Imaging observations in the U band of eight radio galaxies are presented. We find no optical counterpart to the radio jets. For the three radio galaxies 3C 147, 3C 279 and 3C 433, we show that the radio to optical spectral index of the jet is significantly higher than the typical values found in the three best known optical jets (M87, 3C 273 and 3C 66B). We conclude that the cut-off frequencies are lower than $10^{14}$Hz in these cases. For the 3C 31 jet, our data are consistent with the radio to optical spectral index being comparable to the typical values. This result is in contradiction with the detection of the optical jet in the B band by Butcher et al (1980). Finally, the lower limit on the radio to optical spectral index we obtain for the four other radio jets of our sample is still consistent with the typical values.