Transcript: Light and all other forms of electromagnetic radiation travel at 300 thousand kilometers per second or 186 thousand miles per second. This is the speed of light denoted by the small letter “c”. The speed of light is so fast that it was not possible to measure it in ancient times. Four hundred years ago Galileo tried to measure it by positioning a friend on a hill top five miles away with a lantern. Galileo also had a lantern with a shutter. Galileo opened the shutter of his...

Transcript: Faraday showed that the forces of electricity and magnetism were related, but what did this have to do with light? The answer was provided in the 19th century by the Scottish physicist James Clark Maxwell. Maxwell was a theorist who produced an elegant theory of light and electromagnetic radiation. He had a set of four equations relating the two forces of electricity and magnetism, and he showed that changing electric or magnetic forces create a disturbance that behaves like a...

Transcript: Michael Faraday was a brilliant, self taught, English physicist who lived about two hundred years ago. He rose from being a book binder’s apprentice to the director of the Royal Institution in London, the foremost scientific society of its age. Faraday was a brilliant experimenter and a brilliant communicator, conducting a series of public lectures at Christmastime that were extremely popular and continue to the present day two hundred years later. Faraday did an elegant set of...

Transcript: What do the forces of electricity and magnetism have to do with each other? At first sight they could not appear more different. Magnetism is the force that takes the metal in a compass needle and can align it with the magnetic field of the Earth. Electricity might be familiar as a force which when you rub a balloon on your head will cause it to stick to the ceiling against the force of gravity. Yet several hundred years ago, Michael Faraday showed that electricity and...

Transcript: The distinction between intrinsic radiation and reflective radiation is very important. The Sun and all the stars, we see in their intrinsic thermal radiation, which because of their high surface temperature of thousands of degrees Kelvin comes out in the visible part of the spectrum. However the moon, the planets, and all the rocky objects of the solar system are seen in reflected radiation from the Sun, not their own intrinsic radiation. The albedo of an object is defined as...

Transcript: In astronomy it’s important to distinguish between the intrinsic radiation or color of an object and the reflected radiation or color from an object. In the everyday world most objects we see are seen in the reflected light of something very hot, either sun light or the hot filament from an incandescent light for example. So the fundamental source of the radiation is a hot object at a temperature of several thousand Kelvin. A blue book is not hotter then a red book; it is just...

Transcript: We’re used to viewing the world through light, through visible radiation, but visible radiation is only intrinsically emitted from objects that are thousands of degrees Kelvin. Most objects we are surrounded with, the temperature of the Earth itself, is several hundred degrees Kelvin. According to Wien’s Law, thermal radiation from objects at several hundred degrees is 20 times smaller then thermal radiation from the surface of the Sun. This is infrared radiation emerging as...

Transcript: Wien’s Law states that the wavelength of the maximum emission in a thermal spectrum is inversely proportional to the temperature. The mathematical form of this relationship gives the dominate wavelength in a thermal spectrum, although it should be remembered that a thermal spectrum is very broad and spans an order of magnitude more in wavelength. For example, the surface of the Sun is at a temperature of about 5,700 Kelvin. Using Wien’s Law says that the peak of the thermal...

Transcript: Everything with a temperature, which is to say every object in the physical universe, emits a thermal spectrum, a broad spectrum of radiations that result directly from the random microscopic motions of atoms or molecules. This thermal radiation is also sometimes called a blackbody spectrum because of the way that physicists produce such a spectrum in the laboratory by creating a hollow cavity, lining the cavity with black so that radiation emitted through a small entrance...

Transcript: The concept of thermal radiation is fundamental to our understanding of energy and temperature. The basic principles were enunciated a hundred years ago by the German physicist Wilhelm Wien. First, all objects in the universe emit a thermal spectrum spanning a broad range of wavelengths. Second, the amount of radiation and the wavelength of the maximum energy of emission depend on the temperature of an object but not on its chemical composition, and third, the higher the...

Transcript: How is the Kelvin temperature scale related to the velocity of particles in a gas? The theory was worked out a hundred and fifty years ago by James Clark Maxwell and Ludwig Boltzmann. Essentially we are relating the kinetic energy of a particle (0.5mv2) to the temperature measured on the Kelvin scale. The constant of proportionality is called the Boltzmann constant. It has a very small numerical value, 1.4 times 10-23 joules per Kelvin. Using this scaling we can see that the...

Transcript: In the Celsius temperature scale, zero degrees corresponds to the freezing point of water. However the molecules in ice are still moving microscopically, so there is thermal energy in the substance. It’s possible to be much colder then ice. Scientists therefore use a temperature scale where zero corresponds to the absolute cessation of microscopic motion. Zero on the Kelvin scale is much lower then zero on the Celsius scale. Zero on the Kelvin scale corresponds to minus 273...

Transcript: Scientists use a different temperature scale from the one you’re probably used to. The Fahrenheit temperature scale is an archaic system of units that has been abandoned by most scientists. It was invented almost 300 years ago. Scientists and most Europeans use the Celsius measurement for temperature where the boiling point of water is 10 degrees Celsius or centigrade and the freezing point of water is zero. The Celsius scale essentially takes the difference between the...

Transcript: We all have an idea of the concepts of hot and cold, but what aspect of matter does temperature really measure? Temperature measures the microscopic motions of atoms or molecules in any substance. The higher the temperature, the faster the random microscopic motions. This is the scientific definition of temperature that applies on the microscopic scale and the macroscopic scale. Note however that thermal energy and temperature are not the same thing. A drop of boiling water...

Transcript: The visible spectrum is just one slice of a much larger range of radiations. Nearly two hundred years ago two scientists demonstrated this. Around the year 1800, William Herschel took a spectrum of sunlight and placed a thermometer beyond the red end of the spectrum. The temperature rose, demonstrating that energy existed beyond the visible end of the spectrum. A couple of years later, German chemist Johann Ritter placed a sheet of paper soaked in silver-chloride beyond the...

Transcript: Newton was the first to describe the components of radiation emitted by the sun. He took the sun’s light and dispersed it in wavelength with a prism and created the visible spectrum. The visible spectrum runs through the colors of the rainbow: red, orange, yellow, green, blue, indigo, violet, often creating the mnemonic Roy G. Biv. This sequence from red to blue is also a sequence of decreasing wavelength and increasing frequency.

Transcript: Radiation is another mode of heat transfer or way to move energy from one place to another. Every object that has a temperature emits thermal radiation. Unlike the case of convection or conduction, radiation can occur through the vacuum of space. It is the Sun’s radiation that stops the Earth from being in a deep freeze. Radiation is one of the most fundamentally important concepts in astronomy.

Transcript: Convection is heat transfer through the motion of masses of material. It’s a very efficient way to transfer a thermal energy. If you poured boiling water into a bath and just waited, the extra kinetic motion of the molecules in the boiling water would eventually diffuse through the bath, heating up the bath, but if you swirled around the water with your hand the bath would heat up more quickly. The swirling of the water corresponds to convection. Similarly, when you boil a pan...

Transcript: There are three basic modes of heat transfer or ways that energy can be carried from a higher temperature material to lower temperature material. One is conduction which is heat transfer by tiny microscopic motions of the atoms or molecules. Conduction can occur in solids and liquids but not gases because the density is too low. Convection is the large wholesale motion of masses of material, and it can occur most effectively in liquids and gases, although conduction does occur...

Transcript: Wave-particle duality not only means that light has some of the properties of particles, carrying energy from one place to another and acting as if the waves are concentrated in a packet. It also means that particles share properties with waves. Physicist William de Broglie, a hundred years ago, was the first to give a formal description of this fundamental attribute of matter, of matter being wave-like, meaning that it can suffer the properties of diffraction just as a wave...

Transcript: There’s an important sense in which the analogy of an atom as a miniature solar system is wrong. In a solar system a planet can have any distance from the central star and any energy in its orbit. This is not true of an atom. In classical physics atoms can have any energy. However in the quantum theory atoms can only have particular discrete amounts of energy, and they can only change their energy by fixed or discrete amounts. These fixed amounts of change of energy for atoms...

Transcript: How do we know that atoms are real? Most people have never seen atoms. Even the Greeks, when they speculated about the existence of atoms, were just doing a thought experiment. There’s a simple experiment you can do that shows that the fundamental units of nature must be hundreds of thousands of times smaller then the eye can see. If you take the surface of a bath and sprinkle it finely with pepper and then put a single drop of heavy oil on it, an interesting thing will...

Transcript: In the quantum theory the energy of a photon is given by the product of Planck’s constant and the frequency of the photon. Planck’s constant is a fundamental constant of nature and a very tiny number, 6.6 times 10-34 joule-seconds. What does this mean? This means that radiation is grainy just as matter is grainy at the level of atoms. The graininess of radiation corresponds to individual photons. How much energy is there in a photon? A photon of visible light can be...

Transcript: About a hundred and fifty years ago scientists were faced with a profound mystery in their understanding of matter and radiation. Astronomers had observed sharp spectral features in the emission of nebulae and stars. Similarly hot gases in the laboratory showed sharp spectral features in the spectra, and since a spectrum is a map of energy this meant that there were particular energies of transition in atoms. This has no way of being understood in classical physics because in...

Transcript: How does light travel through space? If you had a flashlight it would focus light in the forward direction. A laser beam sends light almost perfectly from one point to another. However, most astronomical sources of light or other forms of electromagnetic radiation are spherical, and they emit equally in all directions. In this situation the intensity of radiation is a function of the distance from the source, diminishing as the inverse square of the distance. This is called...