1. Chapter 14
Light and Reflection

2. 14.1 Characteristics of Light
Objectives
Identify the components of the electromagnetic spectrum
Calculate the frequency or wavelength of electromagnetic radiation
Recognize that light has a finite speed
Describe how the brightness of a light source is affected by distance

3. White Light What appears to us to be white light from
a bulb or the sun is actually a spectrum of
waves
The visible spectrum can be separated
into six primary colors: ROYGBV
The invisible spectrum includes other forms
of radiation (X-rays, microwaves, radio
waves).
Visible or invisible, they are all
electromagnetic waves.

5. Electromagnetic Waves
…are transverse waves consisting of oscillating electric and magnetic fields at right angles to each other.

6. Electromagnetic Waves, cont.
Since the E-field and the B-field oscillate
perpendicular to the direction of wave
travel, electromagnetic waves are
transverse waves
All electromagnetic waves travel at the
speed of light

7. Electromagnetic Wave Speed
Equation for Wave Speed:
c = f λ
where: c = speed of light (m/s)
f = frequency (Hz) or (1/s)
λ = wavelength (m)
c = 3.00 x 108 m/s

8. Question
The AM radio band extends from 5.4x105 Hz to 1.7x106 Hz. What are the longest and shortest wavelengths in this frequency range?
c = f λ, so λ = c / f
Answers: Longest λ = 5.6x102 m
Shortest λ = 1.8x102 m

9. Wave Trivia
Why can radio waves travel through space but sound waves can’t?
Hints: What kind of wave is a sound wave
and what does is need in order to propagate?

10. Wave Trivia, cont.
Why do you see lightening much sooner than you hear the thunder?
Speed of Light
3.0x108 m/s
Speed of Sound
343 m/s

11. Wave Trivia, cont.
Can you get a sunburn on a cloudy day?

12. Waves Huygens’ Principle
Wave fronts are lines of particles that represent a position within any given wave
Wavelet is a spherical or secondary wave produced by point sources along the wave front
Ray approximation is a simplification that treats the propagating wave as a straight line (ray) perpendicular to the wave front

13. Huygens’ Principle

14. Brightness
Since light emits in all directions, the farther the light is you are from the source the more spread out the light will be. Therefore, the light appears less bright as you get farther away from the source
Brightness decreases by the square of the distance from the source

15. 14.2 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

16. Reflection
…the turning back of an electromagnetic wave at the surface of a substance
Most substances absorb at least some of the incoming light wave, and reflect the rest

17. Reflection, cont. Diffuse Reflection Specular Reflection
The manner in which light is reflected from
a surface depends on the surface’s smoothness
Diffuse Reflection
Specular Reflection

18. Reflection, cont.
Angle of incidence is the angle between a ray that strikes s surface and the normal to the surface at that point
Angle of reflection is the angle between the normal to the surface and the direction in which the reflected ray moves
θ1 = θ2

19. Flat Mirrors
If an object is placed a distance in front of a flat mirror, the image of the object will appear to be located behind the mirror

20. Flat Mirrors, cont.
The object distance (p) in front of the mirror is equal to the image distance (q) behind the mirror
The image formed
by light rays that
appear to intersect
behind the mirror,
but never really do, is
called a virtual image

21. Ray Diagrams Draw the first ray perpendicular
…use simple geometry to locate an image formed by a mirror
Draw the first ray perpendicular
to the mirror’s surface. Its
angle of incidence and of
reflection are both zero.
Draw the 2nd ray at an angle that
is not perpendicular to the
mirror’s surface. The angle of
incidence will equal the angle of
reflection.
Trace both reflected rays back to
their image location behind the mirror.

22. Sources of Light (Worksheet Handout)
Luminous bodies: emit light
Illuminated bodies: reflect light
Measurements of light are made by comparing light
sources with the output of known luminous bodies.

23. Sources of Light, cont.
The rate at which light is emitted from a source
is called luminous flux.
Luminous flux is denoted by the letter P, with
units of lumens (lm).
The amount of illumination a light source
provides to an object depends on how far
away from the object the light source is.
The actual amount of illumination onto a surface
is called illuminance. The symbol for illuminance
is E, and units are lm/m2.

24. E = P / (4r2) Sources of Light, cont. Relationship between luminous
flux (P) and illuminance (E):
E = P / (4r2)
This equation works only if the light source is
small enough or far enough away to be considered
a point source, and only if the surface is directly
facing the light source.

25. I = P / (4) Sources of Light, cont.
Another way to measure the output of
a light source is in candelas. The symbol
for candelas is I, and the units are cd.
The relationship between luminous
flux (P) and candelas (I) is:
I = P / (4)

26. Sources of Light, final. Summary of terms:
P: luminous flux (output of a light source)
E: illuminance (the amount of light that hits a surface)
I: candela (another measure of the output of a light source)

27. 14.3 Curved Mirrors Objectives
Calculate distances and focal lengths using
the mirror equation for concave and convex
spherical mirrors
2. Draw ray diagrams to find the image distance
and magnification for concave and convex
spherical mirrors
3. Distinguish between real and virtual images
4. Describe how parabolic mirrors differ from
spherical mirrors

28. Concave Spherical Mirrors
…an inwardly curved, mirrored surface that is a portion of a sphere and that converges incoming light rays
Concave mirrors magnify
the image
Common use is a
dressing table mirror

29. Image from a Concave Spherical Mirror
Determined by
The radius of curvature of the mirror (R), which is the same as the radius of a sphere whose center of curvature is (C) and whose curvature is the same as the mirror
The distance the object is away from the mirror (p)

30. Real and Virtual Images
Real images are formed if the light rays actually intersect at a single point. These images can be displayed on a surface, like the images on a movie screen.
Virtual images are created when the light rays don’t really intersect, but appear to intersect. These images cannot be displayed on a surface.

31. Image Location 1/p + 1/q = 2/R Where: p is the object distance
q is the image distance
R is the radius of curvature

32. Image Location, cont. 1/p + 1/q = 2/R
If the object distance (p) is far enough away from the mirror, then 1/p approaches zero, and the image distance (q) therefore approaches R/2.
This means the image forms at a location
halfway between the mirror and the center
of curvature of the mirror (C).
This location is called the focal point of the
mirror, and is denoted by the letter F.

33. Focal Point When the image is located at the focal point,
image distance is called the focal length (f).

34. 1/p + 1/q = 1/f Mirror Equation
Using the original equation and the new variable for focal length (f), the mirror equation becomes:
1/p + 1/q = 1/f
p is object distance
q is image distance
f is the focal length, which is half
the radius of curvature of the mirror

35. Center of curvature (C) and focal point (f)
1/p + 1/q = 2/R
Where R is the radius of curvature for a
spherical mirror whose center is located at C.
1/p + 1/q = 1/f
R = 2f
2/R = 1/f or
Since R=C, C is located twice as far from the mirror as f.

36. Mirror Equation Conventions
Real images are formed on the front side of the mirror, where rays reflect
Virtual images are formed on the back side of the mirror, where light rays do not really exist
Mirrors are drawn so that the front side is to the left of the mirror
Object and image distances are positive if they are on the front side of the mirror and the image distance is negative if it on the back side of the mirror
Object and image heights are positive when they are above the principle axis, and negative when below

37. Magnification M = h’/h = - q/p
Curved mirrors form images that are not the same size as the object
Magnification is the measure of how
much larger or smaller the image is
compared to the object
Equation for magnification (M):
M = h’/h = - q/p
h = object height q = image location
h’ = image height p = object location

38. Concave Mirror Example
A concave spherical mirror has a focal length
of 10.0cm. An upright object is placed 30.0cm
from the mirror.
Where is the center of curvature (C) located?
What is the image distance?
What is the magnification?
Is the image upright or inverted?
Is the image real or virtual?

39. Magnification, cont. - + Sign conventions for magnification upright
Orientation of image with respect to object
Sign of M
Type of image this applies to
upright +
virtual
inverted -
real

40. Ray Diagrams for Curved Mirrors
…are used to check values calculated from mirror and magnification equations
Similar to ray diagrams for flat mirrors, but use three reference rays instead of two
Need to measure and locate center of curvature (C) and focal point (F) along the principle axis

41. Rules for Drawing Ray Diagrams
Ray Number
Line drawn from object to mirror
Line drawn from mirror to image after reflection 1
parallel to principle axis
through focal point f 2 3
through center of curvature C
back along itself through C

42. Ray 1
Ray 2
Ray 3

43. Convex Spherical Mirrors
…an outwardly curved mirrored surface that is a portion of a sphere that diverges incoming light rays
The image is always virtual
The image distance is always negative

44. Ray Diagram – Convex Mirror

45. Convex Mirror Example An upright object is placed 10.0cm from a convex
spherical mirror with a focal length of 8.00cm.
Where is the center of curvature (C) located?
What is the image distance?
What is the magnification?
Is the image upright or inverted?
Is the image real or virtual?

46. Spherical aberration occurs when parallel rays far from the principle axis converge away from the mirror’s focal point

47. Parabolic Mirrors
Eliminate spherical aberration because their curvature is sharper than a spherical mirror, therefore rays parallel to the principle axis are never far enough away from the axis to be reflected back to a different point
All parallel rays from a parabolic mirror converge at the focal point

48. Parabolic Mirrors - Uses
Radio telescopes – receive radio signals (invisible
electromagnetic waves) from outer space by using a
parabolic metal surface
2. Refracting telescope – a light telescope that uses a
combination of lenses to form an image (CH 15)
3. Reflecting telescope – a light telescope that uses a
parabolic mirror (objective mirror) and small lenses
to form an image

49. 14.4 Color and Polarization
Objectives
Recognize how additive colors affect
the color of light
2. Recognize how pigments affect the
color of reflected light
3. Explain how linearly polarized light is
formed and detected

50. Colors
The color of an object depends on the wavelength of color that reflects from the object. All other wavelengths are absorbed by the object.

51. Additive Primary Colors
Red, green and blue are the additive primary colors
Just like white light can be dispersed
into its primary colors, the primary colors
can be combined to form white light
Can be combined in varying proportions
to form all the colors of the spectrum

52. Additive Primary Colors, cont.
Complementary colors
are formed by adding
2 of the 3 primary colors.
This complementary color
can then be combined with
the 3rd primary color to
form white light.
The three complementary
colors are cyan, magenta
and yellow

53. Subtractive Primary Colors
When mixing pigments, the resulting colors are different
than what you’d get if you mixed light of the same colors.
The mixed pigments subtract certain colors based on the
frequencies of light that aren’t absorbed by the pigment.
The 3 primary pigments (or primary subtractive colors)
are cyan, magenta and yellow.
Mixing the 3 primary pigments in proper proportions
results in black because all of the primary colors are
subtracted from white light.

54. Additive and Subtractive Primary Colors
(mixing light)
Subtractive
(mixing pigments)
Red
Primary
Complementary to cyan
Green
Complementary to magenta
Blue
Complementary to yellow
Cyan (blue green)
Complementary to red
Magenta (red blue)
Complementary to green
Yellow (red green)
Complementary to blue

55. Polarization of Light Waves
Linear polarization is the result of the alignment of electromagnetic waves such that the vibrations of the E-fields are parallel to each other

56. Transmission Axis
The direction in which the electric fields are polarized depends on the arrangement of molecules in the polarizing crystal. The line along which light is polarized is the transmission axis of the substance.

57. Horizontal Polarization
Reflected light is completely polarized parallel to the reflecting surface.
If the surface is parallel to the ground, the polarization is horizontal.
This causes glare if light is reflected at a low angle from roads, bodies of water or car hoods.

58. Reducing Glare
Light that is polarized horizontally causes glare. This can be filtered out by a polarizing substance whose transmission axis is oriented vertically (polarized sunglasses).

59. Polarization by Scattering
Scattering is the absorption and reradiation of light by particles in the atmosphere.
When an unpolarized beam strikes an air
molecule, the electrons
in the molecule vibrate
in the same plane as the E-field of the incoming wave.
The reradiated light is
polarized in the direction
of the electron oscillations.