Ray Tracing and Image Formation Double Convex (Converging) Lens Double Concave (Diverging) Lens Double Convex (Converging) Lens Double Concave (Diverging) Lens Double Convex (Converging) Lens Double Concave (Diverging) Lens Ray tracing to predict image formed by lens: Focal Point Focal points of lenses are determined by focusing a parallel beam of light. Light rays from distant objects enter lenses in parallel beams. Converging Lens (Convex) Ray tracing using the law of refraction: focal point optical Axis light rays focal length Diverging Lens (Concave) optical Axis focal length Ray Tracing Model of Image Formation Ray Tracing Model predicts image location and orientation Need to know: ï lens focal length ï three principal light rays Double Convex Lens (converging lens) Ray 1. The ray parallel to the optical axis is bent by the lens to pass through f2: 1 Object f2 optical axis f1 Ray 2. The ray through the center of the lens continues in a straight line with no change in direction: 1 2 Object f2 optical axis f1 image Ray 3. The ray through f1 is bent by the lens to travel in a path parallel to the optical axis: 1 2 optical axis Object f2 f1 Image 3 The image of the point from the object will be at the intersection of the principal rays. Double Convex Lens: Image Formation WHAT DETERMINES THE KIND AND LOCATION OF AN IMAGE WITH A CONVEX LENS? f1 f2 f1 f2 f1 f2 Predicting Images Fresnel Lenses What is a Fresnel Lens? How Does a Fresnel Lens Magnify an Image? Fresnel Lenses are used to: - Magnify light - Cast parallel beams of light Light Houses -- Warning to ships at sea of dangerous coastal terrain. -- Produced powerful beams of light visible from tens of miles away. -- 1822-Augustin Fresnel (French physicist)--invented the large lens that produced a parallel beam of light rays --very efficiently! Fresnel lens in a Light House How does a Fresnel Lens produce a parallel beam of light? Where is the light source? Why does the lens have a ridged structure instead of a smooth surface? Comparison: Normal Lens vs Fresnel Lens All refraction occurs at interface Thick Converging Lens with Flat Side Air Glass no role in light refraction Fresnel lens all the glass needed to do the refraction Fresnel lenses use less glass or plastic than normal convex lenses, yet they serve the same func- tions. How a Fresnel Lens Magnifies Light FRESNEL LENS (magnifies-- with less glass) eye f1 IMAGE f2 OBJECT enlargement of central area object diverging rays eye How a Fresnel Lens is Used to Cast a Parallel Beam of Light FRESNEL LENS (produces a parallel light beam-- with less glass) Light Source at focal point f1 f2 enlargement of central area f1.