What it shows
A long focal length objective and a short focal length eyepiece make a refracting telescope. In normal adjustment its angular magnification is f0 / fe.
The science
The objective lens forms a real, inverted image of a distant object at its focal plane, a distance f0 behind it. The eyepiece lens is placed so that this image is at its own focus, giving a final image at infinity, which the relaxed eye sees. The distance between the lenses is then f0 + fe, and the angular magnification is M = f0 / fe. Because the image is inverted, this type is used for the sky and not for viewing on the ground.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A spectacle lens of +2 dioptre (focal length 50 cm) as the objective
- A strong magnifier or a lens of about 5 cm focal length as the eyepiece
- Two cardboard or PVC tubes that slide into one another, about 50 cm total length
- Black paper, tape and clay
- A brick wall or a building with bricks at a distance for testing
- A ruler
Safety
For this project: Never look at the Sun through the telescope, even for a moment; it can blind. An adult must supervise any cutting.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
7 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Measure the focal lengths of both lenses by focusing a distant window on a card: note f0 and fe.
Mount the objective at the front of the outer tube and the eyepiece at the end of the inner tube.
Line both tubes with black paper to reduce stray light.
Aim the telescope at a brick wall or a distant window and slide the inner tube until the image is sharp; the tube length should be close to f0 + fe.
Look through the telescope with one eye and directly at the wall with the other eye; count how many bricks seen by the naked eye fit into one brick seen through the telescope.
Compare this counted value with the predicted f0 / fe.
Try a different eyepiece of longer focal length and note that the magnification falls and the field of view widens.
What you should see
The image of the wall is bigger and inverted, with a magnification close to f0 / fe, about 8 to 10 for a 50 cm and 5 cm pair. A longer eyepiece gives less magnification but a wider field.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the magnifying power of a telescope in normal adjustment?
M = f0 / fe, the ratio of the two focal lengths.
2What is the length of the tube in normal adjustment?
The sum f0 + fe.
3Why does an astronomical telescope give an inverted image?
Neither lens turns the image of the objective upright.
4Why is a large objective helpful?
It gathers more light so faint objects appear brighter.
Take it further
Add a third lens of suitable focal length as an erecting lens to make a terrestrial telescope and test it on a distant tree.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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