Ch. 16 Light

Milbank High School

Sec. 16.1Light Fundamentals

• Objectives– 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

Let there be light!

• Main way we sense our surroundings

• Natural light– Sun, flames, sparks, fireflies

• Artificial Light– Bulbs, lamps, TVs, lasers

Light Fundamentals

• Electromagnetic Wave– Stimulates the retina in the eye– Wavelengths between 400 and 700nm

• Exact nature not fully understood– Waves?– Particles?

• 3.00 x 108 m/s

• c = λf

Ray Model

• Represents a straight-line representation of light– Ignores wave properties– Used to describe how light is reflected and

refracted

Sources of Light

• Luminous body

• Illuminated body

• Luminous flux, P– lumen, lm

• Illuminance, E– lux, lx

Luminous Intensity

• Luminous flux that falls on 1 m2 of a sphere 1 m in radius– candela, cd (candle power)

• E = P/4πd2

• E is illuminance• P is luminous flux• d is distance from surface

Questions

• How do we know that light travels through a vacuum?

• What color of visible light has the shortest wavelength?

• What are the units for:– Luminous intensity– Illuminance– Luminous flux

Sec. 16.2Light and Matter

• Objectives– 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

Light and Matter

• Transparent– glass

• Translucent– Lamp shades– Frosted light bulbs

• Opaque– bricks

Color

• Spectrum• Primary Colors

– Red, green, blue

• Secondary Colors– Yellow, cyan, magenta

– White• Same intensities of red, green, and blue

Colors Con’t

• Complementary colors– Yellow made from red and green light– Yellow + blue = white– Thus, yellow is the complementary color

Color by subtraction

• Dye– Molecule that absorbs certain wavelengths of light

and transmits others– Pigment

• Primary Colors– Absorbs one primary color from white light– Yellow, cyan, magenta

• Secondary Colors– Absorbs two primary colors– Red, green, and blue

Formation of Colors in Thin Films

• Soap bubble, oily films

• Results of constructive and destructive interference of light – Thin-film interference

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?