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Finding focal length by displacement method In an experiment for finding focal length of a convex lens the object and screen are kept fixed. There are two position of lens between the object and screen for which an image is obtained on the screen. These positions are separated by 20cm. Also, the magnification in first situation is -2. Find the distance between the object and the screen, that is the focal length of the lens. |
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Answer» Solution :As we can see from the analysis persented before, this refers to the situation of DISPLACEMENT method of finding the focal length of lens (Fig 34-47) Calculation : `(1)/(d-x)+(1)/(x)=(1)/(f)` From Fig. 34-47 , it is clear that in the first situation , object is at a distance x from the lens and in second situation screen is at a distance x from the lens. Therefore, `d-2x=20cm` Also `m_(1)=(d-x)/(-x)=-2` `d=3x` So, `x=20cm` and `d=60cm` . Therefore, `(1)/(40)+(1)/(20)=(1)/(f)` ,br> `f=40//3cm` We stated previously that d must be greater than 4f . This can be easily seen from the ANSWER. |
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