# Compound microscope

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1. Dec 29, 2014

### RachaelD95

is magnification negative for a compound microscope at infinity

2. Dec 29, 2014

### sophiecentaur

welcome to PF
Is this what you wanted? An inverted image implies a negative sign for the magnification.

3. Dec 29, 2014

### RachaelD95

I done a question where the formula was m=(-l0.25/fofe) as the final image was at infinty.
1. The angular magnification of a microscope is 400X when the final image is at infinity. The optical tube length is 16cm and the focal length of the objective is 5mm. What is the focal length of the eyepiece?

They took the magnification to be -400x not sure why? The only thing I could take from it was that when its a compound microscope at infinity the magnification is negative.

4. Dec 29, 2014

### sophiecentaur

Wherever you choose to put the final (virtual) image, the magnification will still have the same sign. Using infinity just makes the calculation easier.

5. Dec 29, 2014

### RachaelD95

So why is the magnification negative? It only seems to be for a compound microscope at infinity the magnification is negative. For a simple magnifier and astrological magnification remains positive

6. Dec 30, 2014

### sophiecentaur

I don't know where you got that from. Both the astronomical telescope (not the galilean telescope) and the microscope produce inverted real images - which is what the sign tells you. Also, I don't understand where you got the bit about 'focussed at infinity' as being special. You can easily focus a telescope at closer than infinity; for people with short sight and no glasses, for instance. The image is still the same way up! (i.e. inverted)
Are you using more than one source for you information? You may find that the sign has been omitted in one source.