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प्रश्न
A concave lens produces an image 20 cm from the lens of an object placed 30 cm from the lens. The focal length of the lens is:
(a) 50 cm
(b) 40 cm
(c) 60 cm
(d) 30 cm
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उत्तर
60 cm
Object distance from the lens, u =-30 cm
Image distance from the lens, v =-20 cm
Using the lens formula, we get:
`1/f=1/v-1/u`
`1/f=1/-20-1/-30`
`1/f=1/-20+1/30`
`1/f=(-3+2)/60=1/60`
∴ f =- 60 cm
Therefore, the focal length of the lens is 60 cm.
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संबंधित प्रश्न
Define the following term in the context of spherical mirrors:- Pole
Explain the following term related to spherical lenses:- principal axis
A concave mirror always forms a real image.
A virtual image larger than the object can be produced by a ______.
What type of image is formed:
in a plane mirror?
What is the relation between the focal length and radius of curvature of a spherical mirror (concave mirror of convex mirror)? Calculate the focal length of a spherical mirror whose radius of curvature is 25 cm.
Name the spherical mirror which can produce a virtual and diminished image of an object.
State the two kinds of spherical mirror and distinguish them with the aid of proper diagrams.
The letters on the front of an ambulance are written laterally inverted like 
. Give reason.
Complete the following diagrams shown in Fig. by drawing the reflected ray for each of the incident ray A and B.
Figure shows a concave mirror with its pole at P, focus F and centre of curvature C. Draw ray diagram to show the formation of image of an object OA.
State whether the image in part (a) is real or virtual?
How is the focal length of a spherical mirror related to its radius of curvature?
Why does a driver use a convex mirror instead of a plane mirror as a rear view mirror?
Illustrate your answer with the help of a ray diagram.
Upto what maximum distance from the pole the image in a convex mirror can be obtained? what will be the location of object then ?
Define the following term in relation to concave mirror.
Focal length
What is “aperture”?
