How is electromagnetic radiation produced

Infrared astronomers use microns millionths of a meter for wavelengths, so their part of the EM spectrum falls in the range of 1 to microns. Optical astronomers use both angstroms 0. Using nanometers, violet, blue, green, yellow, orange, and red light have wavelengths between and nanometers. This range is just a tiny part of the entire EM spectrum, so the light our eyes can see is just a little fraction of all the EM radiation around us. The wavelengths of ultraviolet, X-ray, and gamma-ray regions of the EM spectrum are very small.

Instead of using wavelengths, astronomers that study these portions of the EM spectrum usually refer to these photons by their energies, measured in electron volts eV.

Ultraviolet radiation falls in the range from a few electron volts to about eV. X-ray photons have energies in the range eV to , eV or keV. Gamma-rays then are all the photons with energies greater than keV. Show me a chart of the wavelength, frequency, and energy regimes of the spectrum. Why do we put telescopes in orbit? The Earth's atmosphere stops most types of electromagnetic radiation from space from reaching Earth's surface.

This illustration shows how far into the atmosphere different parts of the EM spectrum can go before being absorbed. Only portions of radio and visible light reach the surface. Most electromagnetic radiation from space is unable to reach the surface of the Earth. For air n is nearly equal to 1, for water n is 1. Most radio waves are emitted by charges oscillating in antennas. The direction of the acceleration of the charges is along the antenna.

A radio wave propagates from the antenna to the receiver along a straight-line path called the line of sight. The direction of the electric field E of the electromagnetic radiation emitted by the antenna lies in a plane that contains the antenna and the line of sight to the receiver, and is perpendicular to the line of sight. The wave is polarized , which means that E has a well defined direction.

The electric field is strongest and the intensity highest in the directions perpendicular to the antenna and goes to zero in the direction along the antenna. You get very poor reception if you stand under the antenna.

To carry information, the electromagnetic wave must be modulated. The information carried by a radio wave is sound. The amplitude of an AM amplitude modulated radio wave represents the pressure variations, which make up the sound. If we take a neon sign and separate out the colors with a prism we would see the following spectrum:.

An observant student might now ask -- I see how light can produce colors now, but where does white light come from? The answer is that it comes from all the colors. When you take all the colors and combine them then you will get white. If we place sunlight or light from an incandecent lightbulb though a prism we would see the following spectrum:.

Now this spectrum looks different from the neon light because it is continuous. It is an entire band of light and not just several different lines. The reason why this spectrum looks different is because it was not generated by electricity exciting particular gases. It was generated by heat exciting atoms. Some familiar phenomena are based on the production of electromagnetic waves by varying currents. Your microwave oven, for example, sends electromagnetic waves, called microwaves, from a concealed antenna that has an oscillating current imposed on it.

There is a relationship between the E — and B -field strengths in an electromagnetic wave. This can be understood by again considering the antenna just described. The stronger the E -field created by a separation of charge, the greater the current and, hence, the greater the B -field created. It can be shown that the magnitudes of the fields do have a constant ratio, equal to the speed of light.

This is true at all times and at all locations in space. A simple and elegant result. To find the B -field strength, we rearrange the above equation to solve for B , yielding. We are given E , and c is the speed of light. Entering these into the expression for B yields. Note that as this wave spreads out, say with distance from an antenna, its field strengths become progressively weaker.

They can be detected in electromagnetic waves, however, by taking advantage of the phenomenon of resonance, as Hertz did. A system with the same natural frequency as the electromagnetic wave can be made to oscillate. All radio and TV receivers use this principle to pick up and then amplify weak electromagnetic waves, while rejecting all others not at their resonant frequency. For your TV or radio at home, identify the antenna, and sketch its shape.

Estimate its size. Try tuning the radio and note the small range of frequencies at which a reasonable signal for that station is received. This is easier with digital readout.

If you have a car with a radio and extendable antenna, note the quality of reception as the length of the antenna is changed. Broadcast radio waves from KPhET. Wiggle the transmitter electron manually or have it oscillate automatically. Display the field as a curve or vectors.