Lecture: Light is a form of energy that is detected by the human eye. Depending on the conditions, light either appears to behave like a stream of particles (
photons ) and in others like a wave. (
See figures 1 & 2 )
Figure 1 - How Photons Move - http://www.cs.utexas.edu/~mechin/photon_mapping/img/photon_map.jpg
Figure 2 - Light Behaving Like A Wave and A Particle at the Same Time - http://static.neatorama.com/images/2012-11/wave-particle-quantum-physics.jpg
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Damn you, quantum physics! Just as we got used to the mind-boggling fact that light can act as either a wave OR as a particle , a new quantum physics experiment has shown that it can act like a wave AND a particle at the same time:
Now, for the first time, researchers have devised a new type of measurement apparatus that can detect both particle and wave-like behavior at the same time. The device relies on a strange quantum effect called quantum nonlocality, a counter-intuitive notion that boils down to the idea that the same particle can exist in two locations at once.
"The measurement apparatus detected strong nonlocality, which certified that the photon behaved simultaneously as a wave and a particle in our experiment," physicist Alberto Peruzzo of England's University of Bristol said in a statement. "This represents a strong refutation of models in which the photon is either a wave or a particle." " -
http://www.neatorama.com/2012/11/06/Quantum-Physics-Experiment-Shows-Light-Behaving-as-a-Wave-and-a-Particle-Simultaneously/ -
Alex • Tuesday, November 6, 2012 at 10:00 AM Unlike sound, a medium is not needed for light to propagate. This can be proven through the example of sunlight. Sunlight travels to the Earth through space. However, there are no mediums in space for any type of wave to travel through. This proves that a medium is not needed for light to travel. Light waves are transverse in nature. (See figure 3 )
Figure 3 - Transverse Wave - http://www.passmyexams.co.uk/GCSE/physics/images/transvers_waves_001.jpg
Transverse waves of light are electromagnetic waves. This in turn gives the conclusion that light is a particular form of radio wave and thus our eyes must be a form of radio receiver. The range of light visible to humans is a small part of the electromagnetic spectrum (See figure 4 ). The frequencies that a human can see in this spectrum are 400 THz to 750 THz (THz stands for Tera-Hertz. A Tera-Hert is 10 ₁₂ Hz ). These frequencies are perceived by the human eye as many different colours, with red at the lowest of frequencies and purple at the highest. White is produced when a mixture of these frequencies (colours) are mixed together. A perfect example of this is a r a i n b o w . The reason a rainbow appears after rain has fell and the sun then shows is because when white light from the sun passes through the falling rain drops, all the frequencies that are mixed together producing the white light are then split. When they split into their separate frequencies, a rainbow is produced with red (the lowest frequency ) at the top and purple (the highest frequency ) at the bottom. (See figure 5 )
Figure 4 - The Electromagnetic Spectrum
Figure 5 - Separate Frequencies of Light and the Result of Mixed Frequencies - http://msprinsendam.files.wordpress.com/2010/11/picture11.png
A rainbow goes in the order red , orange , yellow , green , blue , indigo , violet . When the white light that is produced by the sun splits, it mainly takes the form of the primary colours, but secondary colours are produced because of overlapping frequencies. For example red , red and yellow , yellow . But red and yellow make orange . So thus the first part of a rainbow is red , orange , yellow . With this being said however, each secondary colour still does have its own frequency component. Light that has the frequency 500 THz looks orange . (See figure 6 )
Figure 6 - Rare 'floating rainbow' - http://www.dailymail.co.uk/sciencetech/article-2155361/Rare-flying-rainbow-brightens-skies-southern-China.html
In a vacuum (somewhere like space ) light travels at a constant speed of 300, 000 km/s. It travels at a lower speed (but not much lower ) in air. However, in glass the speed is reduced by about 1/3. The change in velocity in the air/glass interface is why glass makes a useful material for lenses. The speed of light in air is about 1 million times larger than the speed of sound in air. The speed of light in air is 300, 000, 000 m/s. The speed of sound in air is 340 m/s. As known from previous lectures (as well as standard grade and higher physics ) the frequency of a wave is the number of complete cycles every second. The medium in which light travels through does not affect the frequency of the light wave. Also known from previous lectures (as well as standard grade and higher physics ) is that the speed (velocity ) of a wave is the wavelength of the wave multiplied by its frequency. In regards to the relationship between frequency, wavelength and velocity, the formula velocity = frequency x wavelength shows that if velocity changes, either wavelength or frequency must change also. The wavelength will change as frequency does not change when passing through a different medium. (See figure 7 )
Figure 7 - Wavelength Changes With Velocity - How the Frequency Does Not Change Even If the Medium It Travels Through Does
Going back to the subject of white light and its components, here are a few of the slides from the lecture's PowerPoint. (See figures 8, 9, 10, 11 and 12 )
Figure 8 - Components of "White" Light
Figure 9 - The Visible Light Spectrum
Figure 10 - The Visible Light Spectrum - 2
Figure 11 - The Visible Light Spectrum - 3
Figure 12 - The Sun's Spectrum
Waves with frequencies a little less than the human eye can perceive (waves with a wavelength longer than 700nm ) are known as infrared (IR ). An example of this is compact disc (CD ) lasers, which operate at 780nm. Waves with frequencies a little higher than the human eye can perceive (waves with a wavelength shorter than 400nm ) are known as ultraviolet (UV ). Energy in the sunlight at the Earth's surface contains about 50% of electromagnetic waves visible to the human eye, 3% ultraviolet and the rest is infrared . Many animals have different seeing capabilities from humans. For example: a dog cannot see colour; bee's can see UV ; Pit Viper's can see IR . Monochromatic light is very rare. Most light sources do not make use of it. One that does however is a street lamp. It allows us to effectively see in shades of grey and thus the world around us would appear as if we were completely colour blind. The image below basically shows text of what I have just said, but it also has two pictures on it that are useful in further explaining monochromatic light, which is why the picture has been placed in this blog. (See figure 13 )
Figure 13 - Narrow and Broad-band Light
To measure brightness of light, scientists use two measurement units known as candela and lumen . "Photographers don't need to use these measurements because they use light exposure meters that are calibrated (ultimately) against a scientific standard ." - From the PowerPoint lecture An objects brightness is a quality that is very subjective and depends on its reflectance, colour and that of its surroundings, as well as the illumination and the user's eye's adapted state. Light waves from sources in our surroundings are transmitted , reflected , absorbed , scattered and refracted by the atmosphere and objects around us. As in a mirror or a bright point, specular reflected light may appear as an image . An example of this is the highlight on a mirror ball. The type of light that shows no image or highlights (for example, the reflected light from a white sheet of paper) is diffuse reflected light, which scatters in many directions. "Adjacent objects may cast shadows on each other, or tend to colour one another other by their reflected light. " - From PowerPoint lecture In the viewer's perception, an image scene may be altered by the manipulation of the above effects. (See figure's 14, 15, 16 and 17 )
Figure 14 - Transmission and Reflection
Figure 15 - Specular Reflection
Figure 16 - Diffuse Reflection
Figure 17 - Examples of Reflections Using a Harley and a Rider's Helmet
An example below show's what should be done to accommodate the lighting conditions at sunset. (See figure 18 )
Figure 18 - To Accommodate Sunset Light
The final images below go on to explain how refraction works. (See figures 19, 20 and 21 )
Figure 19 - Refraction
Figure 20 - Refraction Continued
Figure 21 - Refraction Continued
"Using our knowledge of the properties of light and its modification due to the environment, we can use the power of modern digital image manipulation software to create mood and environmental effects or to compensate them. "Thus we can alter the viewer's perception of the image scene, to achieve a desired effect ." - From PowerPoint lecture.
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