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SNC 2D 10.1 – Sources and Nature of Light What is Light? A type of radiant energy in the form of a wave that can be detected by the human eye Can be from natural or artificial sources o Natural: The sun, fireflies, lightning o Artificial: Light bulbs, fluorescent tubes, candles, lasers, etc. Sources of Light Objects that emit (give off) their own light are said to be luminous. e.g., the sun, a switched-on flashlight In luminous objects, some form of energy is changed into light energy. This occurs when atoms within the material absorb this energy and enter into an excited state by bumping an electron up one energy level. Then, almost immediately, the excited atoms release the energy in the form of light when the electron falls back down to its normal state. Objects that do not emit light are said to be nonluminous. e.g., the moon, a switched-off flashlight, a pencil, a chair, etc. These objects can reflect light from a luminous source to allow us to see them Light from Incandescence: Things that are extremely hot become luminous The process of emitting light because of a high temperature is called incandescence e.g. light bulb – fine metal wire (usually tungsten) glows white hot o Very inefficient at producing light – only 5% of electrical energy gets converted into light, the other 95% is lost as heat Light from Electric Discharge: When electricity passes through a gas, the gas particles can enter into an excited state and emit light This process is called electric discharge e.g., lightning, “neon” gas tubes (actually many different gases used for different colours), street lights Light from Phosphorescence: Certain materials, called phosphors, will give off light for a short time after you shine light on them The process of emitting light for some time after receiving energy from another source is called phosphorescence e.g., glow-in-the-dark paint (like on watch dials) Light from Fluorescence: Fluorescence is the process of emitting light while receiving energy from another source e.g., fluorescent light bulbs, CFLs o electrical energy hits molecules of mercury vapour, causing them to emit ultraviolet (UV) energy o the UV energy then hits a phosphor coating on the inside of the tube, causing it to glow white o Process is about 20% efficient Light from Chemiluminescence: The process of changing chemical energy into light energy with little or no change in temperature is called chemiluminescence e.g., glow sticks – two chemicals inside separated from one another; when mixed, they give off light until chemicals are used up Light from Bioluminescence: The process of living creatures making themselves luminous by using a chemical reaction Similar to chemiluminescence e.g., fireflies, glow worms, some fish The Nature of Light Light is known as an electromagnetic wave. Electromagnetic waves are very similar to water waves, in that both involve the movement of energy from one point to another. The wavelength of a wave is defined as the distance from one crest (or trough) of a wave to the next crest (or trough). Electromagnetic (EM) waves are invisible and can travel through a vacuum; unlike sound waves, for example, they do not need particles in order to travel from one place to another. EM waves will travel through a vacuum at the speed of light – 3.00 x 108 m/s The Electromagnetic Spectrum: The electromagnetic spectrum is a diagram that illustrates the range, or spectrum, of electromagnetic waves in order of their wavelength. Notice that the range of visible light is just a tiny part of the entire EM spectrum – furthermore, the colours of light are just different wavelengths within the visible range. The colour red has the longest wavelength of visible light, which is 700 nm (nanometers, or 10 -9 metres). Violet has the shortest wavelength of visible light, at 400 nm.