We have measured the far-infrared reflectivity and Kerr angle spectra on a high-quality crystal of pure semimetallic bismuth as a function of magnetic field, from which we extract the conductivity for left-and right handed circular polarisations. The high spectral resolution allows us to separate the intraband Landau level transitions for electrons and holes. The hole transition exhibits 100% magnetic circular dichroism, it appears only for one polarisation as expected for a circular cyclotron orbit. However the dichroism for electron transitions is reduced to only 13 ± 1%, which is quantitatively explained by the large effective mass anisotropy of the electron pockets of the Fermi surface. This observation is a signature of the mismatch between the metric experienced by the photons and the electrons. It allows for a contactless measurement of the effective mass anisotropy and provides a direction towards valley polarised magneto-optical pumping with elliptically polarised light.Circular dichroism, the property of materials to interact differently with left and right circularly polarised light is associated with symmetry breaking due to e.g. molecular chirality, spontaneous magnetisation or an external magnetic field. A charged particle in a magnetic field moves in a circular orbit perpendicular to that field. Radiation propagating parallel to the field is only absorbed by that particle for right or left circular polarisation, leading to 100% magnetic circular dichroism. In condensed matter, the behaviour of charge carriers in a lattice is described using an effective mass, which can be very different from the free-electron mass, but also strongly anisotropic [1]. In the latter case the cyclotron orbits become elliptical which, as we will show, strongly reduces the magnetic circular dichroism. The reduction of magnetic circular dichroism can thus be regarded as a manifestation of the mismatch between the metric experienced by photons (isotropic) and the electrons (anisotropic), which becomes apparent when they interact [2-4]. As a consequence, elliptically polarised light with the same ellipticity as an electron pocket can cause a 100% valley polarised magneto-optical absorption.Bismuth is a canonical semimetal which possesses a rich electronic structure with strong spin-orbit interaction, low carrier density, long mean-free path and small cyclotron mass [5][6][7][8] (Fig. 1a). A hole pocket is oriented along the trigonal axis, where the bands are almost parabolic.Three electron pockets are tilted 6• from the plane perpendicular to the trigonal axis and have a Dirac-like band dispersion. In this plane the dielectric function is isotropic. A magnetic field parallel to the trigonal axis combined with light propagating along the same axis therefore presents an ideal case to measure the dichroism in relation to the effective mass. For holes the mass is isotropic in the plane whereas for electrons it is strongly anisotropic (a factor > 200 [18]), leading to a strong dichroism contrast between electron and hole transi...