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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) Draw a ray diagram to show the formation of image by a concave lens when an object is placed in front of it.
(b) In the above diagram mark the object distance (u) and the image distance (v) with their proper signs (+ve or –ve as per the new Cartesian sign convention) and state how these distances are related to the focal length (f) of the concave lens in the case.
(c) Find the nature and power of a lens which forms a real and inverted image of magnification –1 at a distance of 40 cm from the optical centre.
For what position of an object a real, diminished image is formed by a convex lens?
A convex lens forms an image of an object equal to the size of the object. Draw a diagram to illustrate it.
In the following cases, where must an object be placed in front of a convex lens so that the image formed is of the same size as the object?
A student focussed the image of a distant object using a device ‘X’ on a white screen ‘S’ as shown in the figure. If the distance of the screen from the device is 40 cm, select the correct statement about the device.
(A) The device X is a convex lens of focal length 20 cm.
(B) The device X is a concave mirror of focal length 40 cm.
(C) The device X is a convex mirror of radius of curvature 40 cm.
(D) The device X is a convex lens of focal length 40 cm.
Yesh find out F1 and F2 of symmetric convex lens experimentally then which conclusion is true.
Draw neat diagram to show the
Convergent action of a convex lens,
An object 4.0 cm in size, is placed 25.0 cm in front of a concave mirror of focal length 15.0 cm.
(i) At what distance from the mirror should a screen be placed in order to obtain a sharp image?
(ii) Find the size of the image.
(iii) Draw a ray diagram to show the formation of the image in this case.
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?
