One apple [and a half of an apple] is left on the plate.
2.".. a pair of spectacles is lying on the table." - pair is the subject here and it is in the singular, so it takes a verb in the singular.
3. The glasses are lying on the table.
glasses - subject ; it has a plural form. Glasses is one of the nouns that appear only in the plural form , meaning spectacles- it is called pluralia tantum.
Note :not to be confused with the noun glass , which can be either an uncountable noun, meaning the material glass, or a countable noun[glass - glasses] meaning a drinking vessel from which people drink.
e.g. This plate is made of glass -material.
He poured some wine in that Murano glass - drinking vessel [sg. glass, pl. glasses].
I`ve bought a pair of modern glasses [eyeglasses or spectacles].