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प्रश्न
A student did an experiment with a convex lens. He put an object at different distances 25 cm, 30 cm, 40 cm, 60 cm and 120 cm from the lens. In each case he measured the distance of the image from the lens. His results were 100 cm, 24 cm, 60 cm, 30 cm and 40 cm, respectively. Unfortunately his results are written in wrong order.
What would be the image distance if the object distance was 90 cm?
उत्तर
Lens formula is given by:
`1/f=1/v-1/u`
`1/20=1/v-1/-90`
⇒`1/20=1/v+1/90`
⇒`1/v=1/20-1/90`
⇒`1/v=(9-2)/180`
⇒`1/v=7/180`
⇒`v=180/7`
⇒`v=25.7`cm
The image distance will be 25.7 cm if the object distance is 90 cm.
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संबंधित प्रश्न
Analyse the following observation table showing variation of image-distance (v) with object-distance (u) in case of a convex lens and answer the questions that follow without doing any calculations :
| S. No. | Object-Distance u (cm) |
Image-Distance v (cm) |
| 1 | –100 | +25 |
| 2 | –60 | +30 |
| 3 | –40 | +40 |
| 4 | –30 | +60 |
| 5 | –25 | +100 |
| 6 | –15 | +120 |
(a) What is the focal length of the convex lens? Give reason to justify your answer.
(b) Write the serial number of the observation which is not correct. On what basis have you arrived at this conclusion?
(c) Select an appropriate scale and draw a ray diagram for the observation at S.No. 2. Also find the approximate value of magnification.
A beam of light travelling in a rectangular glass slab emerges into air. Draw a ray-diagram indicating the change in its path.
Explain with the help of a diagram, why the convex lens is also called a converging lens.
A convex lens produces an inverted image magnified three times of an object placed at a distance of 15 cm from it. Calculate focal length of the lens.
What kind of lens can form:
am erect diminished image?
State two applications of a convex lens.
How will you determine the focal length of a convex lens by the plane mirror method?
_______ times larger images can be obtained by using a simple microscope.
The above image shows a thin lens with a focal length of 5m.
- What is the kind of lens shown in the above figure?
- If a real inverted image is to be formed by this lens at a distance of 7m from the optical centre, then show with calculation where should the object be placed.
- Draw a neatly labelled diagram of the image formation mentioned in (ii).
