1. Light and Reflection
Chapter 14

2. Characteristics of Light
Section 14.1

3. Electromagnetic Waves
Light is made of electromagnetic waves.
Take a prism and break up white light into a rainbow like band of colors. These are all in the visible spectrum.
Red, orange, yellow, green, blue, indigo and violet.
ROY 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. Electromagnetic Waves
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
All electromagnetic waves move at the speed of light

9. Electromagnetic Waves
We will use 3.00 X 108 m/s as the speed of light, c.
The wave speed equation is:
c = f
Speed of light = frequency X wavelength

10. Sample Problem
The AM radio band extends from 5.4 X 105 Hz to 1.7 X 106 Hz. What are the longest and shortest wavelengths in this frequency range?
f 1 = 5.4 x 105 Hz f 2 = 1.7 x 106 Hz
c = 3.0 x 108 m/s
c = f
= c/ f
1 = 5.6 x 102 m
2 = 1.8 x 102m

11. Light travels in straight lines.
Show the laser on the wall. Put an index card in the beam. This shows that the light is traveling in a straight line, but you can only see it when it hits something.
Put some chalk dust in the beam to show it is continuous.
Brightness decreases by the square of the distance form the source
Show how the size of the dot the laser makes gets bigger as it gets further from the source.
Laser

12. 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.

13. Flat mirrors
Section 14.2

14. Reflection of Light
Reflection – the turning back of an electromagnetic wave at the surface of a substance

15. 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

16. Part 2 - Reflection Reflection from a mirror: Mirror Normal
Incident ray
Reflected ray
Angle of incidence
Angle of reflection
Mirror

17. 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.

18. Angle of incidence = Angle of reflection
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 !!!

19. 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)

21. 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.

22. Flat Mirrors
Image is VIRTUAL, UPRIGHT, UNMAGNIFIED

23. Chapter 14
14.3 Concave Mirrors

24. 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.

25. 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.

26. 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.

27. 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.

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

29. 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.

30. 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

31. 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.

32. Rules for drawing reference rays
Line drawn from object to mirror
Line draw from mirror to image after reflection 1
Parallel to principal axis
Through focal point F 2 3
Through center of curvature C
Back along itself through C

33. Ray 1

34. Ray 2

35. Ray 3

36. All three rays together

37. Spherical Mirrors - ConcaveF
Image is REAL, INVERTED, and DEMAGNIFIED !!!

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

39. Concave Spherical Mirror
Object’s distance
Type of Image
Location of Image
Greater than focal length
Real and inverted
In front of mirror
At the focal length
Image is infinitely away from mirror and can’t be seen
Between focal point and mirror’s surface
Virtual and upright
Behind mirror

40. Distance greater than focal length

41. Distance = focal length

42. Between focal length and mirror

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

44. Concave Spherical Mirrors

46. 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

47. 1/q = 1/f – 1/p
Substitute:
(1/10 cm) – (1/30 cm) = 1/q
Solve:
cm = 1/q
q= 15 cm

48. 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.

49. Spherical Mirrors - Convex
Convex spherical mirror:
An outwardly curved, mirrored surface that is a portion of a sphere and that diverges incoming light rays
The focal point and center of curvature are situated behind the mirror.

50. Spherical Mirrors - Convex
Convex mirrors take the objects in a large field of view and produce a small image, but give a the observer a complete view of a large area.
Examples:
In stores, the passenger’s side of a car

51. Spherical Mirrors - ConvexF
Image is VIRTUAL, UPRIGHT, and DEMAGNIFIED

52. Colour
White light is not a single colour; it is made up of a mixture of the seven colours 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.

53. Wavelengths of Light Red Light – l = 680 nm Green Light - l = 500 nm
Blue Light - l = 470 nm

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

55. Only red light is reflected
Seeing colour
The colour an object appears depends on the colours of light it reflects.
For example, a red book only reflects red light:
Homework
White
light
Only red light is reflected

56. (or red and blue, as purple is made up of red and blue):
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
A white hat would reflect all seven colours:
White
light

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

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

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