Light travels in straight lines. We all know that from Primary Science. Mo Tzu (470-391 BC) in ancient China knew that when he saw an upside-down image of the outside view on his wall when light rays from outside entered and crossed a pin hole in some blinds. Our primary school teacher also told us that non-luminous objects bounce off light from light sources to our eyes. This knowledge is traced back to Alhazen (AD 965-1038) from the Islamic empire.
Light travels in straight line... Light is bounced off non-luminous objects...
Ancient Greek scholars saw they could use geometry to study light rays because these rays are straight. Euclid (325-265 BC) showed that rays are reflected off the mirror at the same angle as they incident it. Ptolemy (AD 85-165) found that light refracts or bends when it enters a transparent material. Such are covered in our Lower Secondary Science. In O Level physics, trigonometry began to creep into our study of science. An example is the relationship sin i / sin r = constant which is called the law of refraction or Snell's Law. This experimental law was named after Willebrord Snellius (1580–1626) although Ibn Sahl (c. 940–1000) described the law earlier than Snellius. Christian Huygens, by drawing the wave model of refraction, derived the expression:
sin i / sin r = speed of light in vacuum / speed of light in material medium
In 1664, Isaac Newton experimented with a prism and realized that white light from the Sun is composed of a continuous spectrum of colors, contrary to the belief in those days that colors were mixtures of darkness and lightness. This is called the dispersion of white light which we were exposed to in Lower Secondary science. I've briefly described how rainbows are formed here.
White light is made up of a spectrum of colours...
But in O level, we learned there are other invisible light. These were discovered by different people at different times. William Herschel (1738-1822) found infrared when he measured the temperature of each color and saw that the thermometer reading increased when the thermometer was moved to the area beyond the red end. This is the invisible light that makes sunlight feel warm to our skin. Johann Ritter (1776-1810) successfully detected ultraviolet that is the invisible light beyond the violet end by exposing silver nitrate to it. The exposure causes the silver ion to be reduced to silver metal which is dark in color.
Infrared is heat...
In O Level, physics teachers taught us that light is composed of waves. This, so called classical theory of light, was born out of a heated debate between two scientists. Newton believed that light was made of particles he called corpuscles while Christian Huygens (1629-16930) argued that light travels as waves. These scientists used their ideas to explain reflection and refraction. Because Newton was more powerful in the field of science, more scientists believed in him. It took others to prove Huygens' wave theory of light after Huygens' death. In 1801, Thomas Young (1773-1829) showed that when light is directed onto two slits in a piece of card, the light spreads out and produces a series of bright and dark bands (interference pattern) on a screen. This experiment shows two phenomena, diffraction and interference, which could be explained by considering light as waves.
Light waves undergo diffraction...
In 1808, Malus, a French colonel, looked through a crystal and found that it became brighter or darker as it was turned. This is polarization. Augustin Fresnel (1780-1827), a French army engineer, explained this phenomenon by suggesting that light waves vibrate in all directions and the microscopic slots in the crystal cancel out the vibrations that are not lined up with the slots. This leads to the idea that light consists of transverse, not longitudinal, waves.
Light waves can be polarised....
Does light have a finite speed? Aristotle thought light reaches us in no time at all. Ole Romer (1644-1710), an astronomer, found that the further Jupiter was from the Earth, the longer it took for light from its moons to reach him. This shows that light does have a finite speed. What's that speed? Several scientists devised ingenious ways to find it. Jean Foucault (1819-1868) bounced a light beam using a fast-spinning mirror to a stationary mirror. The stationary mirror reflected the light back to the spinning mirror. By then, the mirror would have moved through an angle. By finding this angle, Foucault found the speed of light to be 298,000 km/s.
Light has finite speed...
James Clerk Maxwell (1831-1879) came out with four extremely complicated equations that describe electric field and magnetic field and their interactions. The last equation carries the product of two constants called permittivity ε0 = 8.85 x 10-12 F/m (which indicates how polarized a medium is when subject to electric field) and permeability µ0 = 1.257 x 10 -6 H/m (which indicates the degree of magnetization of a material when subject to a magnetic field) of vacuum. If you calculate the square root of the reciprocal of the product of these constants, you will get a value for the speed of light! It implies that light is made up of an electric field component and a magnetic field component.
Light is electromagnetic waves...
That is why light is called electromagnetic waves. This mathematical result opened up to other scientists to discover other electromagnetic waves in the spectrum: radio waves discovered by Heinrich Hertz (1857-1894), X-rays by Wilhelm Rontgen (1845-1893), gamma rays by Paul Villard (1860-1934), microwaves by Percy Spencer (1894-1970).
While scientists were certain of the wave nature of light, the idea of light as particles re-emerged when Max Planck (185801947) tried to explain Gustav Kirchoff's experimental graph of intensity of light against wavelength of light from a red-hot glowing metal. Scientists found that the shape of the graph couldn't be satisfactorily explained if the body was seen to be emitting any wavelength. Max Planck proposed that light could only be emitted in discrete units (which were later called photons, not corpuscles) given by E = hf (E = energy of photon, h = Planck constant = 6.63 × 10-34 J/s and f = frequency of light). This equation, which is to be learned in A Level physics, marked the birth of quantum physics.
Light consists of photons...
Albert Einstein (1879-1955) then used Planck's idea to explain Philip Lenard's (1862-1947) experiment of shining bright light onto metals causing electrons to be emitted from the metal's surface. This phenomenon was called photoelectric effect, another important concept in A Level. The strange result was that no matter how intense you shine a certain wavelength of light onto a certain metal, the maximum kinetic energy of an electron emitted is the same. Using Planck's idea of light as packets of energy, he came out with the following equation to explain photoelectric effect:
hf = KEmax
You might be tempted to say that Isaac Newton was right after all. But light is like Dr Jekyll and Mr Hyde. Sometimes, they behave as waves, sometimes, they are particles. Newton did not thought of light like that.
Last but not least, Niels Bohr (1885-1962) created a model of the atom to explain why hydrogen emits or absorbs only certain wavelengths of light. He reasoned that the electron in an atom orbits around the nucleus in certain energy levels. The electron can absorb a certain photon and be excited to a certain energy level. The excited electron can also give out the same photon and go back to the previous energy level.
E2 - E1 = hf (E1 = ground state energy level, E2 = excited state energy level)
Electrons absorb and emit photons...
Enjoy this video which covers almost the same thing I have just written: http://www.youtube.com/watch?v=OLCqaWaV6jA
(Chinese translations: Diffraction 衍射, Dispersion 分散, Electric field 电场, Electromagnetic spectrum 电磁波谱, Electron 电子, Gamma rays 伽玛射线, Infrared 红外线, Interference 干涉, Jupiter 木星, Longitudinal waves 纵波, Magnetic field 磁场, Microwaves 微波, Non-luminous 非发光, Nucleus 核子, Phenomenon 现象, Photoelectric effect, Photon 光子, Polarization 极化, Radio waves 无线电波, Reciprocal, Refraction 折射, Silver nitrate 硝酸银, Spectrum, Transparent 透明, Transverse waves 横波, Trigonometry 三角函数, Ultraviolet, Vacuum 真空, Wavelength 波长, X rays X射线)
Excellent expository!
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