Фотохромные линзы темнеют на солнце.

Because фотохромные линзы is the subject of the sentence, so both words are in the nominative plural:

• линза → линзы
• фотохромный → фотохромные

The adjective has to agree with the noun in number, gender, and case. Since линзы is plural, the adjective is plural too.

A singular version would be:

• Фотохромная линза темнеет на солнце.
  A photochromic lens darkens in the sun.

That ending marks the plural nominative form of the adjective.

Russian adjectives change depending on the noun they describe. Here:

• singular masculine: фотохромный
• singular feminine: фотохромная
• singular neuter: фотохромное
• plural: фотохромные

So фотохромные matches plural линзы.

Because the subject is plural.

• линза темнеет = the lens darkens
• линзы темнеют = the lenses darken

So темнеют is the 3rd person plural form of the verb темнеть.

Темнеют means become dark, get darker, or darken.

It describes a change of state, not just a fixed description.

So:

• тёмные линзы = dark lenses
• линзы темнеют = the lenses are becoming dark / darken

In this sentence, it means that the lenses change color when exposed to sunlight.

It is present tense, from the imperfective verb темнеть.

In Russian, the present tense of an imperfective verb can describe:

• what is happening now
• a general fact
• a repeated or habitual action

Here it is most naturally a general fact:

• Фотохромные линзы темнеют на солнце.
  Photochromic lenses darken in the sun.

If you wanted a more clearly future/completed meaning, Russian would normally use a perfective verb, for example:

• Фотохромные линзы потемнеют на солнце.
  The photochromic lenses will darken in the sun.

Literally, на often means on, but in Russian prepositions do not always match English one-to-one.

Here на солнце means:

• in the sun
• in sunlight
• when exposed to the sun

So this is a normal, idiomatic Russian expression. You should understand it as in the sun, not as physically on top of the sun.

It is in the prepositional case, because на is being used to indicate location/condition here.

With location, на usually takes the prepositional:

• на столе = on the table
• на улице = outside / on the street
• на солнце = in the sun

Why does солнце look unchanged? Because for this noun, the nominative and prepositional singular happen to look the same:

• nominative: солнце
• prepositional: о солнце / на солнце

So the case changed, even though the spelling did not.

Because на солнце is the normal expression for being in sunlight or exposed to the sun.

• на солнце = in the sun / in sunlight
• под солнцем = more literally under the sun

Под солнцем can sound more literal, poetic, or broader, like under the sun in English. For the everyday idea of something reacting to sunlight, на солнце is the usual choice.

Yes. Russian word order is fairly flexible.

The version given:

• Фотохромные линзы темнеют на солнце.

is a neutral, natural way to say it.

But you could also say:

• На солнце фотохромные линзы темнеют.

This puts more emphasis on на солнце: in the sun, photochromic lenses darken.

So the basic meaning stays the same, but the emphasis can shift.

Because Russian has no articles.

English uses a/an and the, but Russian usually does not. So:

• линза can mean a lens or the lens
• линзы can mean lenses or the lenses

You understand which one is meant from context.

The stress is:

• фотохро́мные ли́нзы темне́ют на со́лнце

A rough pronunciation guide:

• фотохро́мные → fa-ta-KHROM-ny-ye
• ли́нзы → LEEN-zɨ
• темне́ют → teem-NYE-yut
• со́лнце → SON-tse

A useful pronunciation note: in солнце, the л is typically not pronounced clearly, so it sounds close to сонце.

Yes, it is a correct and natural phrase, especially in a neutral or technical context.

In everyday speech, people may also say things like:

• очки-хамелеоны
• линзы-хамелеоны

But фотохромные линзы is the standard, precise term for photochromic lenses.