An element or compound used to enhance a semiconductor is called a/an

When a small, carefully controlled bit of an impurity is introduced into a super-pure substance, the substance becomes a semiconductor.

The element or compound that was added to the pure substance to make it a semiconductor is called a "dopant".

I tried including some more sentences in the answer, to give you other forms of that word, like the ones that rhyme with 'rope', 'roped', and 'roping', but won't allow the use of the word !

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.