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Chapter 13 Light and Reflection

Chapter 13 Light and Reflection. Ms. Hanan Anabusi. 13-2 Flat Mirrors. Objectives: Distinguish between specular and diffuse reflection of light. Apply the law of reflection for flat mirrors. Describe the nature of images formed by flat mirrors. Reflection of light.

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Chapter 13 Light and Reflection

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  1. Chapter 13 Light and Reflection Ms. Hanan Anabusi

  2. 13-2 Flat Mirrors Objectives: • Distinguish between specular and diffuse reflection of light. • Apply the law of reflection for flat mirrors. • Describe the nature of images formed by flat mirrors.

  3. Reflection of light • Reflection – the turning back of an electromagnetic wave at the surface of a substance. • Most substances absorb at least some of the incoming light and reflect the rest. • A mirror reflects almost all of incoming light.

  4. Angle of incidence: the angle between a ray that strikes a surface and the normal to that surface at the point of contact Angle of reflection: the angle formed by the line normal to a surface and the direction in which a reflected ray moves.

  5. Laws of Reflection • According to the Laws of Reflection, angle of incidence = angle of reflection (θi = θr ) Incident light ray Reflected light ray Normal θi θr

  6. Notice the difference between a smooth and rough surface and how the light rays are reflected. • Angle of incidence (qi) and angle of reflection (qr) angles are equal.

  7. Specular and Diffuse Reflection

  8. Notation for Mirrors and Lenses • The object distance is the distance from the object to the mirror or lens • Denoted by p • The image distance is the distance from the image to the mirror or lens • Denoted by q • The lateral magnification of the mirror or lens is the ratio of the image height to the object height • Denoted by M

  9. Images • Images are always located by extending diverging rays back to a point at which they intersect • Images are located either at a point from which the rays of light actually diverge or at a point from which they appear to diverge

  10. Types of Images • A real image is formed when light rays pass through and diverge from the image point • Real images can be displayed on screens • A virtualimage is formed when light rays do not pass through the image point but only appear to diverge from that point • Virtual images cannot be displayed on screens

  11. Images Formed by Flat Mirrors • Simplest possible mirror • Light rays leave the source and are reflected from the mirror • Point I is called the image of the object at point O • The image is virtual

  12. Images Formed by Flat Mirrors, 2 • A flat mirror always produces a virtual image • Geometry can be used to determine the properties of the image • There are an infinite number of choices of direction in which light rays could leave each point on the object • Two rays are needed to determine where an image is formed

  13. Images Formed by Flat Mirrors, 3 • One ray starts at point P, travels to Q and reflects back on itself • Another ray follows the path PR and reflects according to the Law of Reflection • The triangles PQR and P’QR are congruent

  14. Images Formed by Flat Mirrors, 4 • To observe the image, the observer would trace back the two reflected rays to P' • Point P' is the point where the rays appear to have originated • The image formed by an object placed in front of a flat mirror is as far behind the mirror as the object is in front of the mirror • p = |q|

  15. Lateral Magnification • Lateral magnification, M, is defined as • This is the general magnification for any type of mirror • It is also valid for images formed by lenses • Magnification does not always mean bigger, the size can either increase or decrease • M can be less than or greater than 1

  16. Lateral Magnification of a Flat Mirror • The lateral magnification of a flat mirror is 1 • This means that h' = h for all images

  17. Reversals in a Flat Mirror • A flat mirror produces an image that has an apparent left-right reversal • For example, if you raise your right hand the image you see raises its left hand

  18. Reversals, cont • The reversal is not actually a left-right reversal • The reversal is actually a front-back reversal • It is caused by the light rays going forward toward the mirror and then reflecting back from it

  19. Properties of the Image Formed by a Flat Mirror – Summary • The image is as far behind the mirror as the object is in front • p = |q| • The image is unmagnified • The image height is the same as the object height • h' = h and M = 1 • The image is virtual • The image is upright • It has the same orientation as the object • There is a front-back reversal in the image

  20. Ray Diagrams - are diagrams that use simple geometry to locate an image formed by a mirror. • Let’s try to draw a ray diagram for a pencil in front of a flat mirror. Draw p and q. Let’s use the tip of the pencil. • 1st draw a ray from the tip of the pencil perpendicular to the mirror. • The angle is 0 with the normal, so qior q’ is 0.

  21. 2nd draw a ray that is not perpendicular with the mirror. • Draw in the incidence, and the reflected ray. • Remember qi =qr . • Now use dotted lines to trace both rays behind the mirror. Where these dotted lines meet is the image point, which in this case is where the image of the pencil’s tip forms.

  22. Flat Mirrors Can you explain this? • To view the entire body of a person, a mirror that is half the person’s height is all that is needed.

  23. qr qi Flat Mirror • All you see is what reaches your eyes • You thinkobject’s location is where rays appear to come from. Smooth Mirror All rays originating from peak will appear to come from same point behind mirror! Image Object

  24. qr qi d Flat Mirror (1) Draw first ray perpendicular to mirror 0 = qi = qr (2) Draw second ray at angle. qi = qr (3) Lines appear to intersect a distance d behind mirror. This is the image location. Example Light rays don’t really converge there, so it’s a “Virtual Image” Virtual: No light actually gets here d

  25. Flat Mirror Summary • Image appears: • Upright • Same size • Located same distance from, but behind, mirror • Facing opposite direction: Left/Right inverted • Virtual Image:Light rays don’t actually intersect at image location. • Why do ambulances have “AMBULANCE” written backwards?

  26. Assignments • Class-work: Section Review, page 454, odd questions. • Homework: Section Review, page 454, even questions. http://homepages.ius.edu/kforinas/W/Reflection1.html

  27. Activity 1

  28. Activity 2

  29. Diagram Skills – Flat Mirrors θi = θr = 0 θi = θr = 15 deg θi = θr = 32 deg θi = θr = 32 deg θi = θr = 43 deg Mirror front Mirror front A B C

  30. Diagram Skills – Flat Mirrors θi = 0 θi = 15 deg θi = 32 deg θi = 43 deg θi = 32 deg Mirror front Mirror front A B C

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