Learn English – Is it light or lights

Etymonline has this to say:

-ics
in the names of sciences or disciplines (acoustics, aerobics, economics, etc.) it represents a 16c. revival of the classical custom of using the neuter plural of adjectives with -ikos (see -ic) to mean "matters relevant to" and also as the titles of treatises about them. Subject matters that acquired their names in English before c.1500, however, tend to remain in singular (e.g. arithmetic, logic).

So yes, at some point in history, there were such things as physic (meaning "natural science"), mathematic (meaning "mathematical science"), etc. that were later turned into plural forms but kept being treated as singular.

Edit: having looked in a few more places, it appears that in contemporary English, it still makes some sense to have both the suffix -ic and its plural form -ics. According to the Collins English Dictionary, the former has kind of specialized in forming adjectives, while the latter is happily forming nouns:

-ic
suffix forming adjectives

1. of, relating to, or resembling: allergic, Germanic, periodic. See also -ical.
   [...]

[from Latin -icus or Greek -ikos; -ic also occurs in nouns that represent a substantive use of adjectives (magic) and in nouns borrowed directly from Latin or Greek (critic, music)]

[...]

-ics
suffix forming nouns (functioning as singular)

1. indicating a science, art, or matters relating to a particular subject: aeronautics, politics
2. indicating certain activities or practices: acrobatics

[plural of -ic, representing Latin -ica, from Greek -ika, as in mathēmatika mathematics]

The key here is that they are not just two unrelated suffixes. Much rather, one is etymologically a plural form of the other. As the American Heritage Dictionary succinctly puts it, -ics is "-ic + -s".

To be clear, the headlights of cars today usually have two filaments.

One filament projects the beam (off the mirrors behind) straight ahead (roughly level with the road, say).

The other one projects the beam (off the mirrors) downwards (ie dipped, pointing downwards).

Thus -- when the light is on normal (that is to say, "high" beam, because the beam is pointing up high), it hits the road a long way ahead.

In contrast, when you "dip" the lights, you will see the light shining on the road a fairly short distance ahead.

They are "dipped" ... pointing-low ... the beam is pointing downwards, rather than onwards.

The thing that you dip the lights with is called a "dip switch."

(Before the 60s they were a footswitch, now they are on a stalk, or obviously just automatic.)

Note that "dimming" the lights would not help, you would still dazzle the oncoming driver. You must, of course, point the beam downwards - it's how headlights work.

I mention "bulbs have two filaments.." in the early days of motoring there were different systems to point the headlights downwards, when a car was oncoming..

'Dipping' (low beam) [ie low, downwards onto the ground] headlamps were introduced in 1915 by the Guide Lamp Company, but the 1917 Cadillac system allowed the light to be dipped with a lever inside the car rather than requiring the driver to stop and get out..." etc etc.

Dipping headlights, one of the basic elements of driving, along with brakes, turning indicators, etc!

It would appear that using the word "dip" for dipping the headlights downwards, is now more BrE. It seems that in the USA, the word has transformed to "dim." Moreover, people in the USA tend to think of the downwards-dipped beam as the "normal" one, and you then have "brights", "high" beam, etc. Physically this is completely incorrect; dimming lights would achieve nothing, you have to literally point them downwards, i.e. dip them.