Lesson 1: Reflection and its Importance The Role of Light to Sight The Line of Sight

1. Chapter 11: reflection and refraction Lesson 1: Reflection and its Importance The Role of Light to Sight The Line of Sight The Law of Reflection Specular vs. Diffuse Reflection • Lesson 2: Image Formation in Plane Mirrors • Image Characteristics for Plane Mirrors • What Portion of a Mirror is Required to View an Image? • Lesson 3: Refraction at a Boundary • Boundary Behavior • Refraction and Sight • The Cause of Refraction • Optical Density and Light Speed • The Direction of Bending • Lesson 4: The Mathematics of Refraction • Snell's Law • Ray Tracing and Problem-Solving • Determination of n Values • Lesson 5: Total Internal Reflection • Boundary Behavior Revisited • Total Internal Reflection • The Critical Angle

2. Lesson 1: Reflection and its Importance The Role of Light to Sight The Line of Sight The Law of Reflection Specular vs. Diffuse Reflection

3. The Role of Light to Sight • Without light, there would be no sight. • The objects which we see can be placed into one of two categories: luminous objects and illuminated objects. • Luminous objects are objects which generate their own light. • Illuminated objects are objects which are capable of reflecting light to our eyes. • The sun is an example of a luminous object, while the moon is an illuminated object.

4. The Line of Sight • The line of sight is a straight line between your eye and the object. In order to view an object, you must sight along a straight line at that object; and when you do, light will come from that object to your eye along the line of sight.

5. We use line of sight to determine image location • In order to locate the image, two conditions must be satisfied: • You must be able to see the image. One of the many rays of light from the object that approach the mirror and must reflect along your line of sight to your eye. • Since there is only one image for an object placed in front of a plane mirror, every sight line from different observing places would intersect in a single location. This location of intersection is known as the image location.

7. The image is positioned directly across the mirror along a line which runs perpendicular to the mirror. • The distance from the mirror to the object (known as the object distance) is equal to the distance from the mirror to the image (known as the image distance). For all plane mirrors, this equality holds true: • Object distance = Image distance fro

8. The Law of Reflection • The ray of light approaching the mirror is known as the incident ray (I). The ray of light which leaves the mirror is known as the reflected ray (R). • At the point of incidence where the ray strikes the mirror, a line can be drawn perpendicular to the surface of the mirror. This line is known as a normal line (N). The normal line divides the angle between the incident ray and the reflected ray into two equal angles. • The angle between the incident ray and the normal is known as the angle of incidence (θi). The angle between the reflected ray and the normal is known as the angle of reflection (θr). The law of reflection states that when a ray of light reflects off a surface, the angle of incidence is equal to the angle of reflection. θi = θr

9. example • Identify which angle is angle of incidence and which angle is angle of reflection. • Incident angle is ___ • Reflected angle is _____ B C A D

10. example • A ray of light is incident towards a plane mirror at an angle of 30-degrees with the mirror surface. What will be the angle of reflection?

11. example • A ray of light is approaching a set of three mirrors as shown in the diagram. The light ray is approaching the first mirror at an angle of 45-degrees with the mirror surface. Trace the path of the light ray as it bounces off the mirror. Continue tracing the ray until it finally exits from the mirror system. How many times will the ray reflect before it finally exits?

12. Specular vs. Diffuse Reflection • light reflects off surfaces in a very predictable manner - in accordance with the law of reflection. Once a normal to the surface at the point of incidence is drawn, the angle of incidence can then be determined. The light ray will then reflect in such a manner that the angle of incidence is equal to the angle of reflection. The Law of Reflection is Always Observed (regardless of the orientation of the surface)

13. Specular reflection: Reflection off of smooth surfaces such as mirrors or a calm body of water. • Diffuse reflection: Reflection off of rough surfaces such as clothing, paper, and the asphalt roadway. • Each individual ray obeys the laws of reflection.

14. Why Does a Rough Surface Diffuses A Beam of Light? • For each type of reflection, each individual ray follows the law of reflection. However, the roughness of the material means that each individual ray meets a surface which has a different orientation. The normal line at the point of incidence is different for different rays. Subsequently, when the individual rays reflect off the rough surface according to the law of reflection, they scatter in different directions. The result is that the rays of light are incident upon the surface in a concentrated bundle and are diffused upon reflection.

15. practice • A ray is incident on a plane mirror at 25 degrees (incident angle). Draw a diagram to indicate the • The surface of the mirror • The line of normal • The incident ray • The reflected ray

16. Lesson 2 - Image Formation in Plane Mirrors • Why is an Image Formed? • Image Characteristics in Plane Mirrors • Ray Diagrams for Plane Mirrors • What Portion of a Mirror is Required to View an Image?

17. Why is an image formed? • An image is formed because light gives off from an object in a variety of directions. Some of this light (which we represent by rays) reaches the mirror and reflects off the mirror according to the law of reflection. Each one of these rays of light can be extended backwards behind the mirror where they will all intersect at a point (the image point). Any person who is positioned along the line of one of these reflected rays can sight along the line and view the image - a representation of the object.

18. Image formation from plane mirrors ..\..\RealPlayer Downloads\07PPT03 Image characteristics in a plane mirror.flv

19. Image characteristics in a plane mirror • An image has the same size as the object. • The image is as far behind the mirror as the object is in front of the mirror. • The image has the same orientation as the object. • The image is laterally inverted. (left and right reversal) • The image is virtual, no actual light meet at the image position. Virtual image can not be captured on a screen.

20. the lateral inversion (left-right reversal).

21. example • If Suzie stands 3 feet in front of a plane mirror, how far from the person will her image be located?

22. example • If a toddler crawls towards a mirror at a rate of 0.25 m/s, then at what speed will the toddler and the toddler's image approach each other?

23. Ray diagrams 1. Draw the image of the object. 2. Pick one extreme on the image of the object and draw the reflected ray that will travel to the eye as it sights at this point. 3. Draw the incident ray for light traveling from the corresponding extreme on the object to the mirror. 4. Repeat steps 2 and 3 for another extreme on the object.

24. Practice – draw ray diagram

25. What Portion of a Mirror is Required? • Ray diagrams can be used to determine what portion of a plane mirror must be used in order to view an image. • In order to view his image, the man must look as low as his feet, and as high as the tip of his head. The man only needs the portion of mirror extending between points X and Y in order to view his entire image. All other portions of the mirror are useless to the task of this man viewing his own image. to view an image of yourself in a plane mirror, you will need an amount of mirror equal to one-half of your height.

26. example • In the diagram, a light ray leaves a light source and reflects from a plane mirror. At which point does the image of the source appear to be located? • A • B • C • D

27. example • When a ray of light strikes a mirror perpendicular to its surface, the angle of reflection is • 0° • 45° • 60° • 90°

28. example • Parallel light rays are incident on the surface of a plane mirror. Upon reflection from the mirror, the light rays will • converge • diverge • be parallel • be scattered

29. example • A plane mirror produces an image of an object. Compared to the object, the image appears • inverted and the same size • reversed and the same size • inverted and larger • reversed and larger

30. example • When a student looks into a plane mirror, she sees a virtual image of herself.  However, when she looks into a sheet of paper, no such image forms.  Which light phenomenon occurs at the surface of the paper? • regular reflection • diffuse reflection • polarization • resonance

31. example • In the diagram, a light ray, R,strikes the boundary of air and water. • Look at the graph and estimate the approximate degree of the angle of incidence. • 31° • 61° • 119° • 149°

32. example • A ray of light strikes a plane mirror at an angle of incidence equal to 45°.  What is the angle between the incident ray and the reflected ray?

33. Objective - Refraction at a Boundary • Refraction of Light Waves • Refraction and Sight • The Cause of Refraction • Optical Density and Light Speed • The Direction of Bending

34. Refraction of Light Waves • Refraction is a boundary behavior. When a wave reaches the end of one medium and encounters another medium, the transmitted wave undergoes refraction (or bending) if it approaches the boundary at an angle (obliquely). • When a beam of light approaches a boundary, it changes direction as it crosses the boundary separating two medium. • Light only bends when incident at an angle. Light enters the medium at an angle (obliquely)

35. The Ray Model of Light • We will rely on the use of rays to represent the direction in which light is moving. The ray is constructed in a direction perpendicular to the wave fronts of the light wave; this accurately depicts the light wave's direction. In this sense, we are viewing light as behaving as a stream of particles which head in the direction of the ray. The idea that the path of light can be represented by a ray is known as the ray model of light.

36. Refraction and Sight • Every object that can be seen is seen only because light from that object travels to our eyes. • When light passes from one medium into a second medium, the light path bends. Refraction takes place. When sighting at an object, light from that object changes media on the way to your eye, a visual distortion is likely to occur. This visual distortion is witnessed if you look at a pencil submerged in a glass half-filled with water.

37. Since refraction of light occurs when it crosses the boundary, visual distortions often occur. These distortions occur when light changes medium as it travels from the object to our eyes.

38. The refraction of light explains • mirages • visibility of the sun after it has actually disappeared below the horizon. • Image formed by lenses is refraction • http://www.freezeray.com/flashFiles/eyeDefects.htm

39. The Cause of Refraction • The transmission of light across a boundary between two media is accompanied by a changein both the speed and wavelength of the wave. • When light enters from denser to less dense (water to air), it speedsup. Since the frequency doesn’t change,the light has a longer wavelength. • When light enters from less dense to denser medium (air to water) it slows down andtransforms into a wave with a shorter wavelength. • The only time that a wave can be transmitted across a boundary, change its speed, and still not refract is when the light wave approaches the boundary in a direction which is perpendicular to it. • As long as the light wave changes speed and approaches the boundary at an angle, refraction is observed. Changing speed is the cause of the light wave changes directions at the boundary.

40. Conditions of Refraction • A light wave must enter the boundary at an angle (obliquely) in order to bend. A light wave will not undergo refraction if it approaches the boundary in a direction which is perpendicular to it.

41. example • The diagram shows a ray of light passing from air into glass at an angle of incidence of 0°. Which statement best describes the speed and direction of the light ray as it passes into the glass? • Only speed changes. • Only direction changes. • Both speed and direction change. • Neither speed nor direction changes

42. example • A change in the speed of a wave as it enters a new medium produces a change in • frequency • period • wavelength • phase

43. example • The diagram shows how an observer located at point P on Earth can see the Sun when it is below the observer's horizon.   This observation is possible because of the ability of the Earth's atmosphere to • reflect light • diffract light • refract light • polarize light

44. example • Which phenomenon of light accounts for the formation of images by a lens? • reflection • refraction • dispersion • polarization

45. example • Base your answer to the question on the diagram below, which represents a ray of monochromatic light (f = 5.09 ×1014 hertz) in air incident on flint glass. what happens to the light from the incident ray that is not refracted? • It is reflected and converted to mechanical energy. • It is reflected, absorbed and energy is decreased. • It is all destroyed. • It is slowed down, dissolved and reflected.

46. example • Refraction of a wave is caused by a change in the wave's • amplitude • frequency • phase • speed

47. example • The diagram represents wave fronts traveling from medium X into medium Y. All points on any one wave front shown must be • traveling with the same speed • traveling in the same medium • in phase • superposed

48. example • What happens to the frequency and the speed of an electromagnetic wave as it passes from air into glass? • The frequency decreases and the speed increases. • The frequency increases and the speed decreases. • The frequency remains the same and the speed increases. • The frequency remains the same and the speed decreases.

49. The Direction of Bending • Refraction is the bending of the path of a light wave as it passes from one material into another material. The refraction occurs at the boundary and is caused by a change in the speed and wavelength of the light wave upon crossing the boundary. • The tendency of a ray of light to bend one direction or another is dependent uponwhether the light wave speeds up or slows down upon crossing the boundary. • The speed of a light wave is dependent upon the optical density of the material through which it moves.

50. Light Traveling from a Fast (less dense) to a Slow (denser) Medium • If a ray of light passes across the boundary from a material in which it travels fast into a material in which travels slower, then the light ray will bend towards the normal line. Note: the incident ray and the refracted ray are on the opposite side of the normal line.