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Chap 16-1. Light Fundamentals. What is Light?. A transverse electromagnetic wave. What is Light?. A small part of the electromagnetic spectrum The part which stimulates the retina of the human eye Visible light has wavelengths 400-700 nm. RED Longer λ Lower f Lower E. BLUE

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Chap 16 1

Chap 16-1

Light Fundamentals

What is light
What is Light?

  • A transverse electromagnetic wave

What is light1
What is Light?

  • A small part of the electromagnetic spectrum

  • The part which stimulates the retina of the human eye

  • Visible light has wavelengths 400-700 nm


Longer λ

Lower f

Lower E


Shorter λ

Higher f

Higher E

How does light interact with matter
How does light interact with matter?

  • Light does not require matter (no medium) for transmission

    BUT if light does pass through matter

  • Light travels in a straight line through a

    • Vacuum or a

    • Uniform medium

Ray model
Ray Model

  • A ray is a straight line that represents the path of a narrow beam of light

  • ignores the wave nature of light

  • a useful model in analyzing reflection and refraction (ray diagrams)

Speed of light
Speed of Light

  • Before 17th century: light thought to be instantaneous

  • Galileo: thought speed of light was finite but could not measure it

  • Ole Roemer (~1700)

    • through measurements of the period of Io, a moon of Jupiter,

    • Calculated that light took 22 min to cross the diameter of Earth’s orbit

    • Calculated speed of light at 2.2 x 108 m/s (75% of true value)

Albert michelson
Albert Michelson

Michelson (1926) made a more precise measurement for speed of light, c with the Michelson-Morely Experiment:


  • Won the Nobel Prize

  • Successfully measured the speed of light

  • c = 3.00 x 108 m/s in a vacuum

  • this is a “defined value” for light

  • Now objects lengths are determined by how long it takes for light to travel from one end to the other

    • Examples:

      • Definition of the meter

      • Light year

Sources of light
Sources of Light

  • A luminous body emits light waves.

  • An illuminated body reflects light waves produced by an outside source.

  • Incandescent: light produced by a hot body.

Luminous flux
Luminous Flux

  • The rate at which visible light is emitted by a source

  • Represented by the letter P

  • The unit is the lumen, abbreviated lm

  • A typical 100 W light bulb emits 1750 lm


  • The rate at which light falls on a surface

  • Represented by the letter E

  • The unit is lumens per square meter,

    lm/m2 = lux, abbreviated lx

Consider a 100-W light bulb in the middle of a sphere. What is the illumination (illuminance) of the sphere?

Note that illumination is proportional to 1/r is the illumination (illuminance) of the sphere?2

Inverse-Square Relationship:

Light from a point source spreads out

over an area proportional to the

square of the distance from the source:

Luminous intensity
Luminous Intensity is the illumination (illuminance) of the sphere?

  • the luminous flux that falls on 1 m2 of a sphere 1m in radius.

  • Unit: candela, cd which is the SI unit of light intensity.

  • Luminous Intensity = P/4 π

7. What is the illumination on a surface 3.0 m below a is the illumination (illuminance) of the sphere?

150 Watt incandescent lamp that emits a luminous flux

of 225 lm?

  • A public school law requires a minimum illumination of is the illumination (illuminance) of the sphere?

  • 160 lx on the surface of each student’s desk. An architect’s

  • specifications call for classroom lights to be located 2.0 m

  • above the desks. What is the minimum luminous flux the

  • lights must deliver?

End 16 1
End 16-1 is the illumination (illuminance) of the sphere?