1. Light and Reflection Chapter 14

2. Characteristics of Light

3. Electromagnetic Waves Light is made of electromagnetic waves. A prism will break up a portion of the spectrum, white light, into a rainbow like band of colors. The colors that appear represent the visible spectrum. Red, orange, yellow, green, blue, indigo and violet. ROY G BIVROY G BIV

5. Electromagnetic Waves The spectrum also includes non-visible electromagnetic waves, such as x-rays, microwaves, radio waves, and radiation. Because they all are electromagnetic waves they all have similar properties.

6. Uses of Electromagnetic Waves Radio waves – AM/FM radio and some television stations Microwaves – Radar, aircraft navigation Infrared – physical therapy, IR photography Visible light – photography, microscopy, astronomy Ultraviolet – sterilization of medical instruments X rays – medical examination of bone/teeth/vital organs, some cancer treatments Gamma rays – examination of thick materials for structural flaws, food irradiation, some cancer treatments

7. Electromagnetic waves are transverse waves consisting of oscillating electric and magnetic fields at right angles to each other. Oscillate: to have a periodic vibration

8. Electromagnetic Waves Electromagnetic waves vary depending on frequency and wavelength BUT, all electromagnetic waves move at the speed of light We use 3.00 X 10 8 m/s as the speed of light, c. The wave speed equation is: c = fλ Speed of light = frequency X wavelength

9. Sample Problem The AM radio band extends from 5.4 X 10 5 Hz to 1.7 X 10 6 Hz. What are the longest and shortest wavelengths in this frequency range? f 1 = 5.4 x 10 5 Hz f 2 = 1.7 x 10 6 Hz c = 3.0 x 10 8 m/s c = f = c/ f 1 = 5.6 x 10 2 m 2 = 1.8 x 10 2 m

10. Light travels in straight lines. Light travels in straight lines, but will separate and lose brightness as it moves away from its source. However, a laser is a concentrated form of light and does not separate as much as other light waves. Laser

11. Brightness The brightness of light is inversely proportional to the square of the distance from the light source. Ex. If you move twice as far away from the light source, ¼ as much light falls on the book.

12. Flat mirrors Section 14.2

13. Reflection of Light Reflection – the change in direction of an electromagnetic wave at the surface of a substance Most substances absorb at least some of the incoming light and reflect the rest The texture of an object will affect how the light is reflected

14. Clear vs. Diffuse Reflection Specular reflection: light reflected from smooth shiny surfaces In specular reflection the incoming and reflected angles are equal (  =  ’)‏ Diffuse reflection: light is reflected from a rough textured surface

15. Reflection of Light 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 Normal is a line perpendicular to the reflection surface.

16. Reflection from a mirror: Incident ray Normal Reflected ray Angle of incidence Angle of reflection Mirror

17. The Law of Reflection Angle of incidence = Angle of reflection In other words, light gets reflected from a surface at THE SAME ANGLE it hits it. The same !!!

18. Drawing a Reflected Image Use ray diagrams to show image location We will find the virtual image (the image formed by light rays that only appear to intersect)‏

20. Drawing a Reflected Image Draw the object in front of the mirror Draw a ray perpendicular to the mirror’s surface. Because this is 0  from normal, the angle is the same from the mirror to the virtual object Draw a second ray that is not perpendicular to the mirror’s surface from the same point to the surface of the mirror. Next, trace both reflected rays back to the point from which they appear to have originated, that is, behind the mirror. Use dotted lines when drawing lines that that appear to emerge from behind the mirror. The point at which the dotted lines meet is the image point.

21. Flat Mirrors Image is VIRTUAL, UPRIGHT, UNMAGNIFIED

22. Concave Spherical Mirrors A spherical mirror has the shape of part of a sphere’s surface. The images formed are different than those of flat mirrors. Concave Spherical Mirror – an inwardly curved, mirrored surface that is a portion of a sphere and that converges incoming light rays.

23. Concave Spherical Mirrors The light bulb is distance p away from the center of the curvature, C. Light rays leave the light bulb, reflect from the mirror and converge at distance q in front of the mirror. Because the reflected light rays pass through the image point, the image forms in front of the mirror.

24. Concave Spherical Mirrors If you were to place a sheet of paper at the image point, you would see a clear, focused image of the light bulb (a real image). If the paper was placed in front of or behind the image point, the image would be unfocused.

25. Concave Spherical Mirrors Real image – an image formed when rays of light actually intersect at a single point Focal length – equal to half the radius of curvature of the mirror.

26. Concave Spherical Mirrors Mirror equation: 1/p + 1/q = 2/R 1 + 1 = 2. Object distance Image distance radius of curvature Or: 1/p + 1/q = 1/f 1 + 1 = 1. Object distance Image distance focal length

27. Concave Spherical Mirrors Object and image distances have a positive sign when measured from the center of the mirror to any point on the mirror’s front side. Distances for images that form on the backside of the mirror always have a negative sign.

28. Concave Spherical Mirrors The measure of how large or small the image is with respect to the original object is called the magnification of the image. M = h’/h = -(q/p) ‏ Magnification = image height = image distance object height object distance

29. Concave Spherical Mirrors For spherical mirrors, three reference rays are used to find the image point. The intersection of any two rays locates the image. The third ray should intersect at the same point and can be used to check the diagram.

30. Rules for drawing reference rays Through focal point F Parallel to principal axis1 Back along itself through C Through center of curvature C 3 Parallel to principal axis Through focal point F2 Line draw from mirror to image after reflection Line drawn from object to mirror Ray

31. Ray 1

32. Ray 2

33. Ray 3

34. All three rays together

35. Spherical Mirrors - Concave Image is REAL, INVERTED, and DEMAGNIFIED !!! CF

36. Concave Spherical Mirror When an object changes its location in relation to the mirror, its image changes in location, and form.

37. Concave Spherical Mirror Behind mirrorVirtual and upright Between focal point and mirror’s surface Image is infinitely away from mirror and can’t be seen At the focal length In front of mirror Real and invertedGreater than focal length Location of Image Type of ImageObject’s distance

38. Distance greater than focal length

39. Distance = focal length

40. Between focal length and mirror

41. Spherical Mirrors – Concave Object Inside the Focal Point Image is VIRTUAL, UPRIGHT, and MAGNIFIED C F

42. Concave Spherical Mirrors

44. M= -(q/p) ‏ We have p, but not q, so we need another equation to find q. 1/p + 1/q = 1/f We have p and f, so we can solve for q. 1/q = 1/f – 1/p

45. Substitute: (1/10 cm) – (1/30 cm) = 1/q Solve: 0.06667 cm = 1/q q= 15 cm

46. Now with q we can substitute into the original formula and solve. M= -(q/p) ‏ M= -(15 cm/30cm) ‏ M= -0.50 This means that the image is smaller than the object and inverted. Therefore it is a real image.

47. Spherical Mirrors - Convex Image is VIRTUAL, UPRIGHT, and DEMAGNIFIED C F

48. color White light is not a single color; it is made up of a mixture of the seven colors of the rainbow. We can demonstrate this by splitting white light with a prism: This is how rainbows are formed: sunlight is “split up” by raindrops.

49. Wavelengths of Light Red Light –  nm Green Light -  nm Blue Light -  nm

50. Adding colors White light can be split up to make separate colors. These colors can be added together again. The primary colors of light are red, blue and green: Adding blue and red makes magenta (purple) ‏ Adding blue and green makes cyan (light blue) ‏ Adding all three makes white again Adding red and green makes yellow

51. Seeing color The color an object appears depends on the colors of light it reflects. For example, a red book only reflects red light: White light Only red light is reflected

52. A white hat would reflect all seven colors: A pair of purple pants, in addition to being ugly, would reflect purple light (or red and blue, as purple is made up of red and blue): Purple light White light

53. Using colored light If we look at a colored object in colored light we see something different. For example, consider the outfit below – I mean, from a physics standpoint, not as a fashion choice: White light Shorts look blue Shirt looks red

54. In different colors of light this kit would look different: Red light Shirt looks red Shorts look black Blue light Shirt looks black Shorts look blue

55. Using filters Filters can be used to “block” out different colors of light: Red Filter Magenta Filter