mikroskopisch
microscopic
So small that it can only be made visible with a microscope.
Bakterien sind mikroskopisch kleine Lebewesen.
Bacteria are microscopic living organisms.

microscopic
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microscopic
So small that it can only be made visible with a microscope.
Bakterien sind mikroskopisch kleine Lebewesen.
Bacteria are microscopic living organisms.

The adjective 'mikroskopisch' (microscopic) is derived directly from the noun 'das Mikroskop' (the microscope). The ending '-isch' (-ic/-ical) is often used to form an adjective from a noun.
Für die Untersuchung ist eine mikroskopische Analyse notwendig.
A microscopic analysis is necessary for the examination.
The literal meaning is used in scientific contexts (e.g., with 'Zellen' (cells), 'Bakterien' (bacteria)). The figurative meaning refers to abstract things (e.g., a 'Chance' (chance), an 'Unterschied' (difference)).
Die Zelle ist ein mikroskopischer Baustein des Lebens.
The cell is a microscopic building block of life.
In the literal meaning, 'mikroskopisch' (microscopic) is very often combined with 'klein' (small) to form 'mikroskopisch klein' (microscopically small), in order to further emphasize the small size.
Staub besteht aus vielen mikroskopisch kleinen Partikeln.
Dust consists of many microscopically small particles.
In the figurative sense, 'mikroskopisch' (microscopic) is used to express that something is extremely small or insignificant. Typical nouns here are 'Chance' (chance), 'Anteil' (share), or 'Vorteil' (advantage).
Sein Anteil am Gewinn war leider nur mikroskopisch.
Unfortunately, his share of the profit was only microscopic.
small
'klein' and 'mikroskopisch' can both express that something has a very small size. 'klein' is the normal, general word and works for almost anything: a room, a mistake, or an animal. 'mikroskopisch' is much stronger. Literally, it means that something is so small that you can only see it with a microscope. Figuratively, people also use it when something is extremely small or hardly important. In everyday life, 'klein' is usually enough. 'mikroskopisch' fits when the tininess is especially emphasized or when talking about very small things in nature or science.

Unter dem Mikroskop sieht Selma mikroskopische Teilchen im Wasser.
Under the microscope, Selma sees microscopic particles in the water.
Sie nimmt nur eine kleine Tasche mit, weil sie heute wenig braucht.
She only takes a small bag with her because she doesn't need much today.