I have a 6 inch dobsonian telescope its focal ratio is 7.8 if i use a 2x barlow what happens?

doesnt it make the picture very poor?it has 1200mm focal lenght so with a 10mm eyepiece i have 120x magnification if i use a 2x barlow i would have 240x magnification but does the barlow weaken the image so i cant see the jupiter atmosphere lines???