NATURE OF LIGHT Electromagnetic Spectrum
Electromagnetic Spectrum • The electromagnetic spectrum comprise of the following: • 1. Radio waves • Electromagnetic radiation with wavelengths that range from hundreds of meters to less than a centimeter. • Familiar due to their use in communications. • AM Radio band – 540 to 1, 650 kHz. • FM band – 88 – 108 MHz • Also includes shortwave radio transmissions and television signals.
Electromagnetic Spectrum • 2. Microwaves • Electromagnetic that range from approximately 1 ft (30 cm) in length to the thickness of a paper. • Microwave oven heat food by causing water molecule to rotate at a frequency of 2.45 GHz. • In astronomy radiation of a wavelength of 8.2 inch (21 cm) has been used to map neutral hydrogen (H) throughout the galaxy. • RADAR is also included in this region.
Electromagnetic Spectrum • 3. Infrared • Electromagnetic radiation that comprises the region of the electromagnetic spectrum where the wavelength of light is measured from 1 mm to 400 nanometer. • Discernible to humans as heat. • Discovered by W. Herschel by dispersing sunlight through a prism and measuring the temperature increase just beyond the red end of the spectrum.
Electromagnetic Spectrum • 4. Visible light • Electromagnetic radiation in the range visible to the human eye between approximately 4, 000 and 7, 700 angstroms. • Wavelengths to which the human eye is sensitive. • Easily pass Earth’s atmosphere. • Further broken down into the familiar color of a rainbow. (MR. ROY G. BIV)
Electromagnetic Spectrum • 5. Ultraviolet • Electromagnetic radiation ranging in wavelength from 400 to 10 billionth of a meter. • Has many important effects on Earth. • The ozone absorbs much of the UV radiation from the sun. • UV that reaches the Earth’s surface can cause suntans and sunburns.
Electromagnetic Spectrum • 6. X-rays • Electromagnetic radiation that are highly energetic with wavelengths ranging from about 10 billionths of a meter to 10 trillionths of a meter. • Useful in medical and industrial radiography. • Can pass through the body. • Allows doctors to study bones and teeth. • Do not pass Earth’s atmosphere so astronomers must place X-ray telescopes in space.
Electromagnetic Spectrum • 7. Gamma rays • Electromagnetic radiation that are most energetic and are comprised of light with wavelengths of less than about ten trillionths of a meter and include waves with wavelengths smaller than the radius of an atomic nucleus (1015m). • Produced by nuclear processes during radioactive decay or in nuclear reactions in space.
Spectroscopy • The study of the spectra especially to determine the chemical composition of substances and the physical properties of molecules, ions and atoms. • Study of the properties of light that depend on wavelength. • Newton’s use of prism dispersing the visible light into the rainbow of colors initiated the study of spectroscopy.
Spectroscopy • Spectroscope • The instrument for studying spectra; an instrument for dispersing light, usually light in the visible range, into a spectrum in order to measure it.
Types of Spectrum • 1. Continuous Spectrum • 2. Dark-line Spectrum • 3. Bright-line Spectrum
Types of Spectrum • 1. Continuous spectrum • Is produced by an incandescent solid, liquid or gas under high pressure. • Consists of an unfiltered band of color. • E.g. Common light bulb
Types of Spectrum • 2. Dark-line spectrum (Absorption spectrum) • Is produced when “white” light is passed through the a comparatively cool gas under low pressure. • Gas absorbs selected wavelengths of light, so the spectrum that is produced appears as a continuous spectrum but with a series of darklines.
Types of Spectrum • 3. Bright-line spectrum (emission spectrum) • Is produced by a hot (incandescent) gas under low pressure. • It is a series of bright lines of particular wavelengths depending on the gas that produces them. • These bright lines appear in the exact location as the dark lines that are produced by this gas in a dark-line spectrum (absorption).
Spectrum • The spectrum coming from the sun contains thousands of dark lines. • Over 60 elements have been identified by matching those lines with those elements known on Earth.
Spectrum • TWO FACTORS CONCERNING A RADIATING BODY IS IMPORTANT • 1. If the temperature of a radiating surface is directly proportional to the fourth power of its absolute temperature. • Stefan Boltzman Law • The energy radiated by a body is directly proportional to the fourth power of its absolute temperature. • Eg. Star – 2 times brighter – energy emitted will be 2 raise 4, it means the star released 16 times more energy.
TWO FACTORS CONCERNING A RADIATING BODY • 2. As the temperature of an object increases, a larger proportion of its energy is radiated at shorter wavelengths. • E.g. • Heated metal rod • Hot - Red color – longer wavelength • Hotter – Blue color – shorter wavelength • Red stars – hot • Blue stars - hotter