Ch. 16: Light
Ch. 16: Light
Part II: Part II: Luminous Flux, Illuminance, and Luminous Intensity
Luminous Flux, Illuminance, and Luminous Intensity
Sec. 16.1
Light Fundamentals
• Objectives
Sources of Light
Natural vs. Artificial
• Natural light
– Recognize that light is the visible portion of an entire range of electromagnetic frequencies
– Describe the ray model of light
– Solve problems involving the speed of light
– Define luminous intensity, luminous flux, and illuminance
– Solve illumination problems
– Sun, flames, sparks, fireflies • Artificial Light
– Bulbs, lamps, TVs, lasers
Luminous Body vs. Illuminated Body
• A Luminous Body
– Generates its own light – e.g. (The Sun, Stars, Light bulbs, Fire) • An Illuminated Body
– Reflects light from some other source
– e.g. (The Moon, Reflector, Mirror, A Movie Theater Screen)
P
Luminous Flux
the rate of light emission measured in lumens (lm)
An incandescent (“old fashioned”) 100 Watt Bulb emits about 1750 lumens of light. That’s a measurement of how much light a source emits…
…but how well it lights up your desk depends its distance away…
…so we introduce a measurement the light falling on a surface
…illuminance.
E
Illuminance
the rate that light falls on a surface measured in lux (lx) 1 lux = 1 lumen per square meter
1 lx = 1 lm/m2
Illuminance depends on distance and luminous flux:
P
E
4 d 2
Where:
E = Illuminance (lx)
P = Luminous Flux (lm)
d = distance from the light source to the illuminated surface
1
I
Luminous Intensity
the amount of light that falls on a defined surface measured in candela (cd)
1 candela = 1 “candle power”
the amount of light that a 1 lumen source would shed on 1 m2
of a 1 m radius sphere if the source were in its center.
Luminous Intensity does not involve distance from a source:
Summary Quantity
Symbol
Units
Luminous Flux
P
lumens
(lm)
Illuminance
E
lux
(lx)
Luminous Intensity
I
candela
(cd)
Formula
P  4 I
or
P  4 d E
2
E
I = Luminous Intensity (cd)
P = Luminous Flux (lm)
the rate of light emission
(1 lm = 4 cd)
P
4 d 2
the rate light falls on a flat surface (1 lx = 1 lm/m2)
P
4
the amount of light that falls on a part of a sphere
(S.I. base unit)
Where:
P
I 
4
Definition
I
asdf
Sec. 16.2
Light and Matter
• Objectives
Light and Matter
• Transparent
– Explain the formation of color by light and by
pigments or dyes
– Explain the cause and give examples of
interference in thin films
– Describe methods of producing polarized light
– glass
• Translucent
– Lamp shades
– Frosted light bulbs
• Opaque
– bricks
Color
• Spectrum
• Primary Colors
– Red, green, blue
• Secondary Colors
–
–
–
–
Yellow, cyan, magenta
R+B = M
B+G = C
G+R = Y
• White
Colors Con’t
• Complementary colors
– The primary color that will turn a secondary
color of light white is its complement.
Example:
– Yellow made from red and green light
– Yellow + blue = white
– Thus, yellow is the complementary color to
blue.
• Same intensities of red, green, and blue
2
Color by subtraction
• Dye
– Molecule that absorbs certain wavelengths of light
and transmits others
– Pigments are like dyes, only they are made from large
particles rather than molecules
– (Dyes form solutions, pigments form suspensions)
Formation of Colors in Thin Films
• Soap bubble, oily films
• Results of constructive and destructive
interference of light
– Thin-film interference
• Primary Pigment
– Absorbs one primary color from white light
– Reflects Yellow, cyan, or magenta
• Secondary Pigment
– Absorbs two primary colors
– Reflects Red, green, or blue
Polarization
• The intensity of the light is reduced by half
• Light travels in two planes
– Polarizing material cancels out one of the
planes
Review Questions
• What colors does white light consist of?
• Is black a color?
• Why are the insides of binoculars and cameras
painted black?
• You put a piece of red cellophane over one
flashlight and a piece of green cellophane over
another. You shine the light beams on a white
wall. What color will you see where the two
flashlight beams overlap?
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