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I was solving the following problem, and I've got confused at the second question

http://img407.imageshack.us/img407/7921/43545856.png [Broken]

1) An object is put at the center of curvature C of a convergent lens. Where is the image? Calculate by gauss equation.

It's obvious that the image is in C', and is inverted and has the same size of the object.

By 1/f = 1/s + 1/s', we get 1/s'= 2/R-1/R => s' = +R (so is in the right of the lens)

2) The object now is put at C'. Where is the image? Calculate by gauss equation.

I would say it's also obvious it would be in C, inverted and equal to the object.

Now I have the following property: A lens, if turned changes nothing, s' and i stays the same. A convergent lens if seen from right to left is also convergent and with the same focus.

So I can use 1/R + 1/s' = 2/R, if I consider the light rays are coming from right to left, and I get s' = +R (R left the lens)

But now is the question:

If I consider a virtual object, and the light rays coming from left to right, we have f = +R/2 and s = -R, so we get

1/-R + 1/s' = 1/f => s' = R/3 right the lens. Why I got this wrong result? Can't I consider virtual objects by negative values?

[]'s

John

http://img407.imageshack.us/img407/7921/43545856.png [Broken]

1) An object is put at the center of curvature C of a convergent lens. Where is the image? Calculate by gauss equation.

It's obvious that the image is in C', and is inverted and has the same size of the object.

By 1/f = 1/s + 1/s', we get 1/s'= 2/R-1/R => s' = +R (so is in the right of the lens)

2) The object now is put at C'. Where is the image? Calculate by gauss equation.

I would say it's also obvious it would be in C, inverted and equal to the object.

Now I have the following property: A lens, if turned changes nothing, s' and i stays the same. A convergent lens if seen from right to left is also convergent and with the same focus.

So I can use 1/R + 1/s' = 2/R, if I consider the light rays are coming from right to left, and I get s' = +R (R left the lens)

But now is the question:

If I consider a virtual object, and the light rays coming from left to right, we have f = +R/2 and s = -R, so we get

1/-R + 1/s' = 1/f => s' = R/3 right the lens. Why I got this wrong result? Can't I consider virtual objects by negative values?

[]'s

John

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