PRE-AP PHYSICSLENSES & RAY DIAGRAMSAllen High School

THIN LENSES Mirrors reflect, Lenses Refract!!! Those of you with glasses or contacts are thankful for

lenses, because they redirect light rays to help you see better (shine light on your retina).

Lenses:

1. Converging Lens – converge light to a single point (Focal Point)

2. Diverging Lens – diverge light away from a single point.

We are only going to investigate Converging Lenses!!

CONVERGING LENS How to find the focal point? The focal point is defined as the intersection of parallel

rays (for example, the sun’s rays form a hotspot).

These parallel rays come from an infinite object distance.

Light rays can go through a lens from either side, therefore Converging Lenses have TWO focal points.

CONVERGING LENS

In physics, we use lenses and objects to produce an image. Where is the image?

Two methods to find an image:1. Ray Diagram2. Equations (next PowerPoint)

Done correctly, both give you the right answer!

RAY DIAGRAMS HOLT, 571

Light travels in all directions, however we are not going to draw every single light ray, only 3 Principle Rays.

“Rules”1. A parallel (to the axis) ray refracts through the focal point, F, on the other side.2. A ray that passes through the first focal

point, F, refracts parallel on the other side.3. A ray that passes through the center of the lens, C, and continues on the original path.

RAY DIAGRAMS

RAY DIAGRAMS

Where you put the object determines a lot about the image.

Images can be described with three classifications:

1. Real: Light really passes through the intersection.Virtual: Light does not pass through the

intersection – you have to pretend it does.

2. Upright – Same vertical direction as object.Inverted – Opposite vertical direction as object.

3. Magnification: Larger (than object), smaller (than object), or same size (as the object).

NEW TERMS

Object Distance (do): The distance measured from the lens (x=0) to the object’s location.

Image Distance (di): The distance measured from the lens (x=0) to the image’s location. Typically, images from converging lenses are on

the opposite side as the object. This is what light really wants to do—pass through a lens.

Therefore, If di is on the opposite side of lens = positive (+di)

If di is on the same side of lens = negative (- di)* Only one scenario for this!

NEW TERMS

Object Height (ho): “how tall” the object is. Upright = +ho

Inverted = -ho

Image Height (hi): “how tall” the image is (relative to the object). Upright = +hi

Inverted = -hi

At this point, we will look at the heights to determine the relative magnification.

EXAMPLE 1 Use a Ruler!

EXAMPLE 2