about lens and plants

1. The two positions of the lens are a pair of conjugate points, i.e. the image and object distances are interchanged. The existence of the conjugate points is a result of the Principle of Reversibilty of Light.

Thus using the thin lens equation, 1/u + 1/v = 1/f
where u and v are the object and image distances respectively
f is the focal length of the lens

i.e. v/u + 1 = v/f

For the first siutation, when the image height is [a], v/u = a/h = magnification
where h is the object height
hence, a/h + 1 = v/f ---------------- (1)

For the second situation, b/h + 1 = v'/f
where v' is the new image distance

But v' = u for the two conjugate points
thus, b/h + 1 = u/f ------------- (2)

(1)/(2): [a/h+1]/[b/h+1] = v/u
but v/u = a/h
hence, [a/h+1]/[b/h+1] = a/h
simplifying, we have a/h + 1 = ab/h^2 + a/h
or ab/h^2 =1
i.e. h = square-root(ab)
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2. Using the thin lens equation again,
1/u + 1/v = 1/f
i.e. u/u + u/v = u/f
1+ 1/m = u/f where m = v/u = magnification

thus, 1/m = u/f - 1
You can see that if the object moves farther away from the lens, u increases. The factor (u/f) increases and hence 1/m also increases. That is m would decrease, this implies the magnification is smaller, the image size will be smaller.
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3. Sorry.....cant help. I think this is more a biology question than a physics question.

Hey!question 3 is a biology question !>.<
If minerals are present in the soil,the water potential in the soil will be lower,so that the difference between the water potential inthe soil and that in the plant is higher,thus the transpiration pull is greater,so it can get more water.

2008-07-27 18:42:15 補充：
If minerals are washed away, difference between the water potential inthe soil and that in the plant is lower,so the transpiration pull is lower,so the plant will take less water.
Simple :p