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प्रश्न
A combination of lenses for a camera contains two converging lenses of focal lengths 20 cm and 40 cm and a diverging lens of focal length 50 cm. Find the power and focal length of the combination.
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उत्तर
Focal length (f1) of the 1st converging lens = 20 cm = 0.2 m
Focal length (f2) of the 2nd converging lens = 40 cm = 0.4 m
Focal length (f3) of the diverging lens = −50 cm = −0.5 m (negative sign due to sign convention)
Power = `1/"Focal length of lens (in metre)"`
Power (P1) of 1st lens = `1/0.2` = 5 D
Power (P2) of 2nd lens = `1/0.4` = 2.5 D
Power (P3) of diverging lens = `1/0.5` = −2 D
Net power of the combination = P1 + P2 + P3
= 5 + 2.5 + (−2)
= 7.5 − 2
= 5.5 D
Net focal length of the combination = `1/"Net power"`
= `1/5.5`
= 0.1818 m
= 18.18 cm
संबंधित प्रश्न
What is the focal length of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm? Is the system a converging or a diverging lens? Ignore thickness of the lenses.
A student uses a lens of focal length 40 cm and another of –20 cm. Write the nature and power of each lens
Which of the two has a greater power: a lens of short focal length or a lens of large focal length?
What is the nature of a lens whose power is, −4 D?
The power of a combination of two lenses X and Y is 5 D. If the focal length of lens X be 15 cm :
(a) calculate the focal length of lens Y.
(b) state the nature of lens Y.
Define power of a lens. Write its units. Deduce the relation `1/f =1/f_1 +1/f_2`for two thin lenses kept in contact coaxially.
A thin converging lens is formed with one surface convex and the other plane. Does the position of image depend on whether the convex surface or the plane surface faces the object?
A screen is placed a distance 40 cm away from an illuminated object. A converging lens is placed between the source and the screen and its is attempted to form the image of the source on the screen. If no position could be found, the focal length of the lens
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Assertion and reasoning type
- Assertion: Myopia is due to the increase in the converging power of eye lens.
- Reason: Myopia can be corrected with the help of concave lens.
