Head office, Rana Pratap Bagh, Delhi011 2721 1512+91 97116 99511

Science project idea

Two-lens refracting telescope from a pair of magnifying glasses

A long-focus lens gathers light from a distant object and forms a small image. A second short-focus lens acts as a magnifier for that image, so far-away objects look larger and nearer.

Make it yourself8 numbered steps, the materials and the science, all free.Go to the stepsGet a ready-made kitReady-made models, kits and spare parts in the store under Physics.Browse PhysicsAsk on WhatsAppAsk the head office whether a model or kit for this idea is available.Send a message

What it shows

A long-focus lens gathers light from a distant object and forms a small image. A second short-focus lens acts as a magnifier for that image, so far-away objects look larger and nearer.

The science

The objective lens forms a real inverted image of a distant object at its focal length. The eyepiece is placed one focal length from that image and acts as a magnifying glass. The magnifying power equals the focal length of the objective divided by that of the eyepiece, here about 40 divided by 5, which is eight times.

Materials

Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • One magnifying lens of long focal length, about 40 cm (objective)
  • One magnifying lens of short focal length, about 5 cm (eyepiece)
  • Two cardboard tubes, one sliding inside the other
  • Black paper and glue
  • A metre ruler
  • Tape and a wooden stand

Safety

For this project: Never point the telescope at the Sun, because it can burn your eye. An adult must handle glass lenses.

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 rules

Step by step

8 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.

  1. Find each lens focal length by focusing the image of a distant tree or window on a sheet of paper and measuring lens to paper.

  2. Choose the longer focal length lens as the objective and the shorter one as the eyepiece.

  3. Fix the objective at one end of the larger tube using black paper rings and glue.

  4. Fix the eyepiece at the end of the smaller tube in the same way.

  5. Slide the small tube into the big one, so the lenses are about the sum of their focal lengths apart.

  6. Line the tube up on a distant building and move the tube in or out until the image is sharp.

  7. Line the inside of the tubes with black paper to cut glare and tape the joint.

  8. Calculate the magnification and compare it with what you see on a signboard far away.

What you should see

A telescope that shows distant objects larger but upside down. The magnification you calculate from focal lengths should roughly match how much bigger things appear.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1Which lens is the objective?

The long-focus lens that faces the object.

2Why is the image upside down?

The objective lens forms an inverted image and a simple eyepiece does not turn it back.

3How do you find magnifying power?

Divide the objective focal length by the eyepiece focal length.

4Why is a bigger objective useful?

It gathers more light so faint things look brighter.

Free ebook: Viva Questions for Science Projects

Take it further

Make the telescope with a third lens to turn the image upright and discuss why binoculars use prisms.

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

WhatsApp us