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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 | –60 | +12 |
| 2 | –30 | +15 |
| 3 | –20 | +20 |
| 4 | –15 | +30 |
| 5 | –12 | +60 |
| 6 | –9 | +90 |
(a) What is the focal length of the convex lens? State reason for your answer.
(b) For what object-distance (u) is the corresponding image-distance (v) not correct? How did you arrive at this conclusion?
(c) Choose an appropriate scale to draw a ray diagram for the observation at S. No. 4 and find the approximate value of magnification.
उत्तर
(a) From S.No. 3 we can say that the radius of curvature of the lens is 20 cm because when an object is placed at the centre of curvature of a convex lens its image is formed on the other side of the lens at the same distance from the lens. And, we also know that focal length is half of the radius of curvature. Thus, focal length of the lens is + 10 cm.
(b) S.No: 6 is not correct as for this observation the object distance is between focus and pole, and for such cases the image formed is always virtual but in this case a real image is forming as the image distance is positive.
(c) Approximate value of magnification for object distance - 15 cm and image distance + 30 cm is - 2.
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संबंधित प्रश्न
If you focus the image of a distant object, whose shape is given below, on a screen using a convex lens, the shape of the image of this object on the screen would be:
(a)
(b)
(c)
(d)
A beam of light travelling in a rectangular glass slab emerges into air. Draw a ray-diagram indicating the change in its path.
f the image formed by a convex lens is of the same size as that of the object, what is the position of the image with respect to the lens?
Describe with the help of a ray diagram the nature, size and position of the image formed when an object is placed at infinity (considerable distance) in front of a convex lens. State three characteristics of the image so formed.
A convex lens of focal length 8 cm forms a real image of the same size as the object. The distance between object and its image will be:
(a) 8 cm
(b) 16 cm
(c) 24 cm
(d) 32 cm
A convex lens of focal length 10 cm is placed in contact with a concave lens of focal length 20 cm. The focal length of this combination of lenses will be:
(a) +10 cm
(b) +20 cm
(c) −10 cm
(d) −20 cm
Show by a diagram the refraction of two light rays incident parallel to the principal axis on a convex lens by treating it as a combination of a glass slab and two triangular glass prisms.
List four properties of the image formed by a convex mirror.
An object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm. The distance of the object from the lens is 30 cm. Find (i) the position (ii) the magnification and (iii) the nature of the image formed.
Distinguish between Concave lens and Convex Lens.
