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.
Vowel length In addition, the vowels of Received Pronunciation are commonly divided into short and long, as obvious from their transcription. The short vowels are /ɪ/ (as in kit), /ʊ/ (as in foot), /ɛ/ (as in dress), /ʌ/ (as in strut), /æ/ (as in trap), /ɒ/ (as in lot), and /ə/ (as in the first syllable of ago and in the second of sofa). The long vowels are /iː/ (as in fleece), /uː/ (as in goose), /ɜː/ (as in nurse), /ɔː/ as in north and thought, and /ɑː/ (as in father and start). While a different degree of length is indeed present, there are also differences in the quality (lax vs tense) of these vowels, and the currently prevalent view tends to emphasise the latter rather than the former.