Lensed submillimetre-wave foregrounds and the CMBR

Samples of high-redshift galaxies are easy to select in the millimetre/submillimetre (mm/submm) waveband using sensitive telescopes, because their flux density-redshift relations are expected to be flat, and so the selection function is almost redshift-independent at redshifts greater than 0.5. Source counts are expected to be very steep in the mm/submm waveband, and so the magnification bias due to gravitational lensing is expected to be very large, both for lensing by field galaxies and for lensing by clusters. Recent submm-wave observations of lensed images in clusters have constrained the submm-wave counts directly for the first time. In the next ten years our knowledge of galaxy evolution in this waveband will be greatly enhanced by the commissioning of sensitive new instruments and telescopes, including the CMBR imaging space mission Planck Surveyor. This paper highlights the important features of gravitational lensing in the submm waveband and discusses the excellent prospects for lens searches using these forthcoming facilities.