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Question
Describe in brief how would you determine the approximate focal length of a convex lens.
Solution
The approximate focal length of a convex lens can be determined by using the principle that a beam of parallel rays incident from a distant object is converged in the focal plane of the lens.
In an open space, against a white wall, a metre scale is placed horizontally with its 0 cm end touching the wall.
By moving the convex lens to and fro along the scale, focus a distant object on wall. The image which forms on the wall is very near to the focus of the lens and the distance of the lens from the image is read directly by the metre scale. This gives the approximate focal length of the lens.
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RELATED QUESTIONS
The following diagram shows the experimental set up for the determination of focal length of a lens using a plane mirror.
- Draw two rays from the point O of the object to show the formation of image I at O itself.
- What is the size of the image I?
- State two more characteristics of the image I.
- Name the distance of the objects O from the optical centre of the lens.
- To what point will the rays return if the mirror is moved away from the lens by a distance equal to the focal length of the lens?
A lens forms the image of an object placed at a distance 15 cm from it, at a distance 60 cm in front of it. Find the focal length.
A convex lens forms an inverted image of size same as that of the object which is placed at a distance 60 cm in front of the lens. Find:
- The position of the image, and
- The focal length of the lens.
A concave lens forms an erect image of `1/3`rd the size of the object which is placed at a distance 30 cm in front of the lens. Find:
- the position of image, and
- the focal length of the lens.
The focal length of a convex lens is 25 cm. Express its power with sign.
The power of a lens is -2.0 D. Find its focal length and its kind.
Describe how you would determine the focal length of a converging lens, using a plane mirror and one pin. Draw a ray diagram to illustrate your answer.
