On the camera, when it says 4000, 2000, 1000 that actually means 1/4000, 1/2000, 1/1000... when you turn the main knob in clockwise position and get 1", 2", 3" then that is 1 second, 2 seconds, 3 seconds.
I just check out wiki, if you read carefully, they state 1 stop. Those are just typical 1 stop values, cameras will have much more shutter speed options, in increments like 1/3 stops, instead of the typical 1 stop values shown in wiki.
So 1.5 in your case simply means 1/1.5s. There is nothing to convert, 1/1.5s is still 1/1.5s for your D90 or whatever camera you can think of.
Pushing your topic to Google revealed two hits, stating the shutter speed specs and 2 remarks: increments can be changed in 1/2 stops and 1/3 stops. Read up about exposure stops.
The Wikipedia site about Shutter Speed lists the full stops.
Read your manual, again, slowly
Awesomely said. hahahaha.
TS, you should really read up on some basics of how a dslr/camera work before getting it. but then again, if you have it now, you should read on your User Manual. It has more than enough information for you. lol. :bsmilie::bsmilie::bsmilie: