CHAPTER 29: REFLECTION

Transcription

1 CHAPTER 29: REFLECTION

2 29.1 REFLECTION The return of a wave back to its original medium is called reflection. Fasten a spring to a wall and send a pulse along the spring s length. The wall is a very rigid medium compared with the spring, so all the wave energy is reflected back along the spring. Less rigid material like glass, when light strikes it about 4% is reflected, the rest is transmitted. Waves that travel along the spring are almost totally reflected at the wall.

3 29.1 Reflection What happens when a wave reaches a boundary between two media? Answer: When a wave reaches a boundary between two media, usually some or all of the wave bounces back into the first medium.

4 29.2 The Law of Reflection Incident rays and reflected rays make equal angles with a line perpendicular to the surface, called the normal. The angle between the incident ray and the normal is the angle of incidence. The angle between the reflected ray and the normal is the angle of reflection. Angle of incidence = Angle of reflection The incident ray, the normal, and the reflected ray all lie in the same plane.

5 think! If you look at your blue shirt in a mirror, what is the color of its image? What does this tell you about the frequency of light incident upon a mirror compared with the frequency of the light after it is reflected? Answer: 29.2 The Law of Reflection The color of the image will be the same as the color of the object because the frequency of light is not changed by reflection.

6 29.3 Mirrors If a candle flame is placed in front of a plane (flat) mirror, rays of light from the candle are reflected from the mirror You perceive the candle flame to be located behind the mirror. A virtual image appears to be in a location where light does not really reach. Mirrors produce only virtual images. Your eye cannot ordinarily tell the difference between an object and its virtual image.

7 IMAGE POSITION You think in straight lines! The image is located behind the mirror. Your eye cannot ordinarily tell the difference between an object and its virtual image. Virtual Images are images which cannot be visually projected on a screen. VIRTUAL IMAGES are ALWAYS on the OPPOSITE side of the mirror relative to the object.

8 29.3 Mirrors plane mirrors produce images that are: Virtual and upright left-right reversed the same distance from the mirror as the object's distance the same size as the object.

9 QUESTION oyou might have noticed that emergency vehicles such as ambulances are often labeled on the front hood with reversed lettering (e.g., ECNALUBMA). Explain why this is so. oanswer: Most drivers will view the ambulance in their rear-view mirrors. As such, they will be viewing an image of the lettering. Such images appear with left-right reversal and so will be viewed with the proper orientation - AMBULANCE.

10 29.4 DIFFUSE VS SPECULAR REFLECTION Diffuse Reflection Light incident upon a rough surface (paper) Law of reflection still holds; Normals are not parallel. Specular Reflection Mirror (Polished) like reflection All Normals are parallel A surface may be polished for long wavelengths, but not polished for short wavelengths.

11 Wet Road Glare Slide 11 Driving at night on a wet roadway results in an annoying glare from oncoming headlights.

12 Spherical Mirrors Concave & Convex Also called DIVERGING mirror Also called CONVERGING mirror

13 Converging (Concave) Mirror Reflection o o o A converging mirror is one that is spherical in nature by which it can FOCUS parallel light rays to a point directly in front of its surface. We call this point the FOCAL POINT. Real Images are ones you can project onto a screen that is in front of the mirror. Light from an infinite distance, most likely the sun.

14 Converging (Concave) Mirror Two rules of reflection for concave mirrors. Any incident ray traveling parallel to the principal axis on the way to the mirror will pass through the focal point upon reflection. Any incident ray passing through the focal point on the way to the mirror will travel parallel to the principal axis upon reflection.

15 Converging (Concave) Mirror Since the mirror is spherical it technically has a CENTER OF CURVATURE, C. The focal point happens to be HALF this distance. C f 2 We also draw a line through the center of the mirror and call it the PRINCIPAL AXIS. C 2 f

16 Ray Diagram A ray diagram is a pictorial representation of how the light travels to form an image and can tell you the characteristics of the image. object C f Principal axis Rule One: Draw a ray, starting from the top of the object, parallel to the principal axis and then through f after reflection.

17 Ray Diagrams object C f Principal axis Rule Two: Draw a ray, starting from the top of the object, through the focal point, then parallel to the principal axis after reflection.

18 Ray Diagrams object C f Principal axis Rule Three: Draw a ray, starting from the top of the object, through C, then back upon itself. What do you notice about the three lines? THEY INTERSECT The intersection is the location of the image.

19 Ray Diagram Image Characteristics object C f Principal axis After getting the intersection, draw an arrow down from the principal axis to the point of intersection. Then ask yourself these questions: 1) Is the image on the SAME or OPPOSITE side of the mirror as the object? Same, therefore it is a REAL IMAGE. 2) Is the image ENLARGED or REDUCED? 3) Is the image INVERTED or RIGHT SIDE UP?

20 Real Images

21 Virtual Image

22 No Image