Light

Light Facts

• Light: The range of frequencies of electromagnetic waves that stimulate the retina of the eye.

• 400 nm – 700 nm ( 4 x 10-7 to 7 x 10-7 meters)

• 1 meter = 109 nanometers

• Light Speed: 3.00 x 108 m/s or 186,000 miles / sec

• Seen as c in equations E = mc2

The Electromagnetic Spectrum

High Energy (dangerous) Low Energy (no probs)

Sources of LightWhat’s the difference between the moon and sun? (in terms of light)

Illuminated Body

Luminous Body

Sources of light• Luminous Body:

– A body that emits light.

• Illuminated Body: – A body that reflects light.

• Luminous Flux ( P in equations ): – Rate at which light is emitted from a source.

• Units: Lumens ( lm ) Best Buy’s Digital Projector Line

• Luminous Intensity: – Luminous flux that falls on 1m2 of a sphere 1m in radius.

• Units: Candela, Candle Power ( cd ) Cabela’s Spotlight Line

• Luminsous Flux = Luminous Intensity / 4π

• Illuminance ( E in equations ): – How much light falls on a flat surface. Illumination.

• Units: lm/m2 or lux

The luminous flux is a very basic unit of measurement for light. If a uniform point light source of 1 cd luminous intensity (about the intensity of a normal wax candle!) is positioned at the center of a sphere of 1 m radius, then every area of 1 m2 on the inside of that sphere will receive a luminous flux of 1 lm.

1 cd = 4π lumens or 12.57 lm

• Inverse Square Relationship

– As distance away from light source increases, illumination on the surface goes down by the distance squared. The Luminous Flux from light source stays the same.

Inverse square law for Luminous Intensity

Illuminating a Flat Surface• 2 ways to increase illumination on a flat surface:

– Brighter source (ie brighter bulb)

– Move the source closer to the surface

• Mathematically written as:

24 d

PE

Example ProblemWhat is the illumination on your desktop if it is lighted by a 1750

lm lamp (typical 100 W bulb) that is 2.50 m above your head?

Example 2• WI state law requires 160 lx of the surface of each desk. My

lights are 250 cm above the desk surface. What is the minimum luminous flux the lights must deliver?

• Transparent: Completely clear (glass)

• Translucent: Semi Clear. Transmits light, but not shapes (wax paper)

• Opaque: No light transmission (brick wall)

The Speed of Light

Ole Roemer: Danish astronomer who first quantitatively measured the speed of light in 1676.

Found that Io would emerge 13 s faster at position 1. and 13 s slower at position 3.

He assumed that the moons were orbiting at regular intervals, so something had to account for the lapse.

Theorized light had a finite speed, and estimated it to be near 2.2 x 108 m/s.

Accepted value today: 299,792,458 m/s

Light Speed = 3.00 x 108 m/s

Scale model of Earth and Moon with beam of light travelling to it

Light Speed through History

Color

• Primary Colors: Red, Green, Blue (RGB) – All colors can be made by adding varying intensities of red, green, and blue

• Secondary Colors: Yellow, Magenta, Cyan– A secondary color is one that can be made by adding 2 primary colors.

• Complimentary Colors: Blue & Yellow, Cyan & Red, Yellow & Blue– Two colors that add up to make white light.

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• Pigments & Dyes: Substances that absorb (subtract) certain colors and transmits or reflects others. (Example: Red absorbs blue and green light)

• Primary Pigments: Yellow, Cyan, and Magenta – Pigment that only absorbs primary color. Yellow absorbs blue, but reflects red

and green.

• Secondary Pigments: Red, Blue, and Green– Pigments that absorb 2 primary colors. Red absorbs both green and blue.

– Note if Yellow dye (absorbing blue light) is mixed with Blue dye (absorbing green and red light) all light is absorbed and nothing is reflected. Hence Black is the result.

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Dyes in the dice selectively absorb and reflect various wavelengths of light. The dice are illuminated by a) white light b) red light c) blue light.

Polarization