This root was the basis for words having to do with “light” for most of Tolkien’s life. Its derivatives include Q. cala “light” and S. aglar “glory” which likewise retained the same basic form and meaning throughout Tolkien’s life. The root ᴱ√KALA first appeared in the Qenya Lexicon of the 1910s glossed “shine golden” (QL/44) along with a rejected variant ᴱ√ALA “light” (QL/29). In this early iteration, the root ᴱ√KALA included a number of derivatives having to do with daytime, such as ᴱQ. kala “daytime (sunlight), 12 hours” and G. gala “daylight” (QL/44; GL/38). Also of interest is that most of the Gnomish derivatives from this period began with g-, indicating that the true form of the root may have been ✱ᴱ√GALA (or at least a blending with it), since [[eq|initial [g-] became [k-]]] in Early Qenya.
This G/K variation became explicit in The Etymologies of the 1930s, where ᴹ√KAL “shine” had a Noldorin-only variant ᴹ√GAL (Ety/GAL, KAL). Tolkien did consider having a few Quenya derivatives of ᴹ√GAL, but they were all rejected (EtyAC/GAL¹). In the 1930s, it also seems the “daytime” derivatives were transferred to ᴹ√AR “day” (Ety/AR¹). The root √KAL continued to appear in Tolkien’s later writings, variously glossed “shine”, “light” and “be bright”, with such frequency that it is impractical to list all the references. Likewise Tolkien continued to refer to the Sindarin-only variant √GAL on a regular basis; see that entry for details.
The root √KAL had a couple of extended forms of note, in particular √KALAR glossed “be radiant” in the 1930s (PE18/62), also appearing in the vowel-suppressed variant ᴹ√AKLA(R) “radiance, splendour” which served as the basis for Q. alcar/S. aglar “glory, brilliance, splendour, radiance” (PE17/105, 124; PE18/36, 87; VT47/13; Ety/AKLA-R, KALAR); in the Qenya Lexicon of the 1910s this variant root was given as ᴱ√ḶKḶ instead (QL/30). Tolkien frequently used the extension √KALAT as the representation of triconsontal roots (PE18/33, 86; WJ/392), though the only derivative he ever gave for it was ✶kalatta “a light, lamp” (PE21/80).