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My daughter asked me to buy thin glasses for her eyes. So the power of the lenses are the same, but the glasses are thinner because the glasses use plastic with higher refraction index.

Latter I found out that she does that because she think thinner glasses will make her eyes look more normal rather than small.

I told her that's not possible.

She shows me this link

https://www.theoptical.co/see/why-do-my-eyes-look-smaller-with-glasses#:~:text=Choose%20The%20Right%20Material&text=Generally%20speaking%2C%20the%20thinner%20the,your%20eyes%20to%20look%20smaller

Generally speaking, the thinner the lens, the less minification caused. This doesn’t always mean that every prescription needs to upgrade to the thinnest lens possible, but the wrong eyeglass lens material can cause your eyes to look smaller.

I think she's wrong.

While thinner glasses may look more elegant because the glass itself is thin. For the same dioptri it will not change how your eyes look.

Which one is right?

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    $\begingroup$ Given that the 'look' is an overall impression of the face, and big thick glasses change the overall impression of the face, I'd say that yes, it changes your look. In my case the driving factor is my glasses' weight and balance. $\endgroup$
    – Jon Custer
    Commented Feb 7, 2023 at 14:35

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One thing that could play a role is the validity of the thin lens approximation - this likely guides the reasoning for why thick and thin lenses should produce the same effect, but it is less applicable to thick lenses. However, at the moment, I am not sure how one could estimate the importance of the effect.

Related: Lens Formula for thick lens

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  • $\begingroup$ The easiest way to convince you or your daughter is just let her try it out at a good optician. Even if we give a more physical explanation , she will not believe it. And take some friend of her along tu judge. $\endgroup$
    – trula
    Commented Feb 7, 2023 at 15:51
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Thinner lenses do have a slight effect on reducing the apparent enlarging or shrinking of the eyes behind the lenses, but not that much. Also, people with higher prescriptions are likely to opt for thinner glasses lenses for aesthetic or safety purposes.

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Your daughter might be partially right in that it is possible that due to high dioptre (I would say >6.0 is high), you will see a phenomenon called "facial displacement":

https://www.2020mag.com/article/avoiding-facial-displacement

The website states:

Today’s lighter weight, higher index materials coupled with our ability to produce aspheric, digital lenses have eased some of the problems of using large lenses. Yet neither selecting a different material nor using aspheric curvatures can correct the issue of facial displacement, which is primarily a problem for prescriptions in excess of about 6 diopters.

Also seen from this image, if you want a physical description of what is happening:

concave lens effect on how the face looks

From your description it seems that the displacement is "facial inset":

The best ways to reduce facial inset are to control the amount of lateral prism at the outer edge of the lens, and to reduce the distance from the back of the lens to the patient’s face. On strong minus lenses, you may achieve a somewhat better result by having the lab roll and polish the lens edges. This will also improve the appearance of the thick lateral edge of the lens.

Unfortunately there is nothing that physics can do. A good optometrist should be able to better shape plastic lenses (additionally benefiting from being lighter for high dioptres), but I don't wear glasses so I don't know if this is something that will result in more expensive prescription spectacles.

While not entirely unavoidable, it is possible that plastic lenses can be more comfortable, but the fact of the matter is that regardless of the optical material, facial displacement will occur at high dioptres.

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