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प्रश्न
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
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उत्तर
We cannot see anything in a dark room as there is no light present.
(a) We see the electric bulb because it produces light that reaches our eyes directly.
(b) We see a piece of white paper due to the diffused reflection of light from the surface of the paper.
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संबंधित प्रश्न
State power of a lens S.I. unit.
Define 1 dioptre of power of a lens.
Find the power of a concave lens of focal length 2m.
A student uses a lens of focal length 40 cm and another of –20 cm. Write the nature and power of each lens
A convex lens of power 5 D and a concave lens of power 7.5 D are placed in contact with each other. What is the :
(a) power of this combination of lenses?
(b) focal length of this combination of lenses?
Define the term focal length of a lens.
State the condition for the following a lens has both its focal lengths equal .
On reducing the focal length of a lens, its power ______.
Name the type of lens whose power is positive.
The image of an object formed by a lens is real, inverted and of the same size as the object. If the image is at a distance of 40 cm from the lens, what is the nature and power of the lens ? Draw ray diagram to justify your answer.
A convex lens produces a double size real image when an object is placed at a distance of 18 cm from it. Where should the object be placed to produce a triple size real image?
The following diagram shows the object O and the image I formed by a lens. Copy the diagram and on it mark the positions of the lens LL’ and focus (F). Name the lens.
Complete the diagram to show the formation of the image of the object AB.
(i) Name the Lens LL’.
(ii) Where is the image of the object AB formed?
(iii) State three characteristics of the image.
A convex lens is of focal length 20 cm. Find its power.
If the lens is of focal length 25 cm. Calculate the power of the lens.
If focal length of a convex lens is 20 cm at what is the power of the lens?
Increase in the converging power of eye lens cause ‘hypermetropia'
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.
The focal length of a concave lens is 20 cm. The focal length of a convex lens is 25 cm. These two are placed in contact with each other. What is the power of the combination? Is it diverging, converging or undeviating in nature?
An object is placed in front of a converging lens. Obtain the conditions under which the magnification produced by the lens is
- negative and
- positive.
A point object is placed at O in front of a glass sphere as shown in figure.
Show the formation of the image by the sphere.
