Most optical elements (mirrors, lenses) can be characterized by basically one quantity: their focal length. To understand focal length, we need to know something about the focal point. In the image above, note how the light rays, after they pass through the lens, converge on a single point. Below is a schematic drawing of the above situation. The single point to which the light rays are converging is called the focal point.
Tsk is a sound made by quickly releasing the tongue from the top of the mouth to express disapproval or criticism. An example of tsk is the sound a mother would make while shaking her finger at a child who's done something wrong. (a sound) used to express disapproval, genuine or mock sympathy, etc.: a click, or sucking sound, made by touching the tongue to the hard palate and rapidly withdrawing it.
adjective scheÂ·matÂ·ic \ski-Ëma-tik\. : showing the main parts of something usually in the form of a simple drawing or diagram.