Chapter 33 Chapter 33 -- Light and Light and IlluminationIllumination

A PowerPoint Presentation by

Paul E. Tippens, Professor of Physics

Southern Polytechnic State University

A PowerPoint Presentation byA PowerPoint Presentation by

Paul E. Tippens, Professor of PhysicsPaul E. Tippens, Professor of Physics

Southern Polytechnic State UniversitySouthern Polytechnic State University

© 2007

Objectives: Objectives: After completing this After completing this module, you should be able to:module, you should be able to:

•• DefineDefine lightlight, discuss its properties, and give , discuss its properties, and give the range of wavelengths for visible spectrum.the range of wavelengths for visible spectrum.

•• Apply the relationship between Apply the relationship between frequenciesfrequencies andand wavelengthswavelengths for optical waves.for optical waves.

•• Define and apply the concepts of Define and apply the concepts of luminous luminous fluxflux,, luminous intensityluminous intensity, and, and illuminationillumination..

•• Solve problems similar to those presented in Solve problems similar to those presented in this module.this module.

A Beginning DefinitionA Beginning DefinitionAll objects are emitting All objects are emitting and absorbing EM and absorbing EM radiaradia-- tiontion. Consider a poker . Consider a poker placed in a fire.placed in a fire.

As heating occurs, the As heating occurs, the emitted EM waves have emitted EM waves have higher energy and higher energy and eventually become visible. eventually become visible. First red . . . then white.First red . . . then white.

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Light may be defined as electromagnetic radiation that is capable of affecting the sense of sight. LightLight may be defined as electromagnetic radiation may be defined as electromagnetic radiation that is capable of affecting the sense of sight.that is capable of affecting the sense of sight.

Electromagnetic WavesElectromagnetic Waves

cc

EE

BB

Electric Electric EEMagnetic Magnetic BB

Wave Properties:Wave Properties:

1.1. Waves travel at the Waves travel at the speed of light speed of light cc..

2.2. Perpendicular electric Perpendicular electric and magnetic fields.and magnetic fields.

3.3. Require no medium Require no medium for propagation.for propagation.

For a complete review of the electromagnetic properties, you should study module 32C. For a complete review of the electromagnetic properties, you should study module 32C.

3 x 103 x 1088 m/sm/s

The Wavelengths of LightThe Wavelengths of LightThe electromagnetic spectrum spreads over a tremendous range of frequencies or wavelengths. The wavelength

is related to the frequency f:

The electromagnetic spectrum spreads over a The electromagnetic spectrum spreads over a tremendous range of frequencies or wavelengths. tremendous range of frequencies or wavelengths. The The wavelength wavelength

is related to the is related to the frequency frequency ff::

c = fc = 3 x 108 m/sc = fc = 3 x 108 m/s

Those EM waves that are visible (light) have waveThose EM waves that are visible (light) have wave-- lengths that range from 0.00004 to 0.00007 cm.lengths that range from 0.00004 to 0.00007 cm.

Red, Red,

0.00007 cm0.00007 cm

Violet, Violet,

0.00004 cm0.00004 cm

The EM SpectrumThe EM SpectrumA wavelength of one A wavelength of one nanometer 1 nm is:nanometer 1 nm is:

1 nm = 1 x 10-9 m1 nm = 1 x 10-9 m

Red 700 nm Red 700 nm

Violet 400 nm Violet 400 nm

c = fc = 3 x 108 m/sc = fc = 3 x 108 m/s

1024

1023

1022

1021

1020

1019

1018

1017

1016

1015

1014

1013

1012

1011

1010

109

108

107

106

105

104

Frequency wavelengthf (Hz) nm)

10-7

10-6

10-4

10-3

10-1

1 10 102

103

104

105

106

107

108

109

1010

1011

1012

1013

Gamma rays

X-rays

Infrared rays

Short Radio waves

Broadcast Radio

Long Radio waves

Ultraviolet

400 nm 400 nm

700 nm700 nmVisible SpectrumVisible Spectrum

Example 1. Example 1. Light from a HeliumLight from a Helium--Neon laser Neon laser has a wavelength of has a wavelength of 632 nm632 nm. What is the . What is the frequency of this wave?frequency of this wave?

8

-9

3 x 10 m/s 632 x 10 m

cc f f

f = 4.75 x 1014 Hzf = 4.75 x 1014 Hz Red lightRed light

The Helium Neon LaserThe Helium Neon Laser WavelengthWavelength

= 632 nm= 632 nmLaser

Properties of LightProperties of Light

• Rectilinear propagation: Light travels in straight lines.

• Reflection: Light striking a smooth surface turns back into the original medium.

• Refraction: Light bends when entering a transparent medium.

Any study of the nature of light must explain the following observed properties:

Any study of the nature of light must Any study of the nature of light must explain the following observed properties:explain the following observed properties:

The Nature of LightThe Nature of Light

Physicists have studied light for centuries, finding that it sometimes behaves as a particle and sometimes as a wave. Actually, both are correct!

Physicists have studied light for centuries, finding Physicists have studied light for centuries, finding that it sometimes behaves as a particle and that it sometimes behaves as a particle and sometimes as a wave. Actually, both are correct!sometimes as a wave. Actually, both are correct!

Reflection and Reflection and rectilinear propagation rectilinear propagation

(straight line path)(straight line path)Dispersion of white Dispersion of white

light into colors.light into colors.

Photons and Light RaysPhotons and Light RaysLight may be thought of as little bundles of waves Light may be thought of as little bundles of waves emitted in discrete packets called emitted in discrete packets called photonsphotons..

photonsphotons

The wave treatment uses The wave treatment uses raysrays to show to show the direction of advancing wave fronts.the direction of advancing wave fronts.

Light Light rayray

Light rays are convenient for describing how light behaves.

Light rays are Light rays are convenient for convenient for describing how describing how light behaves.light behaves.

Light Rays and ShadowsLight Rays and ShadowsA geometric analysis may be made of shadows A geometric analysis may be made of shadows by tracing light rays from a point light source:by tracing light rays from a point light source:

shadowshadow

screenscreen

Point Point sourcesource

The dimensions of the shadow can be found by using geometry and known distances. The dimensions of the shadow can be found by using geometry and known distances.

Example 2:Example 2: The diameter of the ball is The diameter of the ball is 4 cm4 cm and it is located and it is located 20 cm20 cm from the point light from the point light source. If the screen is source. If the screen is 80 cm80 cm from the from the source, what is the diameter of the shadow?source, what is the diameter of the shadow?

4 cm

20 cm80 cm

h

The ratio of shadow to the source is same as that of ball to source. Therefore:

The ratio of The ratio of shadow to shadow to the source the source is same as is same as that of ball that of ball to source. to source. Therefore:Therefore:

(4 cm)(80 cm)20 cm

h h = 16 cm

4 cm80 cm 20 cm

h

Shadows of Extended ObjectsShadows of Extended Objects

Extended Extended sourcesource

penumbrapenumbra

umbraumbra

The The umbraumbra is the region where no light reaches is the region where no light reaches the screen. the screen.

•• The The penumbrapenumbra is the outer area where is the outer area where only part of the light reaches the screen.only part of the light reaches the screen.

•• The The umbraumbra is the region where no light is the region where no light reaches the screen. reaches the screen.

The Sensitivity CurveThe Sensitivity Curve

Sensitivity curveSensitivity curve

Wavelength Wavelength Se

nsiti

vity

Sens

itivi

ty

Human eyes are not Human eyes are not equally sensitive to equally sensitive to all colors.all colors.

Eyes are most Eyes are most sensisensi-- tivetive in the midin the mid--range range near near = 555 nm= 555 nm..

555 nm555 nm

400 nm400 nm 700 nm700 nm

40 W40 W 40 W40 WYellow light appears brighter to the eye than does red light. YellowYellow light appears brighter light appears brighter to the eye than does to the eye than does red red light.light.

Luminous FluxLuminous FluxLuminous fluxLuminous flux is the portion of total radiant power is the portion of total radiant power that is capable of affecting the sense of sight.that is capable of affecting the sense of sight.

Typically only about 10% Typically only about 10% of the power (flux) emitted of the power (flux) emitted from a light bulb falls in from a light bulb falls in the visible region.the visible region.

The unit for luminous flux is the The unit for luminous flux is the lumenlumen which which will be given a quantitative definition later.will be given a quantitative definition later.

A Solid Angle: SteradiansA Solid Angle: Steradians

Working with luminous flux requires the use of a solid angle measure called the steradian (sr). Working with luminous flux requires the use of a solid angle measure called the steradian (sr).

AR

The Steradian 2

AR

A solid angle of one steradian (1 sr) is subtended at the center of a sphere by an area A equal to the square of its radius ( R2 ).

A solid angle of one A solid angle of one steradiansteradian ((1 1 srsr) is ) is subtended at the subtended at the center of a sphere center of a sphere by an area by an area AA equal equal to the square of its to the square of its radius ( radius ( RR2 2 ).).

Example 3.Example 3. What solid angle is subtended at What solid angle is subtended at the center of a sphere by an area of the center of a sphere by an area of 1.6 m1.6 m22? ? The radius of the sphere is The radius of the sphere is 5 m5 m..

A 1.6 m2

R 5 m

The Steradian 2

AR

2

2

1.60 m(5.00 m)

2

AR

= 0.00640 sr

= 0.00640 sr

The Lumen as a Unit of FluxThe Lumen as a Unit of FluxOne One lumenlumen (lm)(lm) is the is the luminous fluxluminous flux emitted from emitted from a a 1/60 cm1/60 cm22 opening in a standard source and opening in a standard source and included in a solid angle of included in a solid angle of one steradianone steradian (1 (1 srsr).).

In practice, sources of light are usually rated In practice, sources of light are usually rated by comparison to a commercially prepared by comparison to a commercially prepared standard light source.standard light source.

A typical A typical 100100--WW incandescent incandescent light bulb emits a total radiant light bulb emits a total radiant power of about power of about 1750 lm1750 lm. This is . This is for light emitted in all directions.for light emitted in all directions.

The Lumen in Power UnitsThe Lumen in Power UnitsRecalling that luminous flux is really radiant Recalling that luminous flux is really radiant power allows us to define the lumen as follows:power allows us to define the lumen as follows:

One lumen is equal to 1/680 W of yellow- green light of wavelength 555 nm. One lumen is equal to 1/680 W of yellowOne lumen is equal to 1/680 W of yellow-- green light of wavelength 555 nm.green light of wavelength 555 nm.

Wavelength Wavelength

Sensitivity curveSensitivity curveA disadvantage of this A disadvantage of this approach is the need to approach is the need to refer to sensitivity curves refer to sensitivity curves to determine the flux for to determine the flux for different colors of light.different colors of light.

Luminous IntensityLuminous IntensityThe The luminous intensity luminous intensity II for a light source for a light source is the luminous flux per unit solid angle.is the luminous flux per unit solid angle.

FI

Luminous intensity:FI

Unit is the candela (cd)

A source having an intensity of one candela emits a flux of one lumen per steradian. A source having an intensity of one candela emits a flux of one lumen per steradian.

Total flux for Isotropic SourceTotal flux for Isotropic Source

R R 3 m3 m

An isotropic source emits in An isotropic source emits in allall directions; i.e., over a directions; i.e., over a solid angle of solid angle of 44

steradianssteradians..

Total flux: F = 4ITotal flux: F = 4I

The flux confined to area A is:The flux confined to area A is:

F = I AF = I A

= 4= 4

srsr

Thus, for such Thus, for such a source, the a source, the intensity is:intensity is: 4

F FI

Example 4.Example 4. A A 30 30 cdcd spotlight is located spotlight is located 3 m3 m above a table. The beam is focused on a surface above a table. The beam is focused on a surface area of area of 0.4 m0.4 m22. Find the intensity of the beam.. Find the intensity of the beam.

Total flux: F = 4ITotal flux: F = 4I

FFTT = 4= 4(30 (30 cdcd) = 377 lm) = 377 lm RR 3 m3 m

The luminous intensity of The luminous intensity of the beam depends on the beam depends on

2

2 2

0.4 m ; 0.0444 sr(3 m)

AR

754 lm0.0444 sr

FI

Beam Intensity:Beam Intensity:

I = 8490 cdI = 8490 cd

Illumination of a SurfaceIllumination of a SurfaceThe illumination E of a surface A is defined as the luminous flux per unit area (F/A) in lumens per square meter which is renamed a lux (lx).

The The illumination illumination EE of a surface of a surface AA is defined as is defined as the luminous flux per unit area (the luminous flux per unit area (F/AF/A) in ) in lumens lumens per square meter per square meter which is renamed a which is renamed a luxlux (lx)(lx)..

Unit: lux (lx)FEA

An illumination of one lux occurs when a flux of one lumen falls on an area of one square meter.

An illumination of one lux occurs when a flux of one lumen falls on an area of one square meter.

RR

Area Area AA

Illumination, Illumination, EE

Illumination Based on IntensityIllumination Based on Intensity

R

Area A

; ; F FE I F IA

2 but so thatI AEA R

2Illumination, IER

This equation applies for perpendicular surfaces.

The illumination E of a surface is directly proportional to the intensity I and inversely proportional to the square of the distance R.

The The illuminationillumination EE of a surface is directly of a surface is directly proportional to the proportional to the intensity intensity II and inversely and inversely proportional to the proportional to the squaresquare of the of the distance Rdistance R..

Example 5.Example 5. A A 400400--cdcd light is located light is located 2.4 m2.4 m from a tabletop of area from a tabletop of area 1.2 m1.2 m22. What is the . What is the illumination and what flux illumination and what flux F F falls on the table?falls on the table?

R2 2

400 cd(2.40 m)

IER

E = 69.4 lxIllumination:Illumination:

Now, recalling that Now, recalling that E = F/AE = F/A, we find , we find FF from:from:

F = EA = F = EA = (69.4 lx)(1.20 m(69.4 lx)(1.20 m22)) F = 93.3 lm

The Inverse Square RelationshipThe Inverse Square Relationship

2

IER

E/9

E/4

E3 m

2 m1 m

1 m2

4 m2

9 m2

If the intensity is 36 lx at 1 m, it will be 9 lx at 2 m and only 4 lx at 3 m.

SummarySummaryLight may be defined as electromagnetic radiation that is capable of affecting the sense of sight. LightLight may be defined as electromagnetic radiation may be defined as electromagnetic radiation that is capable of affecting the sense of sight.that is capable of affecting the sense of sight.

• Rectilinear propagation• Reflection• Refraction c = fc = 3 x 108 m/sc = fc = 3 x 108 m/s

General Properties of Light:

Red, Red, 700 nm700 nm

Violet, Violet, 400 nm400 nm

Summary (Continued)Summary (Continued)

Luminous fluxLuminous flux is the portion of total radiant power is the portion of total radiant power that is capable of affecting the sense of sight.that is capable of affecting the sense of sight.

Extended Extended sourcesource

penumbrapenumbra

umbraumbra

The formation of shadows:The formation of shadows:

Summary (Continued)Summary (Continued)

Unit: lux (lx)FEA

Total flux: F = 4ITotal flux: F = 4I

Luminous intensity:FI

Unit is the candela (cd)

AR

The Steradian 2

AR

Summary (Cont.)Summary (Cont.)

RR

Area Area AA

Illumination, Illumination, EE2Illumination, IER

E/9

E/4

E3 m 2 m

1 m

1 m24 m2

9 m2

CONCLUSION: Chapter 33CONCLUSION: Chapter 33 Light and IlluminationLight and Illumination