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I Wear Glasses

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...did you not realize that you get glasses and wear them without corrective lenses? There are people who, for a living, play pretend, and they use alterations to "create a character", and glasses are one prop which is sometimes desired or required, even for those without them. 😉😆

If you want to understand lenses, you could look at the optics section of, say, the older (but more information-packed) Physics undergraduate text by Halliday & Resnick (it is in Part Two of the 2-part version), or the newer Fundamentals of Physics by Halliday, Resnick & Walker.

Or just look up the "thin lenses" or the lense maker's equation, which relates the focal length of a lense to the location of the object & image. There is not much to it, but it shows you how real & virtual images work (real ones are those formed by light going through the image, such as images you see through eyeglasses; virtual ones do not, such as, e.g., the image you see in a mirror, which is not actually formed "back there", feet behind the plane of the mirror).

The equation is

1/i + 1/o = 1/f

where o, i are the distances to the 'o'bject and 'i'mage, respectively, and f is the focal length of the lense.

For a mirror, f -> infinity, so

1/i + 1/o = 0, or

1/i = -1/o, so

o = -i

Since the object distance is real (as you stand in front of the mirror, light actually DOES bounce off you before going toward the mirror),

o > 0

so the image distance is

i < 0

and the image is virtual (because no actual light is passing through the image of you which looks like it is "inside" the mirror).