reflex, reflects

Reflex or reflex ( RE flex ) means an involuntary automatic or physical response; a reaction--
A reflex is an involuntary movement, as when the leg kicks if the kneecap is struck.
Ducking when something moves toward you may be a reflex action.

Reflects or reflects ( re FLECTS ) is the third person singular ( he, she, it ) form of "reflect", meaning to display an image of, as in a mirror; to give back light, heat, or an image, literally or figuratively--
Notice how the mirror reflects the light from the camera's flash.
His liberalism reflects his upbringing; his parents were Roosevelt supporters.
Reflects may also be the third person singular form of "reflect", meaning to meditate, or to think seriously about something--
These days he just sits on the porch and reflects on the state of the world.

Now that you know that, you can say--
"When the mirror reflects an object thrown at it, will a person viewing it step aside as a reflex action?"

Note--"reflex", in a technical or scientific context, may mean something sent back, as light, current, or heat--
He has an old single lens reflex camera.