how does a 3mm thick filter add 1mm to backfocus

[steve51450]

I am hoping this can be explained to me in easy terms

i have just ordered some adapters from precise parts in the USA for my scope, filter wheel & ccd combination and my back focus is 55mm

i Have to say they are very efficient company and the price was not to bad all things said and done for purpose made adapters but i could not get my head around the fact that i use filters in my imaging train, which are about 3mm thick it will add about 1mm to the backfocus of my scope. I have never heard of this before and i would be interested to know why

[brianb]

(filters) which are about 3mm thick it will add about 1mm to the backfocus of my scope.

Yes. Light going through glass slows down to about 2/3 of its speed in air (the exact amount is the inverse of the refractive index of the glass), this brings the focus slightly closer to the objective in the same way that the refractive index of the water in a swimming pool makes the pool look shallower than it really is.

[steve51450]

i see, that does help

many thanks