die Optik
look
The visual appearance or look of something or someone.
Die moderne Optik des neuen Smartphones gefällt mir sehr gut.
I really like the modern look of the new smartphone.

look / optics
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look
The visual appearance or look of something or someone.
Die moderne Optik des neuen Smartphones gefällt mir sehr gut.
I really like the modern look of the new smartphone.

You can recognize the meaning of 'Optik' (look/optics) from the context. If it is about design and appearance, the general meaning is intended. In connection with physics or school, it refers to the science. With cameras or binoculars, it means the lens system.
Die sportliche Optik des Autos gefällt vielen jungen Leuten.
Many young people like the sporty look of the car.
When referring to appearance, 'Optik' (look) is often combined with adjectives such as 'modern' (modern), 'klassisch' (classic), or 'ansprechend' (appealing). You can also say 'auf die Optik achten' (to pay attention to the look).
Bei der Renovierung haben wir sehr auf eine moderne Optik geachtet.
During the renovation, we paid a lot of attention to a modern look.
The meanings that refer to physics and to lens systems are typical of technical contexts in science and technology.
Für das Experiment benötigen wir spezielle Kenntnisse der Optik.
For the experiment, we need special knowledge of optics.
design
'das Design' and 'die Optik' both have to do with how something looks. The important difference is: 'Optik' only means how something looks. 'Design' means deliberate design and can also include function. That is why you might say, for example, that a chair has a nice look, but with the design, the shape, seating comfort, and use are also important. Learners may confuse the words because both occur with clothing, devices, websites, or furniture. As a simple guide: 'Optik' is the visible impression; 'Design' is the planned design behind it.

Elif mag die Optik der Jacke, obwohl sie ihr etwas zu groß ist.
Elif likes the look of the jacket, even though it is a little too big for her.
Beim neuen Fahrrad ist das Design schön, aber auch sehr praktisch.
The design of the new bicycle is attractive, but also very practical.