8b: Properties of Light
1. What is light?
a. Light is an arbitrarily defined as electromagnetic waves we can see
b. A typical human can see EM radiation with wavelengths between 390 nm (high energy violet light) and
700 nm (low energy red light)
c. For perspective, 1 nm = 10-9 m or 0.000000001 m (radio waves often have wavelengths in meters)
d. Many organisms can see what we classify as infrared (wavelength higher than 700nm) or UV (below 390
nm)
2. Color
a. Primary colors of light are Red, Green, Blue
i. They roughly correspond to the three color photoreceptors in our eyes (cones)
ii. Red light stimulates mostly the red cones, yellow both the green and red, etc.
b. How we perceive as color is a biological phenomenon
i. what we see is the product of our brains, not physics
ii. Combining light colors is considered additive, because additional wavelengths stimulate
additional color receptors
iii. Example
1. light with a wavelength of 580 nm is yellow light, and is perceived as yellow by our
brains
2. However, an equal mixture of red light at 680 nm and green light at 520 nm also
appears yellow to our brains.
c. Light color additive properties
i. Red + Green = Yellow (notice this is NOT INTUITIVE)
ii. Green + Blue = Cyan
iii. Red + Blue = Magenta
iv. Red + Green + Blue = Perceived as White
d. pure spectral colors
i. A color found on the EM spectrum with a single specific wavelength (ROYGBIV and colors in
between)
e. Non-spectral colors
i. not found on the EM spectrum
ii. our brain “makes them up” (although this could be argued for all colors)
iii. Examples: pink, white, magenta, black
iv. Explanations
1. Magenta is not found on the EM spectrum, but is our brain’s attempt at creating a
transition between red and purple (red and purple are at opposite ends of the
spectrum)
2. White is not on the EM spectrum, but is what we see with all three primary colors
balanced
3. Black is simply the absence of light, and so is also not a color
4. Colors like pink are under-saturated white light (white light with less green and blue),
and are also created by our brains, and browns are under-saturated red, yellow, or
orange.
3. Pigments
a. substances that absorb certain wavelengths of light and reflect others
b. pigments mix together in subtractive manner
i. When mixed together, more wavelengths are absorbed and fewer are reflected
ii. Primary pigment colors: cyan, yellow, and magenta
iii. A mixture of cyan, magenta, and yellow pigments results in a total lack of reflection, creating
the appearance of black objects
c. examples
Perceived Pigment Color
White
Black
Red
Green
Blue
Yellow
Cyan
Magenta
Absorbs
None
Entire spectrum
B, G
R, B
R, G
B
R
G
Reflects
Entire spectrum
nothing
R
G
B
R, G
G, B
R, B
Notes
Absorbs least light E
Absorbs most light E
Look darker because
they reflect less light
Primary Pigment Colors
(subtractive secondaries)
4. Optical density
a. The ability of a material to absorb or block the passage of light
b. Opaque materials do not let light escape
c. Translucent materials let light pass through, but scatter it (like ground glass)
d. Transparent materials let light pass through without scattering (like a window)
5. Reflection
a. Law of reflection applies if light is not absorbed or scattered
b. Regular reflection
i. incoming parallel rays of light reflect at the same angle because of smooth surface
ii. reflection can form sharp images (like a mirror)
c. Diffuse reflection
i. incoming parallel rays of light reflect in different directions because of irregular surface
ii. Scratches on surface must be smaller than wavelength for reflection to be sharp
6. Refraction
a. Light is slowed down in matter
b. Mirages
i. Formed because refractive index of air is dependent on temperature
c. Light dispersion
i. shorter wavelength (higher energy/frequency) waves refract more than longer wavelength
ii. the different photons of different wavelengths making up white light separate
iii. Apparent in refraction of white light through prisms
iv. rainbows
7. Coherent and incoherent light
a. Coherent light
i. Only one wavelength of light comes from a source
ii. All waves are traveling in the same direction (and in phase)
iii. Add up to single wave
iv. Example: laser light source
b. Incoherent light
i. More than one wavelength from a source
ii. Travel in different directions
iii. Example: flashlight
8b: Properties of Light
1. What is light?
a. Light is an __________________ defined as electromagnetic waves we can ________
b. A typical human can see EM radiation with wavelengths between 390 nm (______ energy __________
light) and 700 nm (________ energy _______ light)
c. For perspective, 1 nm = _________ or 0.000000001 m (radio waves often have wavelengths in meters)
d. Many organisms can see what we classify as ___________ (wavelength _________ than 700nm) or ____
(___________ 390 nm)
2. Color
a. Primary colors of light are ___________, ______________, __________
i. They roughly correspond to the three color __________________ in our eyes (_________)
ii. Red light stimulates mostly the red cones, yellow both the green and red, etc.
b. How we perceive as color is a biological phenomenon
i. what we see is the product of our __________, not ___________
ii. Combining light colors is considered _____________, because additional wavelengths stimulate
additional color receptors
iii. Example
1. light with a wavelength of 580 nm is ___________ light, and is perceived as yellow by
our brains
2. However, an equal mixture of red light at 680 nm and green light at 520 nm also
appears ____________ to our brains.
c. Light color ______________ properties
i. Red + Green = _________ (notice this is NOT INTUITIVE)
ii. Green + Blue = _________
iii. Red + Blue = ___________
iv. Red + Green + Blue = Perceived as _________
d. pure spectral colors
i. A color found on the EM spectrum with a ___________ specific wavelength (ROYGBIV and
colors in between)
e. Non-spectral colors
i. ________ _____________ on the EM spectrum
ii. our brain “makes them up” (although this could be argued for all colors)
iii. Examples: pink, white, magenta, black
iv. Explanations
1. ______________ is not found on the EM spectrum, but is our brain’s attempt at creating
a transition between red and purple (red and purple are at opposite ends of the
spectrum)
2. ____________ is not on the EM spectrum, but is what we see with all three primary
colors balanced
3. ___________ is simply the absence of light, and so is also not a color
4. Colors like ____________ are under-saturated white light (white light with less green
and blue), and are also created by our brains, and ___________ are under-saturated
red, yellow, or orange.
3. Pigments
a. substances that ___________ certain wavelengths of light and ___________ others
b. pigments mix together in ____________ manner
i. When mixed together, __________ wavelengths are absorbed and _____________ are reflected
ii. Primary pigment colors: _________, ____________, and ________________
iii. A mixture of cyan, magenta, and yellow pigments results in a total ___________ of reflection,
creating the appearance of ___________ objects
c. examples
Perceived Pigment Color
White
Black
Red
Green
Blue
Yellow
Cyan
Magenta
Absorbs
Reflects
Notes
4. Optical density
a. The ability of a material to absorb or block the passage of light
b. ______________ materials do not let light escape
c. _______________ materials let light pass through, but _____________ it
(like ground glass)
d. _______________materials let light pass through __________ scattering
(like a window)
5. Reflection
a. Law of reflection applies if light is ______ _____________ or
______________
b. Regular reflection
i. incoming _______________ rays of light reflect at the __________ ____________ because of
_______________ surface
ii. reflection can form sharp images (like a mirror)
c. Diffuse reflection
i. incoming parallel rays of light reflect in ______________ _______________ because of
_________________ surface
ii. Scratches on surface must be smaller than wavelength for reflection to be sharp
6. Refraction
a. Light is ______________ down in matter
b. Mirages
i. Formed because refractive index of air is dependent on temperature
c. Light dispersion
i. ___________ wavelength (higher energy/frequency) waves refract __________ than longer
wavelength
ii. the different photons of different wavelengths making up white light ______________
iii. Apparent in refraction of white light through _______________
iv. rainbows
7. Coherent and incoherent light
a. Coherent light
i. Only __________ wavelength of light comes from a source
ii. All waves are traveling in the ___________ ________________ (and ____ _______________)
iii. Add up to ______________ wave
iv. Example: laser light source
b. Incoherent light
i. ___________ than __________ wavelength from a source
ii. Travel in ____________ directions
iii. Example: flashlight