Bandwidth in frequency domain is defined as the difference between highest and lowest frequencies where the power is inbetween some defined limits e.g 3dB bandwidth means range of frequencies where power is more than half(3 dB) but in time domain we also define bandwidth as the rate of change of signal with respect to time i.e. faster the rate of change, more the bandwidth, so can we say that an impulse signal has infinite bandwidth because the rate of change from zero to infinity takes 0 seconds