wavelength 2.2 μm was only announced in 1996. The appearance of the λ ring changed dramatically when it was observed in forward-scattered light in 1986. In this geometry the ring became the brightest feature of the Uranian ring system, outshining the ε ring. This observation, together with the wavelength dependence of the optical depth, indicates that the λ ring contains significant amount of micrometre-sized dust. The normal optical depth of this dust is 10−4–10−3. Observations in 2007 by the Keck telescope during the ring plane-crossing event confirmed this conclusion, because the λ ring became one of the brightest features in the Uranian ring system. Detailed analysis of the Voyager 2 images revealed azimuthal variations in the brightness of the λ ring. The variations appear to be periodic, resembling a standing wave. The origin of this fine structure in the λ ring remains a mystery. 1986U2R/ζ (zeta) ring In 1986 Voyager 2 detected a broad and faint sheet of material inward of ring 6. This ring was given the temporary designation 1986U2R. It had a normal optical depth of 10−3 or less and was extremely faint. It was thought to be visible only in a single Voyager 2 image, until reanalysis of Voyager data in 2022 revealed the ring in post-encounter images. The ring was located between 37,000 and 39,500 km from the centre of Uranus, or only about 12,000 km above the clouds. It was not observed again until 2003–2004, when the Keck telescope found a broad and faint sheet of material just inside ring 6. This ring was dubbed the ζ ring. The position of the recovered ζ ring differs significantly from that observed in 1986. Now it is situated between 37,850 and 41,350 km from the centre of the planet. There is an inward gradually fading extension reaching to at least 32,600 km, or possibly even to 27,000 km—to the atmosphere of Uranus. These extensions are labelled as the ζc and ζcc rings respectively. The ζ ring was observed again during the ring plane-crossing event in 2007 when it became the brightest feature of the ring system, outshining all other rings combined. The equivalent optical depth of this ring is near 1 km (0.6 km for the inward extension), while the normal optical depth is again less than 10−3. Rather different appearances of the 1986U2R and ζ rings may be caused by different viewing geometries: back-scattering geometry in