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प्रश्न
An object 3 cm high is placed at a distance of 8 cm from a concave mirror which produces a virtual image 4.5 cm high:
(i) What is the focal length of the mirror?
(ii) What is the position of image?
(iii) Draw a ray-diagram to show the formation of image.
उत्तर
Distance of the object from the mirror 'u' = -8 cm
Height of the object 'ho' = 3 cm
Height of the image 'hi' = 4.5 cm
We have to find the focal length of the mirror 'f' and distance of the image 'v'.
Using the magnification formula, we get
`m=h_i/h-o=-v/u`
`m4.5/3=(-v)/(-8)`
`v=4.5xx8/3=12`cm
Therefore, the distance of the image 'v' is 12 cm behind the mirror.
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Figure
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A student wants to obtain an erect image of an object using a concave mirror of 10 cm focal length. What will be the distance of the object from mirror?
A student took three concave mirrors of different focal lengths and performed the experiment to see the image formation by placing an object at different distance with these mirrors as shown in the following table.
| Case No. | Object-distance | Focal length |
| I | 45 cm | 20 cm |
| II | 30 cm | 15 cm |
| III | 20 cm | 30 cm |
Now answer the following questions:
(a) List two properties of the image formed in Case I.
(b) In which one of the cases given in the table, the mirror will form real image of same size and why?
(c) Name the type of mirror used by dentists. Given reason why do they use such type of mirrors.
OR
(c) Look at the table and identify the situation (object distance and focal length) which resembles the situation in which concave mirrors are used as shaving mirrors? Draw a ray diagram to show the image formation in this case.
