[edit] Introduction

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Holding a magnifying glass against a wall at the proper distance usually will provide a picture of the lit-up parts of the room. Note that that distance varies, depending on the distance of the object concerned from the lens. Objects that are far away will require a distance called the focal length, while objects that are closer to the lens require a longer distance for a sharp picture of that object on the wall.
Eye glasses restore proper sight to very many people, also eyepieces of optical instruments include lenses.
Photography has many uses

[edit] Refraction at a spherical interface