Hp laptop brightness increasing and decreasing

I bought a new Core i7 HP Pavilion 15-r122ne with Windows 8.1 preinstalled and Nvidia Graphics. I am new to Windows 8. When I am using laptop on battery and I launch different apps or pages the brightness increases and then returning to desktop screen it gets dim again. Please tell me is that software problem or hardware. I am really worried because it is a new laptop.