אור אולטרה-סגול מראה את הסמל.

A simple pronunciation guide is:

or ultra-sagol mar'eh et ha-semel

More roughly by word:

• אור = or
• אולטרה-סגול = ultra-sagol
• מראה = mar'eh
• את = et
• הסמל = ha-semel

Because in Hebrew, adjectives normally come after the noun they describe.

So:

• אור אולטרה-סגול = literally light ultraviolet
• natural English order = ultraviolet light

This is very normal Hebrew word order.

Because אור here is being used in a general / indefinite way: ultraviolet light.

But הסמל means the symbol, which is definite.

So:

• אור אולטרה-סגול = ultraviolet light / UV light
• הסמל = the symbol

If you wanted to say the ultraviolet light, Hebrew would usually say:

האור האולטרה-סגול

Notice that with a definite noun + adjective, both usually take ה־.

Here מראה means shows or reveals.

It is the present tense form of the verb להראות = to show.

So the sentence structure is:

• אור אולטרה-סגול = subject
• מראה = shows
• את הסמל = the symbol

את is the Hebrew marker for a definite direct object.

Since הסמל means the symbol and is definite, Hebrew puts את before it:

• מראה את הסמל

English has no direct equivalent word here, so את is usually not translated.

A useful rule:

• definite object → usually use את
• indefinite object → usually no את

For example:

• מראה את הסמל = shows the symbol
• מראה סמל = shows a symbol

Yes. In Hebrew, the verb in the present tense agrees with the subject in gender and number.

Here:

• אור is masculine singular
• so the verb is מראה, also masculine singular

If the subject were plural, the verb would be plural too. For example:

• האורות מראים את הסמל = the lights show the symbol

Yes, it can.

In this sentence it matches אור, which is masculine singular, so we get:

• אור אולטרה-סגול

Other forms include:

• קרינה אולטרה-סגולה = ultraviolet radiation
• אורות אולטרה-סגולים = ultraviolet lights
• קרניים אולטרה-סגולות = ultraviolet rays

So even though it is a compound adjective, it still behaves like an adjective and can agree with the noun.

Yes. They are closely related.

• לראות = to see
• להראות = to show

So להראות is basically the causative idea: to cause someone to see → to show.

That is why מראה here means shows.

Usually not in the present tense.

מראה can mean:

• shows
• is showing

The exact meaning depends on context.

So this sentence could be understood as either:

• Ultraviolet light shows the symbol
• Ultraviolet light is showing the symbol

English makes that distinction more explicitly than Hebrew does.

Yes, it is natural and clear.

But Hebrew could also express the idea in other natural ways, depending on emphasis. For example:

• אור אולטרה-סגול חושף את הסמל = Ultraviolet light reveals the symbol
• באור אולטרה-סגול אפשר לראות את הסמל = In ultraviolet light, you can see the symbol

The original sentence is straightforward and perfectly fine.