Physics
Describe in brief how would you determine the approximate focal length of a convex lens.
Refraction Lens
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Answer
The principle used for determination of approximate focal length of a convex lens is as follows —
A beam of parallel rays from a distant object incident on a convex lens gets converged in the focal plane of the lens.
In an open space, against a white wall, place a metre rule horizontally with its 0 cm end touching the wall, with its other end towards the illuminated object at a very large distance.
By moving the convex lens to and fro along the length of the metre rule, focus the object on the wall. Since, the light rays incident from a distant object are nearly parallel, the image of it formed on the wall is almost at the focus of the lens.
So, from the image, the distance of the lens is read directly from the metre scale. This gives the approximate focal length of the lens.
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Related Questions
Draw a neat labelled ray diagram to show the formation of an image by a magnifying glass. State three characteristics of the image.
The diagram in figure shows experimental setup for determination of the focal length of a lens using a plane mirror.
(i) Draw two rays from the point O of the object to show the formation of image I at O itself.
(ii) What is the size of the image I?
(iii) State two more characteristics of the image I.
(iv) Name the distance of the object O from the optical centre of the lens.
(v) 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?
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.