Unit 10 Reflection. Grading: Show all work, keeping it neat and organized. Show equations used and include all units.

Transcription

1 Name: Hr: Unit 0 Relection Grading: Show all work, keeping it neat and organized. Show equations used and include all units. REFLECTION Vocabulary Relection: The bouncing o light. The angle a beam o light makes when it strikes a surace is described with respect to the normal, an imaginary line drawn perpendicular to the surace. When light shines onto a mirror, the angle at which the light enters the mirror (angle o incidence) is exactly equal to the angle at which the light leaves the mirror (angle o relection). This is called the law o relection and is easily observed in a plane (lat) mirror. Due to the curvature o a spherical or parabolic mirror, light relected rom its surace behaves somewhat dierently than it does when relected rom a plane mirror. There are two types o spherical mirrors: concave (or converging) and convex (or diverging). Concave Convex The ollowing terminology is used when describing how light is relected rom concave and convex mirrors. Vocabulary Object distance: The distance rom the mirror to the object. This value is always a positive number. Vocabulary Image distance: The distance rom the mirror to the image. An image can be real (able to be projected on a screen), or virtual (not able to be projected on a screen). Vocabulary Focal point: The point where parallel rays meet (or appear to meet) ater relecting rom a mirror. The distance rom this ocal point to the mirror is called the ocal length. The ocal length o a concave mirror always has a positive value while the ocal length o a convex mirror always has a negative value. Vocabulary Mirror Equation: = + ocal length object distance image distance = d o + d i Note: Many situations involving mirrors can also be solved using ray diagrams. CONCAVE (CONVERGING) MIRRORS I an object is located more than one ocal length rom a concave mirror as shown in igure A, the image it orms is real, inverted, and in ront o the mirror. You can actually project this image onto a piece o paper. Both d o and d i have positive values. I the object is at the ocal point as in igure B, no image is ormed because the relected rays are parallel.

2 I an object is located less than one ocal length rom a concave mirror as in igure C, the image it orms is virtual, upright, enlarged, and behind the mirror. In other words, you must look into the mirror to see the image. Here, d o has a positive value and d i has a negative value. Figure A Figure B Figure C CONVEX (DIVERGING) MIRRORS The image ormed by a convex mirror is always virtual, upright, smaller, and behind the mirror. The image can be seen only by looking into the mirror. Here d o has a positive value while d i has a negative value. Solved Examples Example : Sitting in her parlor one night, Gerty sees the relection o her cat, Whiskers, in the living room window. I the image o Whiskers makes an angle o 40 with the normal, at what angle does Gerty see him relected? Solution: Because the angle o incidence equals the angle o relection, Gerty must see her cat relected at an angle o 40. Example 2: Wendy, a ortune teller, is polishing her crystal ball. It is so shiny that she can see her relection when she gazes into it rom a distance o 5 cm. a) What is the ocal length o Wendy s crystal ball i she can see her relection 4.0 cm behind the surace o the crystal ball? b) Is this image real or virtual? a) Given: d o = 5 cm d i = -4.0 cm Unknown: =? Original Equation: Solve: = + d o d i = + = d o d i + 5 cm -4.0 cm Getting a common denominator o 60 cm gives = cm 60 cm = - 60 cm To ind, take the reciprocal o this sum. = -60 cm = -5.5 cm. The minus sign beore the answer means that this is the ocal length o a convex mirror. b) The image seen behind a curved surace is always a virtual image.

3 Example 3: With his ace 6.0 cm rom his empty water bowl, Spot sees his relection 2 cm behind the bowl and jumps back. a) What is the ocal length o the bowl? b) What was surprising about Spots s relection that may have caused him to jump? a) Given: d o = 6 cm d i = -2.0 cm Unknown: =? Original Equation: Solve: = + d o d i = + = d o d i + 6 cm -2 cm Getting a common denominator o 2 cm gives = 2-2 cm 2 cm = 2 cm To ind, take the reciprocal o this sum. = 2 cm. The positive answer means that the bowl was acting as a concave mirror. b) The surprising thing Spot noticed about his relection was that it appeared larger than lie! Exercises Draw a sketch or each exercise! Exercise : a) I you are standing.5 meters away rom a plane mirror, how ar rom you is your image? b) What is the ocal length o a plane mirror?

4 Exercise 2: Ivan is in a house o mirrors with one o his riends when he comes to two mirrors situated at an angle o 90. Ivan stands so that light shining on his ace is incident on one mirror at an angle o 50, as shown. At what angle will this light relect rom the second mirror? Draw the rays on the diagram, and explain your reasoning below. Exercise 3: A popular lawn ornament in the 960s was a colored relecting sphere that sat in the yard as a decoration. a) I a bird is 0.0 cm rom a blue relecting sphere and sees its image relected 5.0 cm behind the surace o the sphere, what is the ocal length o the spherical relector? b) Sketch a ray diagram. c) Would the bird s image appear larger or smaller than the bird itsel?

5 Exercise 4: Polly applies her mascara while looking in a concave mirror whose ocal length is 8 cm. She looks into it rom a distance o 2 cm. a) How ar is Polly s image rom the mirror? b) Sketch a ray diagram. c) Does it matter whether or not Polly s ace is closer or arther than one ocal length? Explain. Exercise 5: A riend is wearing a pair o mirrored sunglasses whose convex surace has a ocal length o 20.0 cm. a) I your ace is 40.0 cm rom the sunglasses, how ar behind the sunglasses is your image? b) Sketch a ray diagram.

6 Exercise 6: a) Where must an object be placed with respect to a concave mirror with a ocal length o meter in order or its image to be ocused on a screen 6 meters rom the mirror? b) What type o image is ormed? (Real or virtual? Upright or inverted?) c) Sketch a ray diagram.