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Lenses Refraction • Light moves through different substances at different speeds EX light moves more slowly through water than air • When light passes from one material into another, it changes speed and bends • Refraction is the bending of light Refraction Video Refraction Video Types of Lenses Two Types: 1. Converging 2. Diverging Converging Lenses • A converging lens causes light rays that are parallel to the principal axis to converge(meet) at one point (i.e., the principal focus, F) lens Incident rays F Refracted rays • Optical Centre (O) centre of the lens • Principal Axis line through the centre – – optical • Principal Focus (F) – point on principal axis where light rays that are parallel to principal axis converge after refraction • Secondary Principal Focus (F’) – different • F is on same side of lens as incident rays • F’ is on opposite side as incident rays Converging Lens – Labeling the Parts Diverging Lenses • A diverging lens causes parallel light rays to diverge (spread apart). lens Incident rays Refracted rays • • • • same as Converging Lens: Optical Centre (O) Principal Axis Principal Focus (F) • Secondary Principal Focus (F’) – different • F is on same side of lens as incident rays • F’ is on opposite side as incident rays Diverging Lenses – Labeling the Parts The Rules for Locating Images in Lenses 1. A ray parallel to the principal axis is refracted through the principal focus (F). 2. A ray through the secondary principal focus (F’) is refracted parallel to the principal axis. 3. A ray through the optical centre (O) continues straight through without being refracted. NOTE: Any two rules can be used to determine the characteristics of an image through a lens. Showing the Rules for Converging Lenses Showing the Rules for Diverging Lenses Drawing Diverging Lens Diagrams 1. Draw a ray from the tip of the object parallel to the principal axis. When it reaches the centre of the lens, it appears to refract through the principal focus. 2. Draw a 2nd ray from the tip of the object through the optical centre (not refracted). 3. The VIRTUAL image appears where the rays cross.