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:

24 d

PE

Luminous Intensity

• Luminous Intensity –– The luminous flux that falls on 1 square

meter of a sphere 1 meter in radius

– Measured in candela, cd• SI unit for all light intensity

44 2

P

d

PE

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