  Webster et al (1995) claimed that up to 80% of QSOs may be obscured by dust.
They inferred the presence of this dust from the remarkably broad range of B-K
optical-infrared colours of a sample of flat-spectrum PKS radio QSOs. If such
dust is typical of QSOs, it will have rendered invisible most of those which
would otherwise been have detected by optical surveys. We used the William
Herschel Telescope on La Palma to obtain K infrared images of 54 B3 radio
quasars selected at low frequency (mainly steep-spectrum), and we find that
although several have very red optical-infrared colours, most of these can be
attributed to an excess of light in K rather than a dust-induced deficit in B.
We present evidence that some of the infrared excess comes from the light of
stars in the host galaxy (some, as previously suggested, comes from synchrotron
radiation associated with flat-spectrum radio sources). The B-K colours of the
B3 QSOs provide no evidence for a large reddened population. Either the Webster
et al QSOs are atypical in having such large extinctions, or their reddening is
not due to dust; either way, the broad range of their B-K colours does not
provide evidence that a large fraction of QSOs has been missed from optical
surveys.
