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प्रश्न
Define the terms principal foci and focal lengths as applied to a convex lens, and show them with the help of proper diagrams.
उत्तर
A convex lens has two principal foci:
(i) first focus, and
(ii) second focus.
First focus: First focus of a convex lens is a point F1 on the principal axis of the lens such that the rays of light starting from it after refraction from the lens become parallel to the principal axis. This is shown in figure. The distance of the first focal point Fj from the optical centre O is called first focal length. In the figure, it has been labelled by f1 (i.e., OF1 = f1)
Second focus: Second focus of a convex lens is a point F2 on the principal axis of the lens such that the rays of light incident parallel to the principal axis of the lens after refraction from the lens passes through it. This is shown in figure. The distance of the second focal point F2 from the optical centre O is called the second focal length. In the figure, it has been labelled by f2 (i.e., OF2 = f2).
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संबंधित प्रश्न
A student wants to project the image of a candle flame on the walls of school laboratory by using a lens:-
(a) Which type of lens should be use and why?
(b) At what distance in terms of focal length 'F' of the lens should be place the candle flame so as to get (i) a magnified, and (ii) a diminished image respectively on the wall?
(c) Draw ray diagram to show the formation of the image in each case?
"A lens can form a magnified erect image as well as magnified inverted image of an object placed in front of it." State the nature of this lens and draw ray diagrams to justify the above statement. Mark the positions of O, F and 2F in the diagram.
A burning candle whose flame is 1.5 cm tall is placed at a certain distance in front of a convex lens. An image of candle flame is received on a white screen kept behind the lens. The image of flame also measures 1.5 cm. If f is the focal length of convex lens, the candle is placed:
(a) at f
(b) between f and 2f
(c) at 2f
(d) beyond 2f
A beam of parallel light rays is incident through the holes on one side of a box and emerges out through the holes on its opposite side as shown in the diagram below:
Which of the following could be inside the box?
(a) a rectangular glass block
(b) a concave lens
(c) a convex lens
(d) a glass prism
In the following cases, where must an object be placed in front of a convex lens so that the image formed is inverted and enlarged?
State the position of object, position of image, nature of image when: Convex lens is used in cine projector.
We can burn a piece of paper by focussing the sun rays by using a particular type of lens. Name the type of lens used for the above purpose. Draw a ray diagram to support your answer.
Find the odd one out and give its explanation.
Observe the given figure and answer the following questions.
- Where is the above type of lens construction used?
- What type of image is formed by an objective lens?
- What happens instead of placing at Fo if the object is placed in between O and Fo?
Distinguish between:
Concave lens and Convex lens
