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प्रश्न
If there is a convex lens of focal length 75 cm and a concave lens of focal length 40 cm, then calculate their combined power and combined focal length.
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उत्तर
Power of convex lens = `100/("f"("cm"))`
= `100/75` = + 1.33 D.
Power of concave lens = `100/("f"("cm"))`
= `100/40` = − 2.5 D.
Their combined power P = + 1.33 D + (− 2.5 D)
P = − 1.17 D.
Their combined focal length F = `100/"P"=100/(-1.17)`
F = − 85.47 cm.
Negative sign indicates that the nature of their combination is concave.
संबंधित प्रश्न
What is meant by a power of a lens? Define its SI unit.
Kavita from 10th is using spectacles. The power of the lenses in her spectacles is –2.5 dioptre. Answer the following questions:
1) Which lenses are used in her spectacles?
2) State the defect of vision Kavita is suffering from.
3) Find the focal length of the lenses used in her spectacles
What can you see in a completely dark room? If you switch on an electric bulb in this dark room as a light source, explain how you could now see:
(a) the electric bulb
(b) a piece of white paper
Consider two statements A and B given below:
A: real image is always inverted
B: virtual image is always erect
Out of these two statements:
What is the nature of a lens whose power is, −4 D?
The optician's prescription for a spectacle lens is marked +0.5 D. What is the:
(a) nature of spectacle lens?
(b) focal length of spectacle lens?
Mohan obtained a sharp inverted image of a distant tree on the screen placed behind the lens. He then moved the screen and tried to look through the lens in the direction of the object. He would see:
(1) a blurred image on the wall of the laboratory.
(2) an erect image of the tree on the lens.
(3) no image as the screen has been removed
(4) an inverted image of the tree at the focus of the lens.
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 double convex lens has focal length 25 cm. The radius of curvature of one of the surfaces is double of the other. Find the radii, if the refractive index of the material of the lens is 1.5.
In what unit is the power of a lens expressed?
