Editing Thorium dioxide (section)

===Glass manufacture===
[[File:Yellowing of thorium lenses.jpg|left|thumb|alt=Three lenses from yellowed to transparent left-to-right|Yellowed thorium dioxide lens (left), a similar lens partially de-yellowed with ultraviolet radiation (centre), and lens without yellowing (right)]]
When added to [[glass]], thorium dioxide helps increase its [[refractive index]] and decrease [[dispersion (optics)|dispersion]]. Such glass finds application in high-quality [[lens (optics)|lenses]] for cameras and scientific instruments.<ref name=CRC>{{cite book| last= Hammond| first= C. R.| title= The Elements, in Handbook of Chemistry and Physics| edition= 81st| publisher= [[CRC Press]]| isbn= 978-0-8493-0485-9| date= 2004| url-access= registration| url= https://archive.org/details/crchandbookofche81lide}}</ref> The radiation from these lenses can darken them and turn them yellow over a period of years and degrade film, but the health risks are minimal.<ref>{{Cite web|author=Oak Ridge Associated Universities|year=1999|title=Thoriated Camera Lens (ca. 1970s)|url=https://orau.org/health-physics-museum/collection/consumer/products-containing-thorium/camera-lens.html|access-date=29 September 2017}}</ref> Yellowed lenses may be restored to their original colourless state by lengthy exposure to intense ultraviolet radiation. Thorium dioxide has since been replaced by rare-earth oxides such as [[lanthanum oxide]] in almost all modern high-index glasses, as they provide similar effects and are not radioactive.<ref>{{cite book |first=W. |last=Stoll |chapter=Thorium and Thorium Compounds |doi=10.1002/14356007.a27_001 |title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=Wiley-VCH |year=2005 |isbn=978-3-527-31097-5 |page=32}}</ref>
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