Hey,
There is no fixed pattern for the questions when it comes to Physics. The questions asked in the boards by CBSE is generally simple and if you are thorough with the syllabus and NCERT text,then you can easily score good marks.
Descriptive questions can be asked from anywhere. However, some chapters which generally have more chances are: Wave optics,Dual Nature of Radiation and Matter,Atoms and Nuclei,Semiconductor Electronics,Communication Systems.
Go through the previous years papers and practice as much as possible as most of the times,the questiona being asked are based on similar pattern.
Hope this helps.
Good luck.