1Chapter 26Properties of Light22Electromagnetic Waves• A moving charge creates a changing magnetic field• The changing magnetic field induces a changing electric field• If you move a charge up and down, you produce a changing electric field and its magnetic counterpart• These coupled oscillating fields are called electromagnetic (EM) waves– The electric field is perpendicular to the magnetic field– Since the electric field and the magnetic field are always perpendicular to the direction of motion, the EM wave is a transverse wave33The EM Wave44The Speed of Light• The speed of light in a vacuum is constant – no matter what!• The speed of light is defined to be:• The speed of light is unvarying because of conservation of energy– If the wave were to slow down, the rate of change of the fields would slow down and energy would be lost– If the wave were to speed up, the rate of change of the field would increase and we would get energy from nothingm/s 102.9979 c8×=55Problem Solving: The Speed of Light• A light year is not a measure of time, but a measure of distance. Using the speed of light, calculate the distance light travels in a year.• The sun is 1.5*1011m away from the Earth. How long ago did the sunlight we see now leave the sun?myrssmvtd15781042.9)/10)(/103(~ ×=××=πsec20min8500/103105.1811==××== ssmmvdt66The Electromagnetic Spectrum• Gamma rays, X rays, light, microwaves and radio waves are all forms of EM waves• Since the speed of light is constant, then the only other thing that can change is the frequency/wavelength• All of these different frequencies form the electromagnetic spectrum77The Electromagnetic Spectrum8Chapter 27Color99Selective Reflection• Everything our eyes see is a reflection• Therefore, if we see the wall to be blue, that is only because that is the only color of light that the wall reflects– The rest of the colors are absorbed1010Problem Solving: Selective Reflection• You shine a blue light on a banana. What color will it look like?–The banana will look black. The banana only looks yellow in white light because it only reflects yellow light. Therefore, if you do not shine any yellow light on the banana, it will not reflect any light and it will look black.1111Selective Transmission• Objects that transmit light are clear if they transmit all light• If an object that transmits light appears red, it is because red is the only color of light not absorbed by the material• Objects that absorb all colors will be opaque1212Mixing Colored Light• White light is the combination of all colors of light• The minimum number of colors needed to perceive white light is three: red, green and blue– These are called the additive primary colors• Colors formed from adding two primary colors together make the additive complimentary colors– Red + Blue = Magenta– Red + Green = YellowYellow– Blue + Green = Cyan1313• What color light is produce when you mix magenta, cyan and yellow light?Problem Solving: Mixing Colored Light–White. Since the additive complimentary colors are made up of two of the primary colors, if we mix all of the complimentary colors together, we get white light just like if we mixed all of the primary colors.1414Mixing Colored Pigments• Before, we added colors together to make white light• Since reflection involves subtracting color from white light, we will have different rules for combining pigments• Subtractive primaries: magenta, cyan, yellowyellow• The absence of pigment is looks white• The combination of all the subtractive primaries is black1515• Magenta light and cyan light illuminate an object. If the object then appears to be yellow, what color of light is absorbed by the object?– Blue. Magenta is made of red and blue light while cyan is made of blue and green light. If we subtract the blue light, we are left with equal parts of red and green light, which make the color yellow.Problem Solving: Selective