If we position the key next to the subject so that it shines from the side then this affect is called side lighting effect.
According to the law of reflection of light, when light falls on a smooth and shiny surface it bounce back making the same angle with the normal on the plane which is made while falling on the surface.
As the key is made up of metal itself as the required Shiny and smooth surface. When the key is put next to the subject the light directly falls on one side of the key, that side of the key shines while the other side of the key is still in the dark.
This effect is called side lighting effect.
The side lighting effect is extensively used in professional photography.
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