The Nature of Light

The Physics Hypertextbook™
© 1998-2008 by Glenn Elert -- A Work in Progress
All Rights Reserved -- Fair Use Encouraged

prev | up | next

---

Discussion

introduction

Light is a transverse, electromagnetic wave that can be seen by humans. The wave nature of light was first illustrated through experiments on diffraction and interference. Like all electromagnetic waves, light can travel through a vacuum. The transverse nature of light can be demonstrated through polarization.

• In 1678, Christiaan Huygens (1629-1695) published Traité de la Lumiere, where he argued in favor of the wave nature of light. Huygens stated that an expanding sphere of light behaves as if each point on the wave front were a new source of radiation of the same frequency and phase.
• Thomas Young (1773-1829) Augustin Fresnel (1788-1827) disproved Newton's corpuscular theory.

sources

Light is produced by one of two methods …

• Incandescence is the emission of light from "hot" matter (T ≳ 800 K).
• Luminescence is the emission of light when excited electrons fall to lower energy levels
  (in matter that may or may not be "hot").

speed

Just notes so far. The speed of light in a vacuum is represented by the letter c from the Latin celeritas -- swiftness. Measurements of the speed of light.

Galileo Galilei (1564-1642) Italy (Tuscany): In fact I have tried the experiment only at a short distance, less than a mile, from which I have not been able to ascertain with certainty whether the appearance of the opposite light was instantaneous or not; but if not instantaneous it is extraordinarily rapid.

Ole Christensen Rømer (1644-1710) Denmark. "Démonstration touchant le mouvement de la lumière trouvé par M. Roemer de l'Académie des Sciences." Journal des Scavans. 7 December 1676. Rømer's idea was to use the transits of Jupiter's moon Io to determine the time. Not local time, which was already possible, but a "universal" time that would be the same for all observers on the earth, Knowing the standard time would allow one to determine one's longitude on the earth -- a handy thing to know when navigating the featureless oceans.

Unfortunately, Io did not turn out to be a good clock. Rømer observed that times between eclipses got shorter as earth approached Jupiter, and longer as earth moved farther away. He hypothesized that this variation was due to the time it took for light to travel the lesser or greater distance, and estimated that the time for light to travel the diameter of the Earth's orbit, a distance of two astronomical units, was 22 minutes.

• The speed of light in a vacuum is a universal constant in all reference frames.
• The speed of light in a vacuum is fixed at 299,792,458 m/s by the current definition of the meter.
• The speed of light in a medium is always slower the speed of light in a vacuum.
• The speed of light depends upon the medium through which it travels.The speed of anything with mass is always less than the speed of light in a vacuum.

other characteristics

The amplitude of a light wave is related to its intensity.

• Intensity is the absolute measure of a light wave's power density.
• Brightness is the relative intensity as perceived by the average human eye.

The frequency of a light wave is related to its color.

• Color is such a complex topic that it has its own section in this book.
• Monochromatic light is described by only one frequency.
  • Laser light is effectively monochromatic.
  • There are six simple, named colors in English (and many other languages) each associated with a band of monochromatic light. In order of increasing frequency they are red, orange, yellow, green, blue, and violet.
  • Light is sometimes also known as visible light to contrast it from "ultraviolet light" and "infrared light"
  • Other forms of electromagnetic radiation that are not visible to humans are sometimes also known informally as "light"
• Polychromatic light is described by many different frequencies.
  • Nearly every light source is polychromatic.
  • White light is polychromatic.

A graph of relative intensity vs. frequency is called a spectrum (plural: spectra).
Although frequently associated with light, the term can be applied to any wave phenomena.

• A continuous spectrum is one in which every frequency is present within some range.
  • Blackbody radiators emit a continuous spectrum.
• A discrete spectrum is one in which only a well defined set of isolated frequencies are present.
  (A discrete spectrum is a finite collection of monochromatic light waves.)
  • The excited electrons in a gas emit a discrete spectrum.

The wavelength of a light wave is inversely proportional to its frequency.

• Light is often described by it's wavelength in a vacuum.
• Light ranges in wavelength from 400 nm on the violet end to 700 nm on the red end of the visible spectrum.

Phase differences between light waves can produce visible interference effects.
(There are several sections in this book on interference phenomena and light.)

Leftovers about animals.

• Falcon can see a 10 cm. object from a distance of 1.5 km.
• Fly's Eye has a flicker fusion rate of 300/s. Humans have a flicker fusion rate of only 60/s in bright light and 24/s in dim light. The flicker fusion rate is the frequency with which the "flicker" of an image cannot be distinguished as an individual event. Like the frame of a movie … if you slowed it down, you would see individual frames. Speed it up and you see a constantly moving image. Octopus' eye has a flicker fusion frequency of 70/s in bright light.
• Penguin has a flat cornea that allows for clear vision underwater. Penguins can also see into the ultraviolet range of the electromagnetic spectrum.
• With eyes mounted laterally on their heads, pigeons can view 340 degrees … everywhere except in back of their heads.
• Sparrow Retina has 400,000 photoreceptors per square. mm.

Summary

• Light is a transverse, electromagnetic wave that can be seen by humans.
  • The wave nature of light was first illustrated through experiments on diffraction and interference.
  • Like all electromagnetic waves, light can travel through a vacuum.
  • The transverse nature of light can be demonstrated through polarization.
  • Light is sometimes also known as visible light to contrast it from "ultraviolet light" and "infrared light".
  • Other forms of electromagnetic radiation that are not visible to humans are sometimes also known informally as "light"
• Light is produced by one of two methods.
  • Incandescence is the emission of light from "hot" matter (T ≳ 800 K).
  • Luminescence is the emission of light when bound electrons fall to lower energy levels.
• The speed of light depends upon the medium through which it travels.
  • The speed of light in a vacuum is a universal constant in all reference frames.
  • All electromagnetic waves propagate at the speed of light in a vacuum.
  • The speed of light in a medium is always slower the speed of light in a vacuum.
    (The difference is usually negligible when the medium is air.)
  • The speed of anything with mass is always less than the speed of light in a vacuum.
    (The speed of light in a vacuum is the universal speed limit.)
  • The speed of light in a vacuum is fixed at 299,792,458 m/s by the current definition of the meter.
• The amplitude of a light wave is related to its intensity.
  • Intensity is the absolute measure of a light wave's power density.
  • Brightness is the relative intensity as perceived by the average human eye.
• The frequency of a light wave is related to its color.
  • Color is such a complex topic that it has its own section in this book.
  • Monochromatic light can be described by only one frequency.
    • Laser light is very nearly monochromatic.
    • There are six simple, named colors in English (and many other languages) each associated with a band of monochromatic light. In order of increasing frequency they are red, orange, yellow, green, blue, and violet.
  • Polychromatic light is compused of multiple frequencies.
    • Every light source is essentially polychromatic.
    • White light is very polychromatic.
  • A graph of relative intensity vs. frequency is called a spectrum (plural: spectra).
    Although frequently associated with light, the term can be applied to many phenomena.
    • A continuous spectrum is one in which every frequency is present within some range.
      • Blackbody radiators emit a continuous spectrum.
    • A discrete spectrum is one in which only a set of well defined and isolated frequencies are present.
      (A discrete spectrum is a finite collection of monochromatic light waves.)
      • The excited electrons in a gas emit a discrete spectrum.

  condition	description	spectrum
  hotter than red hot	incandescent	continuous
  excited electrons	luminous	discrete

• The wavelength of a light wave is inversely proportional to its frequency.
  • Light is often described by it's wavelength in a vacuum.
  • Light ranges in wavelength from 400 nm on the violet end to 700 nm on the red end of the visible spectrum.
    • Wavelengths slightly shorter than 400 nm are said to be ultraviolet.
      (They are "beyond violet" in terms of frequency.)
    • Wavelengths slightly longer than 700 nm are said to be infrared.
      (They are "below red" in terms of frequency.)
• Phase differences between light waves can produce visible interference effects.
  (There are several sections in this book on interference phenomena and light.)

Problems

practice

1. Use Rømer's method and Rømer's numbers to determine the speed of light in a vacuum.

   If you're comfortable reading Seventeenth Century French, here's the paragraph that reports Rømer's measurement of a 22 minute delay as the light from Jupiter's moon Io traverses the extra distance equal to the diameter of earth's orbit (represented by HE on a diagram in the report).

   Il ne s'ensuit pas pourtant que la lumière ne demande aucun temps : car après avoir examiné la chose de près, il a trouvé que ce qui n’était pas sensible en deux révolutions devenait très considérable à l'égard de plusieurs prises ensemble, et que par exemple quarante révolutions, observées du côté F, étaient sensiblement plus courtes que quarante autres, observées de l'autre côté en quelque endroit du zodiaque que Jupiter se soit rencontré ; et ce à raison de 22 pour tout l’intervalle HE, qui est le double de celui qu’il y a d'ici au soleil [expand].

   I couldn't find any astronomical measurements from Rømer's day, so here are the currently accepted values.

   	jupiter	earth
   semimajor axis (106 km)	778.57	149.60

   • Answer it.
2. A common measure of astronomical distances is the light-year. This is the distance a beam of light would travel in a vacuum in one year. Determine the size of a light-year in meters.
   • Answer it.
3. The Speed of Dark
   What if one fine evening, as the sun was setting and a full moon was rising, the sun suddenly quit emitting light?
   1. How soon after the sun went black would we know about it on earth?
   2. How soon before or after we saw the sun go dark would the moon cease shining? (It is best to start this problem by drawing a sketch showing the relative positions of the three bodies.)
   3. What is the speed of dark?
   • Answer it! What is the speed of dark?
4. Write something completely different.
   • Answer it.

numerical

1. Here is an excerpt from Galileo's report of his attempt to determine the speed of light in a vacuum.

   Let each of two persons take a light contained in a lantern, or other receptacle, such that by the interposition of the hand, the one can shut off or admit the light to the vision of the other. Next let them stand opposite each other at a distance of a few cubits and practice until they acquire such skill in uncovering and occulting their lights that the instant one sees the light of his companion he will uncover his own …. Having acquired skill at this short distance let the two experimenters, equipped as before, take up positions separated by a distance of two or three miles and let them perform the same experiment at night, noting carefully whether the exposures and occultations occur in the same manner as at short distances; if they do, we may safely conclude that the propagation of light is instantaneous; but if time is required at a distance of three miles which, considering the going of one light and the coming of the other, really amounts to six, then the delay ought to be easily observable ….
   
   In fact I have tried the experiment only at a short distance, less than a mile, from which I have not been able to ascertain with certainty whether the appearance of the opposite light was instantaneous or not; but if not instantaneous it is extraordinarily rapid …. [expand]

   1. Estimate the time for a light wave to travel the distance in Galileo's speed of light experiment. (Buona nota: Un miglio italiano è di 1,873 chilometri.)
   2. How does this compare to the reaction time of a typical human?
2. Determine the size of a …
   1. light-day
   2. light-hour
   3. light-minute
   4. light-second
   5. light-millisecond
   6. light-microsecond
   7. light-nanosecond
   8. light-picosecond
3. Cutting lasers now exist that emit pulses of laser light so brief that they must be measured in femtoseconds. One particular femtosecond laser emits radiation with a wavelength of 1053 nm for only 350 fs.
   1. What type of electromagnetic radiation does this laser emit?
   2. How many wavelengths of this radiation are present in each pulse?
4. Read the following passage from the Lunar and Planetary Institute website.

   
   Source: NASA

   The Laser Ranging Retroreflector experiment was deployed on Apollo 11, 14, and 15. It consists of a series of corner-cube reflectors, which are a special type of mirror with the property of always reflecting an incoming light beam back in the direction it came from. A similar device was also included on the Soviet Union's Lunakhod 2 spacecraft. These reflectors can be illuminated by laser beams aimed through large telescopes on Earth. The reflected laser beam is also observed with the telescope, providing a measurement of the round-trip distance between Earth and the Moon. This is the only Apollo experiment that is still returning data from the Moon ….

   Laser beams are used because they remain tightly focused for large distances. Nevertheless, there is enough dispersion of the beam that it is about 7 kilometers in diameter when it reaches the Moon and 20 kilometers in diameter when it returns to Earth. Because of this very weak signal, observations are made for several hours at a time. By averaging the signal for this period, the distance to the Moon can be measured to an accuracy of about 3 centimeters (the average distance from the Earth to the Moon is about 385,000 kilometers).

   Determine …
   1. the angular spread of the laser beam used in this experiment
   2. the round trip light time for the laser pulse sent to the moon
   3. the error in the round trip light time measurements …
      1. in absolute terms (in nanoseconds)
      2. in relative terms (in parts per trillion)

investigative

1. Determine the one-way transit time for a signal sent from the earth to …
   1. a satellite in geosynchronous orbit
   2. the moon
   3. the sun
   4. mars
   5. saturn
   6. Voyager 1 and/or Voyager 2
   7. Proxima Centauri
   8. the Large Magellanic Cloud
   9. the Andromeda Galaxy (a.k.a. M81)
   10. the edge of the observable universe
2. What is your height in light-nanoseconds?

Resources

• general
  • Lightwave: Light Education Page, University of Southampton
  • The Mechanical Universe and Beyond (video on demand, login required)
    • Optics, Many properties of light are properties of waves, including reflection, refraction, and diffraction.
• speed of light
  • Data and Story Library
    • Estimating the Speed of Light Story and Speed of Light Datafile, Simon Newcomb
    • Speed of Light Story and Michelson Datafile, A. A. Michelson, 1879
  • Démonstration touchant le mouvement de la lumière, Ole Rømer, Journal des Sçavans. Lundi 7 Décembre 1676: 223-236 [Club Astro Centrale Paris].
  • Light May Have Slowed Down, New Scientist, 15 August 2001
  • Speed of Light, University of Virginia
• laser ranging
  • Apollo 11 Laser Ranging Retroreflector Experiment, Lunar and Planetary Institute
  • International Laser Ranging Service, NASA Goddard Space Flight Center
  • McDonald Laser Ranging Station, University of Texas McDonald Observatory
• infrared
  • near infrared
• ultraviolet
  • Sunglass Blues, Nigel Bunce & Jim Hunt, University of Guelph

---

prev | up | next

Another quality webpage by Glenn Elert

home | contact bent | chaos | eworld | facts | physics