Chapter 16 Light

Objectives Chapter 16

• 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

Objectives Chapter 16

• Explain the formation of color by light and pigments and dyes

• Explain the cause and give examples of interference in thin films.

• Describe methods of producing polarized

Electromagnetic Waves

Visible Spectrum

• Red has longest wavelength (700 nm)• Violet has shortest (400 nm)• ROYGBIV• Green is most sensitive to eye• Visible Light is right on the bubble of Ionizing

Radiation

Speed of Light

• 3.00 x 108 m/s, c (symbol for speed of light)– In a vacuum. Light slows down when it has to

weave its way through matter• C = wavelength times frequency• What is the frequency of red light? Green

light?• Who hears a baseball hit first? Radio listener

or fan in stands? Satellite 22,300 miles away

Sources of Light

• Everything gives off EM radiation, not everything gives off visible light. The hotter an object, the higher the frequency of EM radiation

• Produce visible light: Luminous• Reflect visible light: Illuminated

Light Terms

• Luminous flux: Rate at which visible light is emitted from a source (symbol P)– The unit is the lumen, seen on shelves.

• Illuminance: The illumination of a surface – How well does it reflect the light that its it off?

Measured in lux• Different types of paper have different lux

values (how white is the paper)

Light Terms

• Candela: SI unit of light. Measures the luminous intensity of something in a 1 meter squared sphere 1 meter away

Light and Matter

• Transparent: Transmit Light Waves without distorting image

• Translucent: Allow light through but distort image (lamp shade, frosted glass)

• Opaque: No light waves– All above discussing VISIBLE spectrum. An object

can be transparent to visible and opaque to another

Color• Color of material depends just as much on the

light that hits it as the material itself (if not more)

Primary ColorsRED + GREEN =YELLOW

RED+ BLUE = MAGENTA

GREEN + BLUE = CYAN

When red, blue, and green light are projected onto a screen, the overlapping areas appear different colors. Where all the three overlap, white is produced.

Additive primary colors are red, blue, and green because these colors produce the highest number of different colors.

28.6: Complementary Colors• When two colors are added together

to produce white, they are called complementary colors.

• YELLOW + BLUE =WHITE ( Yellow a combination of Green + Red )

• MAGENTA + GREEN = WHITE ( Magenta a combination of red + blue)

• CYAN + RED = WHITE ( Cyan a combination of green+ blue)

Color Subtraction

• Dyes: Capable of absorbing/reflecting certain types of light

• Pigment: Same as dye, just larger– Distinction is molecular

• A Red pigment looks red under white light because it absorbs Blue and Green

• A Yellow pigment looks Yellow under white light because it absorbs Blue

Blue

BLUE

An Impure Blue pigment reflects not only blue light, but also colors to either side of blue namely, green and violet. It absorbs red orange and yellow light.

Red , orange, and yellow have been subtracted from the incident light.

Light transmitting through pure blue glass

Light Models

Other Information

Nitrogen and oxygen molecules ring like tiny bells with high frequency when energized by sunlight.

Reemit light in all directions like the sound of a bell.

Atom

Scattered Radiation

Incident Beam

Other Information

Ultraviolet light from the sun is absorbed by the protective layer of the ozone gas.

Visible frequencies of violet light is scattered.

Although the violet light is scattered more, our eyes are more sensitive to blue. That’s why we see a blue sky.

Other Information

Other Information

• Lower frequencies of light are scattered the least by nitrogen and oxygen molecules

• Red, orange, yellow are transmitted more readily through the atmosphere

• Light of lower frequencies is transmitted while light of higher frequencies are scattered

Other Information

• At dawn and sunset, the sunlight reaches the earth at a longer path

• At noon, the light travels the shortest distance

• Red light (and OY) is scattered as the path of the sunlight becomes longer

Other Information

• Water is transparent to almost all the visible frequencies of light.

• The color is actually the reflected color of the sky

• Red is absorbed (more)by the molecules in the water, leaving a reflection of greenish-blue dominant hue