How a thin prism brings two images together
Double vision can happen when the eyes are aimed at slightly different places. Each eye then sends the brain a view from a different angle, and the images may not merge into one. A Fresnel prism helps by changing the direction of incoming light: it shifts the image seen through the affected lens toward the other one.
A Fresnel prism is a thin, flexible sheet with many tiny ridges. Those ridges bend light much like a thicker, curved prism would, but without adding much weight or bulk to the glasses. The sheet is usually stuck to the lens rather than built into it, which makes the correction easy to adjust.
Why use a removable sticker?
The amount and direction of correction depend on how the eyes are misaligned, and that can change. A clinician can try a prism on the existing glasses, then alter its strength or placement if the first setting does not bring the images together comfortably. This offers a way to test symptom relief without immediately replacing the lenses or choosing a permanent procedure.
The tradeoff is that the ridged film can blur or scatter some light, so vision through it may look less crisp than through a smooth lens. It also shifts the image rather than changing the eye muscles themselves. The prism can make two views easier for the brain to combine, while leaving the underlying cause of the misalignment untouched.
