CH 2

Chapter 2: How We See the Invisible World THE PROPERTIES OF LIGHT Learning Objectives + Identify and define the characteristics of electromagnetic radiation (EMR) used in microscopy » Explain how lenses are used in microscopy to manipulate visible and ultraviolet (UV) light The Electromagnetic Spectrum The entire range of electromagnetic radiation is the electromagnetic spectrum. The distance between crests of electromagnetic waves is called the wavelength (A). Wavelengths of electromagnetic radiation range from less than a nanometer (gamma rays) to more than a kilometer (radio waves). Visible light is a narrow band of the electromagnetic radiation between 380 to 750 nm. Like other forms of electromagnetic energy, light travels in rhythmic waves. While light travels as a wave, many of its properties are those of a discrete particle, the photon. Photons are not tangible objects, but they do have fixed quantities of energy. The amount of energy packaged in a photon is inversely related to its wavelength. Photons with shorter wavelengths have more energy which photons with longer wavelengths have less energy. Visible light has enough energy that it can excite electrons. When an electron is hit by a photon of the correct wavelength, the electron will absorb the energy and will move to a higher orbital. The electron will give off the energy eventually and will go back to its original energy state. That energy can be used to do work. Ultraviolet light (UV light) has shorter wavelengths and more energy than does visible light. It has enough energy that in addition to exciting electrons, it can damage molecules. This is why exposure to UV light can damage DNA and lead to skin cancer. Interactions of Light When light meets matter, it may be reflected, transmitted, absorbed or refracted. Reflection occurs when light strikes an object and bounces off of it. Usually the object absorbs some of the wavelengths and reflects others. The object appears to be the color of the wavelength that is reflected. Transmission refers to the passage of light through an object. Absorption occurs when light is not reflected or transmitted through an object but is taken up by the object. The light absorbed may be used as an energy source (as occurs during photosynthesis). In some causes the light is absorbed and given back off as a longer wavelength. The reemission of light is luminescence if it occurs while the light is