Transcript Early Work – Feb. 20

Early Work – Feb. 20
• Explain the uses of light
• Definitions from Ch. 16
Test Back
Ch 16
Light
Light
• Light is the range of frequencies of
electromagnetic waves that stimulates the
retina of the eye
Light
• Wavelength range
– 400 nm – violet
– 700 nm – red
• ROYGBIV
• Light travels in straight lines
– Which makes shadows possible
Light
• Light behaves like both a wave and a
particle
• Use ray model to model the path of light
(which thinks of it as a particle)
Speed of Light
• Ole Roemer (1644 – 1710)
• First to determine light travels
at a measurable speed
• Calculated velocity based on
distance and time (Io’s orbit is
42.5 hours)
• c = 299, 792, 458 m/s
• c = 3 x 108 m/s
Practice Problems
• P 376: 1, 4
Early Work – Feb. 22
• Explain how Roemer first calculated the
speed of light.
• Turn in vocab and PP 1, 4
Sources of Light
• A luminous body emits light waves
– The sun
• An illuminated body reflects light waves
– The moon
Luminous Flux
• Luminous Flux, P
– The rate at which visible light is emitted from
a source
– Measured in lumens, lm
– A standard 100-watt light bulb emits 1750 lm
Illuminance
• Illuminance, E
– The rate at which light falls on a
surface
• Something we can use more than
luminous flux
– Measured in lumens per square
meter, lm/m2
• Or lux, lx
– Since light is emitted radially,
use 4πr2 to find the surface
area of a sphere
Illuminance
• Two ways to increase illumination
– Get a brighter source – increase luminous flux
– Move object closer
• Both represented with:
P
E=
2
4p d
Luminous Intensity
• Luminous Intensity –
– The luminous flux that falls on 1 square meter
of a sphere 1 meter in radius
P
P
=
E=
2
4p d
4p
– Measured in candela, cd
• SI unit for all light intensity
Example
• What is the illumination on your desktop if
it is lighted by a 1750-lm lamp that is 2.50
m above your desk?
Practice Problems
• 6, 7, 9 – 11
16.2
Light and Matter
Materials
• Transparent materials
– Light waves are transmitted without distortion
• Translucent materials
– Light waves are transmitted with distortion
• Opaque materials
– Transmit no light, or reflect all light
Color
• Isaac Newton, at 24, did experiments on
what he called the spectrum after
observing it from a prism
• Through it unevenness in the glass
Color by Addition
• White light can be formed
from the correct
combination of red, green,
and blue lights
• Primary Colors – red,
green, and blue
– Used in TVs
• Secondary Colors – purple
(magenta), cyan, yellow
– Red and green  yellow
– Red and blue  magenta
– Blue and green  cyan
Color by Addition
• If you have a color made by
two other colors, then you
already have two of the
three colors needed to
make white light.
• Complementary Colors – a
primary and a secondary
that produce white light
– Magenta and green
– Yellow and blue
– Cyan and red
Colors by Subtraction
• Dye – a molecule that absorbs certain
wavelengths and transmits or reflects
others
• Pigment – a colored material that absorbs
certain colors and transmits or reflects
others
– Difference is a pigment is larger and can be
seen with a microscope
– Often pigments are ground inorganic
materials
Colors by Subtraction
• Primary Pigment – absorbs one primary
color
– Yellow – absorbs blue and reflects red and
green
– Cyan – absorbs red and reflects blue and
green
– Magenta – absorbs green and reflects blue
and red
Colors by Subtraction
• Secondary Pigment – absorbs two primary
colors and reflects one
– Red – absorbs green and blue
– Green – absorbs red and blue
– Blue – absorbs red and green
Note
• Primary pigments are secondary lights
• Primary lights are secondary pigments
Mixing Pigments
• If the primary pigment yellow (which
absorbs blue) is mixed with the secondary
pigment blue (which absorbs everything
but), then you get black because no
wavelength is being reflected
• Remember, when you mix light you get
white
Bkwk
• P 389: 2 – 21, 24 – 44
• Hmwk due Wed. Feb. 28
• Ch. 16 Test – Feb. 28