Ray Diagrams for Lenses

As with images formed by mirrors, it is convenient to locate the image of lenses by graphical methods. For the sake of simplicity, we consider the rays to bend at the plane through the center of the lens. For a converging lens, the principal rays are:

1.) The parallel ray, drawn parallel to the axis. This ray is bent through the second focal point of the lens.

2.) The central ray, drawn through the center (the vertex) of the lens. This ray is undeflected. (The faces of lens are parallel at this point, so the ray emerges in the same direction but displaced slightly. Since the lens is thin, the displacement is negligible.)

3.) The focal ray, drawn hrough the first focal point. This ray emerges parallel to the axis.

The principal rays for a diverging lens are:

1.) The parallel ray, drawn parallel to the axis. This ray diverges from the lens as if it came from the second focal point.

2.) The central ray, drawn through the center (the vertex) of the lens. This ray is undeflected.

3.) The focal ray, drawn toward the the first focal point. This ray emerges parallel to the axis.

Ray Diagrams for Mirrors

A useful method to locate images is by geometric construction of a ray diagram. By the judicious choice of rays from the head of the figure, we can quickly locate the image. There are three principal rays that are convevievt to use:

1.) The parallel ray, drawn parallel to the axis. This ray is reflected through the focal point.

2.) The focal ray, drawn through the focal point. This ray is reflected parallel to the axis.

3.) The radial ray, drawn through the center of curvature. This ray strikes the mirror perpendicular to its surface and is thus reflected back on itself.

The intersection of any two rays locates the image point of the head. The third ray can be used to provide a check. Ray diagrams are easier to draw if the mirror is replaced by a straight line that extends as far as necessary to intercept the rays.

Convex mirrors

The central ray heading toward the center of curvature C is perpendicular to the mirror and is reflected back on itself. The parallel ray is reflected as if it came from the focal point F behind the mirror. The focal ray is drawn toward the focal point and would be reflected parallel to the axis.

The Onslaught Starts... (Not again!)

Okay, maybe that was an overstatement. But what was once a lesson that seemed easy (waves) now grew into another whopper. Delving deeper into the intricacies of waves makes things more complicated. It is also infuriating. Infuriating for our mentor's part (Mr. Mendoza) because his students seem to have no clue on what to do on the equations. It is also infuriating for us because we sometimes really have no clue on what to do on the equations. But still I have to strive hard so I can fully understand our new undertaking in the complex world of Physics.

On Waves

At last, a topic in Physics I can fully comprehend. After some tedious endeavors on Physics, the concept of waves are easier to grasp, thanks partly to the instructional video Physics in Everyday Life. Previous lessons on Physics were just plain toilsome. Then came the lesson on the background of waves. In the activities involving waves, I fared very well and I hope that I would have consistently high grades on the coming lessons and lectures on Physics III.