Not every word in English that ends in -s is plural.
Consider:
gas
bus
canvas
news
atlas
The why is probably because the word lens was taken from the Latin word lens, which is also singular. See Online Etymology. The Oxford English Dictionary says the same.
If you see "police" as an organisation, as a whole, you can use it as a singular, when you see "police" as a group of civil servants you can use it as a plural.
Measure phrases are special noun phrases that we use to explain how long or big or heavy or expensive something is:
The programme was ten minutes.
The walk was five miles.
The meal was twenty pounds.
The word is only three letters
These measure phrases all include a number, like one or seven and a noun afterwards. In the examples above these measure phrases are Complements of the verb. You will notice that the nouns are all in the plural, as we expect.
We can also use measure phrases like these to modify nouns:
a ten minute programme.
a five mile walk.
a twenty pound note.
a three letter word.
Here, these measure phrases are modifying the nouns programme, walk and note. They are in the same position that we find adjectives in. When we use measure phrases in this way, the noun in the measure phrase is not plural. We see no S on the ends of the words in the measure phrases.
Really this is not very surprising. Why? Well, when we use a noun to modify another noun, we don't usually use plurals (there are exceptions of course). So we usually say:
Best Answer
Not every word in English that ends in -s is plural.
Consider:
gas
bus
canvas
news
atlas
The why is probably because the word lens was taken from the Latin word lens, which is also singular. See Online Etymology. The Oxford English Dictionary says the same.