using oil immersion: how, why and tips

Next practical many of you will use the immersion oil lens (i.e. the x100 lens) for the first time.... This can be tricky, so before you try this in your next lab, I though it would be a good idea to post you some helpful tips on this procedure....

First of all, why are you bothering with immersion oil? 

Immersion oil has the same diffractive index as glass. This means that the light travelling through your specimen won't diffract when it passes between the slide and the x100 lens because it is passing through a drop of oil rather than the air. As a result more light is collected by the lens and you have a clearer image......

image taken from : http://en.wikipedia.org/wiki/File:Immersionsvorteil.svg
In this diagram, the yellow region represents oil. So you can see that on the left, when the light rays (black arrows) pass from the sample to the lens, they do not bend at all. On the other hand, if you look at the right side of the image, you can see that when there is no oil, i.e. when the light rays pass through air between the specimen and the lens, the rays bend when they enter the air and as a result fewer light rays are collected by the lens.

Steps to take when using immersion oil....

1. Focus first with the lower objective lenses, one after the other starting with the weakest and working your way up to the x40 lens 
2. Rotate the lenses out of the way and place a small drop of immersion oil on your specimen
3. Rotate the x100 lens into the immersion oil
4. Very carefully adjust the focus with the fine focus

TIPS

• If you are finding it hard to focus, move the stage backwards and forwards a little so you know that you have to focus on whatever you see moving
• After you have focused with the x40 lens, the focus should be correct or very close to being correct when you use the oil with the x100 lens - so don't use the coarse focus with the x100 lens
• You will probably need more light when you use the x100 lens
• Make sure you know which side of your slide has the specimen - you can't focus if your slide is up-side-down! - you could write something on the edge of the slide to help with this.

DO NOT 

• Use oil with any other lens 
• Use the x100 lens without oil
• Forget to clean the oil off the x100 lens when you are done
• Get oil on any of the other lenses - it can damage them. Once you have put the oil on the specimen, don't rotate back to other lenses, only use the x100 lens.

I hope that helps