Pre-AP Physics Lenses & Ray Diagrams

1. Pre-AP PhysicsLenses & Ray Diagrams Allen High School

2. 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!!

3. 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.

4. 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 Diagram 2. Equations (next PowerPoint) • Done correctly, both give you the right answer!

5. 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.

6. Ray Diagrams

7. Ray Diagrams • Where you put the object determines a lot about the image. • Images can be described with three classifications: • Real: Light really passes through the intersection.Virtual: Light does not pass through the intersection – you have to pretend it does. • Upright – Same vertical direction as object.Inverted – Opposite vertical direction as object. • Magnification: Larger (than object), smaller (than object), or same size (as the object).

8. 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!

9. 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.

10. Example 1 Use a Ruler!

11. Example 2