.

Syllabus sections

Option C: Imaging » Option C: Imaging (Core topics) » C.1 – Introduction to imaging

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Option C: Imaging » Option C: Imaging (Core topics)

Option C: Imaging

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