Lecture 31 Chapter 28 Reflection Midterm 2 – Monday Nov. 15 6-Nov-10 Reflection • We say light is reflected when it is returned into the medium from which it came—the process is reflection. • When light illuminates a material, electrons in the atoms of the material move more energetically in response to the oscillating electric fields of the illuminating light. • The energized electrons re-emit the light by which you see the material. Specular and Diffuse Reflection Reflection from most surfaces is diffuse, with incoming light scattered in all directions. Reflection from smooth, mirror surfaces is specular, with light reflecting in one direction. Mirror 6-Nov-10 Diffuse reflection Specular reflection Check Yourself What objects in this room radiate light? Is the projector screen radiating visible light? Is the screen diffuse or specular reflector? 6-Nov-10 Diffuse Surface is Not a Mirror Light rays diffusely scatter in every direction from every spot on the surface. Look at this spot on a diffuse surface See a light ray from the top of your head reflected from this spot. Also see light ray from the tip of your nose reflected from this spot. 6-Nov-10 Law of Reflection CHECK YOURSELF Diffuse reflection occurs when the sizes of surface irregularities are A. small compared with the wavelength of reflected radiation. B. large compared with the wavelength of reflected radiation. C. Both A and B. D. None of the above. Law of Reflection CHECK YOUR ANSWER Diffuse reflection occurs when the sizes of surface irregularities are A. small compared with the wavelength of reflected radiation. B. large compared with the wavelength of reflected radiation. C. Both A and B. D. None of the above. Explanation: Diffuse reflection occurs for rougher surfaces. Transparent, Reflecting Surfaces Water and glass are transparent but reflect some of the light striking their surface. Can see some of the Can see inside the cab through the right riverbed through the window. Strong reflection of the sun on water6-Nov-10 surface. thePhysics left 1 (Garcia) window. SJSU One Way Mirror One way mirror is just clear glass window. Bright Room Dark Room Observation Discussion GLASS Room (Bright) Room (Dark) Reflected light from bright room masks transmitted light from the 6-Nov-10 darkened observation room Angle of Reflection • Angle of incidence – Angle made by the incoming ray and the perpendicular • Angle of reflection – Angle made by the reflected ray and the perpendicular •Normal – Imaginary line perpendicular to the plane of the reflecting surface – Lies in the same plane as the incident and reflected rays Law of Reflection Angle of incidence equals angle of reflection SPECULAR REFLECTION 6-Nov-10 Physics 1 (Garcia) SJSU Law of Reflection CHECK YOURSELF The law of reflection applies to A. light. B. sound. C. Both A and B. D. None of the above. Law of Reflection CHECK YOUR ANSWER The law of reflection applies to A. light. B. sound. C. Both A and B. D. None of the above. Ray Tracing Light ray from point A is reflected at point B and arrives at point B. Notice the relationship between point B (where the ray is reflected to) and point B’ (where the ray would go if the mirror wasn’t there). 6-Nov-10 Mirror Image Reflection To observer at point B, the light from point A seems to come from point A’, within mirror. 6-Nov-10 Law of Reflection Virtual image • is same size as object, formed behind a mirror, and located at the position where the extended reflected rays converge. • is as far behind the mirror as the object is in front of the mirror. Plane (Flat) Mirror Tracing light rays from original, to mirror, to eye allows us to construct image. HAIR NOSE CHIN Original Image Mirror 6-Nov-10 Demo: Pocket Mirror See full image of your head in a plane mirror that’s half the size of your head. Mirror Original Image 6-Nov-10 Law of Reflection Plane mirror • Note: the only axis reversed in an image is the front-back axis. Image in the Mirror Image is seen an equal distance from original within a plane mirror. Velazquez, Venus at her Mirror Utamaro, Woman Powdering Her Neck Incorrect Incorrect Helmut Newton, Bergstrom over Paris Correct 6-Nov-10 Physics 1 (Garcia) SJSU Law of Reflection CHECK YOURSELF Light reflecting from a smooth surface undergoes a change in A. frequency. B. speed. C. wavelength. D. None of the above. Law of Reflection CHECK YOUR ANSWER Light reflecting from a smooth surface undergoes a change in A. frequency. B. speed. C. wavelength. D. None of the above. Non-Flat Mirrors Image in a curved mirror is distorted from original. Determine image location and size by ray tracing 6-Nov-10 Physics 1 (Garcia) SJSU Law of Reflection Shape of mirror forms a different virtual image. • Convex mirror (that curves outward): virtual image is smaller and closer to the mirror than the object. • Concave mirror (that curves inward): virtual image is larger and farther away than the object. Demo: Convex Mirrors Mirrored surface on a sphere or similar rounded shape. Image is smaller & closer. MC Escher Hand with Reflecting 6-Nov-10 Sphere Ray Tracing for Convex Mirror Image in convex mirror is smaller & closer. Image Original Mirror 6-Nov-10 Demo: Concave Mirrors Mirror is on the inner curved surface of sphere or similar rounded shape. Image size and location depends on object’s distance from the mirror. Telescope 6-Nov-10 Ray Tracing for Concave Mirror Image in concave mirror is larger when original is close to the mirror. Original Mirror Image 6-Nov-10 Ray Tracing for Concave Mirror Image in a concave mirror is inverted if the original is far away from the mirror. Image is formed in front of the mirror, not within it. Image Original Mirror 6-Nov-10 Reflecting Telescopes Large, modern telescopes form images using concave mirrors. Magellan Telescope 6-Nov-10Hubble’s mirror KeyKey PointsPoints ofof LectureLecture 3131 • Specular and Diffuse Reflection • Law of Reflection • Plane Mirror • Ray Tracing • Real and Virtual Images • Curved Mirrors z Before Wednesday, read Hewitt Chap. 28 (first half). z Homework #22 due by 11:00 PM Tuesday Nov. 9. z Homework #23 due by 11:00 PM Friday Nov. 12.