Ciro Santilli @cirosantilli 37

Alice and Bob communicate to one another their randomly chosen measurement bases over the unencrypted classic channel.

This channel must be authenticated to prevent man-in-the-middle. The only way to do this authentication that makes sense is to use a pre-shared key to create message authentication codes. Using public-key cryptography for a digital signature would be pointless, since the only advantage of QKD is to avoid using public-key cryptography in the first place.

if there is an eavesdropper on the line, the results of measurements on the same basis can differ.

Unfortunately, this can also happen due to imperfections in the system.

Alice and Bob must decide what level of error is above the system's imperfections and implies that an attacker is listening.

converges parallel light to a point at center at distance $f$ known as the focal length.

This is for example why you can use lenses to burn things with Sun rays, which are basically parallel.

Conversely, if the input is a point light source at the focal length, it gets converted into parallel light.

image formation: it converges all rays coming from a given source point to a single point image. This amplifies the signal, and forms an image at a plane.

The source image can be far away, and the virtual image can be close to the lens. This is exactly what we need for a camera.

For each distance on one side, it only works for another distance on the other side. So when we set the distance between the lens and the detector, this sets the distance of the source object, i.e. the focus. The equation is:where $S_1$ and $S_2$ are the two distances.