Light

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Light
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The Physics Classroom

• http//www.physicsclassroom.com/Class/light/lightt
  oc.html

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Light

• Light is the range of electromagnetic frequencies
  that stimulates the retina of the eye
• Range from 400 nm to 700 nm
• Red light 700 nm
• Violet light 400 nm

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Light

• Light travels using the Ray Model
• Travels in a straight line
• Light can be bent refracted
• Passes through different densities
• Eye glasses

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Speed of Light

• Pre 1600s thought to travel instantaneously
• Galileo hypothesizes that light had finite speed.
• Albert Michelson first to develop an Earth
  based system to measure speed of light Nobel
  prize

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Speed of Light

• C 299 792 458 m/s
• (round to 3 x 108 m/s)

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Homework

• Read Chapter 16 to page 376
• P. 376 1-5

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Sources of Light

• Luminous light emitting body
• Sun, star, light bulb, glow stick
• Illuminated body that reflects light
• Planets, mirror, bicycle reflector

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Sources of Light

• Luminous Flux the rate at which light is
  emitted from a light source
• Lumen(lm) unit of luminous flux
• Ex a typical 100-watt bulb 1750 lm
• Illuminance (E) illumination of a surface

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Inverse Square Relationship

• Illuminance of a surface varies inversely as the
  square of its distance from a light source (p.
  379)

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Luminous Intensity

• Some light sources are specified in candela, cd,
  or candle power
• Measurement of luminous intensity not luminous
  flux

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How to Illuminate a Surface

• There are two ways to increase the illumination
  on a surface
• 1. Use brighter light source
• 2. Move light source closer to surface
• Mathematically the illuminace is represented by
• E P
• 4 ?d2

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Sample Problem

• What is the illumination on your desktop if it is
  lighted by a 1750-lm lamp that is 2.50 m above
  the desk?
• P 1750 lm (luminous flux), D 2.50 M
• E P , 1750 LM
• 4 ?d2 4? (2.50)2 22.3 LX

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Homework

• Read to p. 381
• P 381 6-11

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Light Matter

• Transparent materials that allow light to pass
  through with out distorting the images
• Translucent materials that allow light to pass
  through but distorts the image
• Opaque does not allow light to pass through

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Color

• Visible light is made up of a range of
  frequencies making up the visible spectrum
• Each of these colors has its own properties
• Colors can be separated by use of a prism

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Color

• Primary colors red, green, blue
• Red blue green white
• Secondary colors made by adding primary colors
  in pairs
• Red green yellow
• Blue green cyan
• Red blue magenta

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Color

• Color by addition is used in color television

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Color

• Color by subtraction
• Use of dyes pigments to produce colors
• When an object is colored (dyed) a pigment is
  added to the substance that reflects that color
  of light.
• All other light is absorbed

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Homework

• Read to the end of the chapter
• P 389 Reviewing Concepts
• 10-22
• Weekend HW Problems p 390
• 31 - 39

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Law of Reflection

• The angle of incidence equals the angle of
  reflection

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Law of Reflection

• Law of Reflection always applies

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Law of Reflection
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Law of Refraction

• When light changes media with different densities
  at an angle other than the normal, the light will
  change direction as it changes velocity.

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Law of Refraction

• When changing from a less dense media to a more
  dense media at an angle other than the normal,
  the light will refract towards the normal.

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Law of Refraction

• When changing from a more dense media to a less
  dense media at an angle other than the normal,
  the light will refract away from the normal.

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Snells Law

• The ratio of the sine of the angle of incidence
  to the sine of the angle of refraction is
  constant.
• The constant n is called the index of
  refraction.

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Snells Law

• Ni sin ?i Nr sin ?r

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Homework

• P 400 1-4