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प्रश्न
Solve the following example.
An object kept 60 cm from a lens gives a virtual image 20 cm in front of the lens. What is the focal length of the lens? Is it a converging lens or diverging lens?
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उत्तर
Given:
Object distance, u = −60 cm
Image distance, v = −20 cm
Using lens formula,
\[\frac{1}{v} - \frac{1}{u} = \frac{1}{f}\]
\[ \Rightarrow \frac{1}{f} = \frac{1}{- 20} - \frac{1}{- 60} = - \frac{1}{30}\]
\[ \Rightarrow f = - 30 \text{ cm }\]
Since, the focal length is negative, the lens is a diverging lens or a concave lens.
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संबंधित प्रश्न
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(A) 20 cm on the other side of the lens and is of the same size, real and erect.
(B) 40 cm on the other side of the lens and is magnified, real and inverted.
(C) 20 cm on the other side of the lens and is of the same size, real and inverted.
(D) 20 cm on the other side of the lens and is of the same size, virtual and erect.
The image formed by a spherical mirror is real, inverted and is of magnification −2. If the image is at a distance of 30 cm from the mirror, where is the object placed? Find the focal length of the mirror. List two characteristics of the image formed if the object is moved 10 cm towards the mirror.
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(B) lens and the screen only
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(a) Write the type of mirror he should use.
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(c) What is the distance between the object and its image?
(d) Draw a ray diagram to show the image formation in this case.
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| S. No. |
Object distance (cm) |
Image distance (cm) |
| 1 | 60 | 15 |
| 2 | 48 | 16 |
| 3 | 36 | 21 |
| 4 | 24 | 24 |
| 5 | 18 | 36 |
| 6 | 16 | 48 |
After checking the observations table the teacher pointed out that there is a mistake in recording the image distance in one of the observations. Find the serial number of the observations having faulty image distance.
(A) 2
(B) 3
(C) 5
(D) 6
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Write an expression for magnification for a lens, explaining the meaning of the symbols used.
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