why w and y are called semi vowels
Semivowel In phonetics and phonology, a semivowel or glide, also known as a non-syllabic vocoid, is a sound that is phonetically similar to a vowel sound but functions as the syllable boundary, rather than as the nucleus of a syllable.[1] Examples of semivowels in English are the consonants y and w, in yes and west. Written /j w/ in IPA, y and w are near to the vowels ee and oo in seen and moon, written /iː uː/ in IPA.
Phoneme The number of phonemically distinct vowels can be as low as two, as in Ubykh and Arrernte. At the other extreme, the Bantu language Ngwe has 14 vowel qualities, 12 of which may occur long or short, making 26 oral vowels, plus 6 nasalized vowels, long and short, making a total of 38 vowels; while !Xóõ achieves 31 pure vowels, not counting its additional variation by vowel length, by varying the phonation. As regards consonant phonemes, Puinave and the Papuan language Tauade each have just seven, and Rotokas has only six. !Xóõ, on the other hand, has somewhere around 77, and Ubykh 81. The English language uses a rather large set of 13 to 21 vowel phonemes, including diphthongs, although its 22 to 26 consonants are close to average.