? asked in 科學及數學其他 - 科學 · 1 decade ago

physics (light)

Describe briefly a simple laboratory method to measure directly the focal length of a lens.

The answer is :Receive a sharp image of a distant object(eg object outside the windows) on a screen, measure the distance between the lens and the image, it is the focal length.

my question is :How can we know the object is "distant" enough to make the image project on the focus??

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  • 1 decade ago
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    I think whenever you move your len and screen together backward, the length of image doesn't change much, then you know it is far enough.

    While theoretically it requires an infinite long object, it is not practical but if it is "far enough", it would give you a good approximated value, what we need and can obtain is just that approximated one.

    For example, the actual focal length of a len is 2.216 cm. Then in your experiment, if you just use a ruler for the measurement, which can give the best answer to the first decimal point. Then once you obtain 2.2 as your result, you can say you have had a far enough object. On the other way round, if you move your setup backward and found it still keeps at 2.2, you can end it and you should know that, even you move farther from that, what you can get may be 2.201 or 2.202 and soon 2.2151 and so on.

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