I have been trying to measure the voltage coming from my boost converter to see how clean the signal is but i only have a cheap oscilloscope so when i set the range to the millivolts the signal jumps out of shot. Is using a voltage divider to reduce it down to a level i can see going to give me an accurate visual indicator of the ripple even though its been reduced by a large amount? I will be a 5v signal i'm testing. many thanks.
Switch the scope input to something called AC coupled. That only shows you the near-term deviations from the average. That's exactly what you want if you are trying to see noise on a power supply. The AC coupled setting should be a choice by the input, sometimes in the "vertical" section.
If you would use a voltage divider to make the signal small enough to fit on your millivolt scale, the ripple will have decreased by the same factor and thus be equally unrecognizable. Worse yet, the typical noise of the scope will increase on the millivolt scale making it even harder to see the ripple at all.
Like Olin said the AC coupling should be used to investigate the ripple on a DC signal.
Then you don't need any reduction and can switch directly to the millivolt range.