Investigating Science 11–12 · Year 12

Refraction to telescope: building a two-lens telescope

Module 6: Technologies

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

The laws of refraction that shape a lens also fix a telescope’s design: two converging lenses separated by the sum of their focal lengths magnify by the ratio of those lengths.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

permanent eye damage if the telescope or a lens is pointed at the Sun
focused sunlight can start fires

Controls

never point lenses or the telescope toward the Sun
store lenses covered and out of direct sunlight
work indoors or in shade

Note

No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.

What you need

converging lenses of focal length 500 mm (+2 dioptres) and 50 mm (+20 dioptres), and a 100 mm lens as a second eyepiece
optical bench or two sliding cardboard tubes, lens holders, metre rule, white card
a printed chart of large letters fixed about 20 m away

How to do it

Measure each focal length by focusing the image of an object at least 50 m away onto the card and measuring the lens-to-card distance (at 50 m the 500 mm lens forms its image 505.1 mm away, 1 % long; a window 5 m away gives 555.6 mm), or measure the object and image distances and use 1/f = 1/u + 1/v.
Mount the 500 mm lens as the objective and the 50 mm lens as the eyepiece, start 550 mm apart and slide the eyepiece out until the chart is sharp; record the separation.
Estimate the magnification by looking at the chart through the telescope with one eye and directly with the other, comparing letter heights.
Swap in the 100 mm eyepiece and repeat.
Compare measured values with M = f_objective/f_eyepiece and explain how the refraction law made the design predictable.

What you should see

The focal lengths come out close to their marked values. The chart is 20 m away, not at infinity, so the 500 mm objective forms its image 512.8 mm behind it and the 50 mm eyepiece gives a sharp relaxed-eye view at a separation of 562.8 mm, not the 550 mm of a telescope focused at infinity; the 100 mm eyepiece focuses at 612.8 mm. The magnification is close to f_o/f_e, 10 and 5 for a distant object (10.3 and 5.1 for the chart, v_o/f_e), with an inverted image. The two-eye comparison gives a magnification near the predicted value, and the learner states the reading uncertainty of that method.

What changes

What you change: eyepiece focal length
What you measure: magnification and tube length
What you keep the same: same objective
same chart and distance
same observer

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

A bigger lens always magnifies more.
A telescope makes objects brighter by adding light.
The image in a simple telescope is upright.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS12-13 INS11/12-1 INS11/12-2 INS11/12-4
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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