optisch
visual
Relating to the outer appearance; visual. It describes how something looks.
Optisch macht das neue Handy einen sehr guten Eindruck.
Visually, the new phone makes a very good impression.

visual / optical
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visual
Relating to the outer appearance; visual. It describes how something looks.
Optisch macht das neue Handy einen sehr guten Eindruck.
Visually, the new phone makes a very good impression.

The adjective 'optisch' (visual/optical) is directly connected with the noun 'die Optik' (optics/appearance). 'Die Optik' refers both to the appearance of something and to the scientific study of light.
Die Optik dieses neuen Autos gefällt mir sehr gut.
I really like the look of this new car.
When 'optisch' (visual) describes general appearance, it is about aesthetics. When it refers to seeing itself or to technical devices, it is about the function of the eye or physics.
Optisch ist die Wohnung sehr schön, aber sie ist unpraktisch.
Visually, the apartment is very nice, but it is impractical.
In the meaning 'relating to appearance', 'optisch' (visually) is often used like an adverb at the beginning or in the middle of a sentence to comment on the whole sentence or on an adjective.
Optisch hat sich hier in den letzten Jahren viel verändert.
Visually, a lot has changed here in recent years.
In the technical or scientific meaning, 'optisch' (optical) often appears with certain nouns. A very common and important collocation is 'optische Täuschung' (optical illusion).
Die Linien auf dem Bild scheinen krumm zu sein, aber das ist nur eine optische Täuschung.
The lines in the picture seem to be crooked, but that is only an optical illusion.
visible
'sichtbar' and 'optisch' both have to do with seeing. 'sichtbar' means that you can see something or that something is clearly recognizable. 'optisch' more often describes how something looks or that it relates to the area of vision. So you can say: a stain is 'sichtbar' if you can see it. A room seems 'optisch größer' (visually larger) if it only looks larger. The words can be confused because both are visual. Memory tip: 'sichtbar' asks whether you can see something; 'optisch' asks how it looks.

Der helle Teppich macht das Zimmer optisch größer.
The light-colored rug makes the room look visually larger.
Auf dem weißen Hemd ist der kleine Fleck sofort sichtbar.
The small stain is immediately visible on the white shirt.