What was the active medium used in the first working laser ever constructed

To determine the active medium used in the first working laser ever constructed, we need to understand the concept of an active medium in a laser.

The active medium in a laser is the material that provides the energy necessary to create laser light. It is typically a solid, liquid, or gas that can be excited to produce a population inversion, which is the condition required for stimulated emission of photons.

Now, let's examine each option:

A. A diamond block: While diamond is a material that can be used as an active medium in certain types of lasers, it was not used in the first working laser ever constructed.

B. Carbon dioxide gas: Carbon dioxide gas was not used in the first working laser ever constructed.

C. Helium-neon gas: Helium-neon gas is commonly used in helium-neon lasers, but it was not used in the first working laser ever constructed.

D. A ruby rod: The correct answer is D. The first working laser ever constructed, known as the ruby laser, used a ruby rod as its active medium. The ruby rod was excited using a flashlamp, which created a population inversion and allowed for the production of laser light.

Therefore, the correct option is D. A ruby rod was the active medium used in the first working laser ever constructed.