The "bandwidth" of a typical oscilloscope is that frequency at which the input amplifiers attenuate the signal by 3 db.

You need to consider how that will impact your work. The timebase may display a 5 ns (100 MHz) pulse, but that's just a characteristic of the sweep circuit. If the input amplifier is 3 db down at 40 MHz, you probably will not be able to see much of a 5 ns pulse. They generally don't give derating information about frequencies above the -3db bandwidth. I suspect a 40 MHz 'scope won't display much of a 5 ns pulse, perhaps 10% of its amplitude, and certainly no useful information about rise and fall.

RE