Light travels at different speeds based on the medium it passes through. For example, in a vacuum, it travels at 3.0 × 10^8 m/s in a straight line. But when directed towards glass, air, diamond, water ...

Plano concave lens bend parallel input rays so they diverge from one another on the output side of the lens and hence have a negative focal length. The spherical aberration of the plano concave lenses ...