Learn Portuguese (Portugal): É no laboratório onde o físico trabalha. - It is in the laboratory where the physicist works.

This is a cleft sentence used to emphasize exactly where the action happens. By placing no laboratório between É and onde (It is ... where), you highlight the location, often to clarify or correct someone (for example, emphasizing he works in the lab, not in a classroom).

Why is it no laboratório and not just o laboratório after É?

In Portuguese emphatic structures, the focused part must keep the preposition it would need in a normal, flat sentence. Because you work in a place (trabalhar em), the preposition em merges with the article o to become no, and it must stay attached to the noun even when moved to the front for emphasis.

Could I use que instead of onde in this sentence?

Yes, you absolutely can. In everyday Portuguese, it is very common to hear É no laboratório que o físico trabalha. Both are correct and mean exactly the same thing, but using onde is a great way to explicitly highlight that you are talking about a physical location.

How do I know físico means 'physicist' here and not 'physical'?

While físico is indeed the adjective for 'physical', grammatically it is acting as a noun here because it is directly preceded by the definite article o (the). The context of someone working in a laboratory also confirms it refers to the profession.