Observed Properties of Light

Transcription

1 It would seem very strange if there were not a sharp distinction between objects and waves in our everyday world. Yet this appears to be the nature of LIGHT.

2 Observed Properties of Light

3 Observed Properties of Light Shadows (rays of light travel in straight lines) Speed (the highest known) Intensity (brightness) Reflection (bouncing off surfaces) Transmission/absorption (coming/not coming through substances) Refraction (slowing down in media) Diffraction (bending at an opening or edge) Interference (adding/subtracting intensity) Color and dispersion (white=all colors combined)

4 Speed of Light Measurement History Empedocles (finite) vs Aristotle (infinite) 1638, Galileo (two lanterns): extraordinarily rapid 1676, Ole Roemer (moons of Jupiter): 214,000 km/s 1704, Isaac Newton: different colors travel same speed 1729, James Bradley (stellar aberration): 301,000 km/s 1849, Hippolyte Fizeau (toothed wheel): 314,000 km/s 1862, Léon Foucault (rotating mirror): 299,796 km/s (laser method): 299,792 km/s 1983, the 17th CGPM: defined as exact constant

5 Speed of Light The speed of light in a vacuum, denoted c, is constant throughout the Universe. c is the maximum speed at which all matter and information in the Universe can travel. c = 299,792,458 meters/second (~186,000 mps) Scale sense: it takes ~8 minutes for light to travel all the way from the Sun to the Earth. When light travels through matter, its speed can change, but can never be larger than c (inside a diamond, light is slowed down to less than 80,000 mps).

6 Decomposition of Sunlight Isaac Newton, 1665 Common (Aristotle) wisdom: white light is the purest form - colored light must therefore have been altered somehow Newton shined a beam of sunlight through a glass prism and showed that it decomposed into a spectrum cast on the wall therefore all the colors were together in the sunlight. He thought he then should be able to combine the colors of the spectrum and make the light white again: he placed another prism upsidedown in front of the first prism. The band of colors combined again into white sunlight. Newton was the first to prove that white light is made up of all the colors that we can see.

7 Invisible rays" in this area had the highest temperature of all. First time anyone had demonstrated that there were forms of radiation that humans couldn't see. Infrared Light Discovery Friedrich Herschel, 1800 Measured temperature of different colors of light. Observed the increase in temperature as he moved the thermometer from violet through blue, green, yellow, and orange to red where it reached its peak and moved the thermometer just outside the red portion of the spectrum in an area that to the human eye contained no light at all. Infrared (from Latin below )

8 Ultraviolet Light Discovery Johann Ritter, 1801 Measured the effect of different colors of light on a light-sensitive chemical, silver chloride. In the red portion of the spectrum darkening of the chemical was relatively slow. Progressing through orange, yellow, green, blue, and violet, he observed that each new batch of silver chloride grew darker faster and placed the chemical just outside the violet portion of the spectrum in an area that to the human eye contained no light at all Invisible rays" in this area had the greatest effect (fastest darkening) of all. Same experiment can be done using a sheet of photographic paper. Ultraviolet (from Latin beyond ) Prism

9 Rays of Light what are they made of?

10 Double-Slit Experiment Thomas Young, 1803 Light passing through two parallel slits will interfere, producing a pattern of bright and dark fringes. wave-like behavior

11 Arago Spot (aka Fresnel bright spot or Poisson spot) A bright on-axis spot exists in the shadow of a circular obstacle blocking a point source of light. 1817, Augustin Fresnel presented his wave theory of light. Simeon Poisson (the leading opponent) predicted the spot based on Fresnel s theory and declared this result absurd as there should be complete darkness at the shadow center according to the particle theory of light Dominique Francois Arago carefully performed the proposed experiment and succeeded in observing the spot! Caused by diffraction of light. convinced most scientists of the wave-nature of light

12 Light as a Wave wavelength wavelength

13 Photoelectric Effect The photoelectric effect is the emission, or ejection, of electrons from the surface of a material (usually metal) in response to incident light. Heinrich Hertz, 1887: discovered that electrodes illuminated with ultraviolet light create electric sparks more easily (i.e. at lower applied voltage). Philipp Lenard, 1902: demonstrated that electrons are liberated from a metal surface when it is illuminated. For each material, it occurs only for light beyond a certain color. Infrared no electrons Visible/Ultraviolet electrons may be emitted X-rays electrons always emitted Gamma -rays Could not be explained by classical physics (light as an electromagnetic wave). Einstein, 1905: photons, the particles of light (1921 Nobel Prize in Physics).

14 Nature of Light Debate Isaac Newton, 1675: light is made of particles of energy (corpuscles). Explained reflection, shadows, traveling in straight lines. Christiaan Huygens, 1678: light is made of waves in ether. Explained diffraction, interference. Michael Faraday, 1847: light is a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium. Nothing is too wonderful to be true if it be consistent with laws of Nature. Albert Einstein, 1905: a beam of light is not a continuous wave propagating through space, but rather a collection of discrete wave packets, photons. James Maxwell, 1864: light is an electromagnetic wave.