does not free space

Hi,

I have T-log file in one partition say(L) which is 500GB. We have T-log backup after every 10 minutes. When I check free space in T-log file it is showing only 10GB occupied, but when I check on OS level, it shows the partition(L) has 0.0% free space. 

After the t-log backup, if the log file is shrink, why does not it release space on OS?

REgards