Telescope
By Telescopes are used to view distant objects with a larger angular magnification. An important aspect of telescope is resolution for which one needs a convex lens of large aperture as the objective. Here we make a very simple telescope to give the feel and excitement.
You need a concave lens of focal length 5 cm, a convex lens of focal length 20 cm and same radius, a transparency sheet, double sticky tape, and cellophane tape to make a simple telescope.
- Put the transparency sheet on a horizontal surface, peel off a length of the double sticky tape (15 cm) and stick it on the transparency sheet. The length should be slightly more than the perimeter of the lenses.
- Now stick another piece of tape of same length, parallel to the first piece at a separation of exactly 15 cm (20cm - 5cm). Remove the upper non sticky cover to expose the sticky surface.
- Carefully put one of the lenses on a tape in vertical orientation and the other lens on the other tape parallel to the first one.
- Now roll the sheet very carefully, keeping the two lenses parallel to each other. After one complete roll, use cellophane tape to securely fix up the roll.
- The telescope is ready. Keeping the concave lens towards eye, look at distant objects and enjoy. Write your experience with this telescope.
Role of bigger objective lens
Bigger objective helps in getting better resolving power of telescope. Magnification and resolving power are different characteristics of a telescope. In this experiment we try to give this feeling.
You need two convex lenses of nearly same focal length \(f_o\) but of different apertures, a concave lens of smaller focal length \(f_e\).
- Choose a sheet of paper where matter is written in different fonts. Posters with heading, subheading and text will be ok. Fix it on a wall at working height.
- At about 30 ft or more, put up the convex lens with bigger aperture in a stand. The axis of the lens should be passing through the paper.
- Place the concave lens at a distance \(f_o-f_e\) from the convex lens. In the present case \(f_o\approx20\,{cm}\), \(f_e=5\,{cm}\) so that separation will be \(\approx 15\,{cm}\). Make sure they are coaxial.
- Look through the concave lens. You will see the circular boundary of the convex lens and within that enlarged image of objects beyond the lens system. Adjust the separation between the lenses and lateral positions so that the printed paper comes in view.
- Try to read the headings and sub headings. Focus on the matter (heading/sub heading) in which you are just able to differentiate the letters and they are not mixed up. Just concentrate on one such word.
- Now replace the convex lens with the other convex lens with smaller aperture. Do the same exercise. Focus on the same word. Are the letters clearly distinguishable?
- It is the personal eye conditions that play important role. But in general you will find the smaller aperture of convex lens (object) has somewhat blurred the image.
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