You may see that printed letters on the page of the text book appear blurred or you feel strain in the eye. Now slowly move the book backwards to a
clear printed letters without straining your eye. Ask your friend to measure the distance between your eye and text book at this position. Note down its value. Repeat the activity with other friends and note down the distances for distinct vision in each case. Find the average of all these distances of clear vision.From this activity you will come to know that to see an object comfortably and distinctly, you must hold it at a distance about 25 cm from your eyes. This distance is called least distance of distinct vision. This varies from person to person and with age. At a young age (say below 10 years) the muscles around the eye are strong and flexible and can bear more strain. Therefore the least distance of distinct vision at this age is as close as 7 to 8 cm. Generally for a healthy person the least distance of distinct vision is around 25 cm. In old age the muscles cannot sustain more strain hence the least distance of distinct vision shifts to a larger value, say, about 1 to 2 m or even more.
| Activity 2 |
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Collect a few wooden sticks used in cloth roller in clothes store (or) collect waste PVC pipes that are used for electric wiring. Prepare sticks or pipes of 20 cm, 30 cm, 35 cm, 40 cm, 50 cm from them. Place a retort stand on a table and stand near the table such that your head is beside the vertical stand (see fig 1). Adjust the clamp on the horizontal rod and fix it at a distance of 25 cm from your eyes. Ask one of your friends to fix a wooden stick of 30 cm height to the clamp in a vertical position as shown in fig1.
Now keeping your vision parallel to horizontal rod of the stand, try to see the top and bottom of wooden stick kept in vertical position.