Last time we ended scratching the topic of focal length, and I told you that this is going to be complicated. Don’t worry it will be. Yet, focal length in itself is rather easy and straightforward to determine and also to explain, if you’re not going into details. It is a characteristic of the lense you’re using. So just imagine having a magnifying glass and you use it to focus sunlight, as if you would try to use it for starting a fire. If you take all the light the lense is capturing and focus it on a single spot on the other side, the distance between the lense and the point that lightspot is most concentrated, that is the focal length or focal distance. It is a physical property of the lense, based on the lenses curvature.
On a berfectly biconvex lens the focal distance would be equal on both sides of the lense. In the context of microscopy we will assume it is that way to make things easier.
Theoretically spoken, and inlcuding some equations, one could express that as following
1/S1 + 1/S2 =1/f
with f being the focal distance and S1 being the distance between object and lens and S2 being the distance between lens and eye/image.
Assuming we are producing a real image here, and with both distances being the same, the image produced would be exactly the same size as the object observed.
The magnification comes into play as following.
M= – (S2 / S1) = f / (f – S1)
That sounds complicated, but what it really tells is,if M is bigger than 1, the image will be magniefied, if it is smaller, than it will be shrinked down. Which is exactly what is happening within a binocular, basically.
What happens in a microscope is turning the image upside down for a real image, which you would not see though as it would be same size as your microscopically small object. But, of course light rays don’t just stop there, so on the far side of the focal distance, on our side of the lens, the image is crossed again and the lightrays spread out, creating a virtual image that is upright again, and magnified, using another lens in the eyepiece to sharpen and focus it again back onto our eyes. and to the image we see.
See the image below to get a better idea of that, if your brain works as visual as mine.