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Light and Electromagnetic Waves Mark Lesmeister Dawson High School

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ELECTROMAGNETIC WAVES PART I

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Introduction to Light & Other EM Waves. Visible light is one type of electromagnetic wave. Radio waves are another. We have a lot of experience with these types of waves. What do you feel as you move your hand closer to a bright light source? What happens to radio stations when you get farther away?

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Lab: Light Intensity Work in groups of 2-3. Make sure the source lines up with the sensor, and that you know where the front of the sensor is on the scale. Start with sensor 5 cm from the light source and measure the illumination. Move the sensor 2 cm farther away, and measure the illumination again. Repeat this step until you have 10 readings.

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Analyzing the Brightness Lab Decide what equation you think fits the graph. (In addition to the ones we already studied, you may want to consider 1/d 2 ). Confirm your hypothesis by linearizing your data and drawing a new graph. Do you get a straight line? Discuss the results with the other groups in your section.

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Brightness The brightness of a light wave is the power transmitted by the wave per unit of area. The brightness decreases by the square of the distance from the source.

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Electromagnetic Radiation

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Electromagnetic radiation consists of oscillating electric and magnetic fields with different wavelengths. Source: Wikipedia

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Electromagnetic Spectrum Each group will examine one portion of the electromagnetic spectrum. For your part, make a whiteboard showing Frequency range Wavelength range Uses of the radiation. _

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The Electromagnetic Spectrum

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Radio waves > 0.1 m Communications; MRIs Microwaves 0.1m> > 0.0001 m Communications; navigation; ovens Infrared waves 0.0001m> > 7 x 10 -7 m Night-vision, thermometers, therapy Visible light 7 x 10 -7 m> > 4 x 10 -7 m Human sight Ultraviolet light 4 x 10 -7 m> > 10 -9 m Sterilization; blacklight X-rays 10 -7 m > > 10 -11 m Medical diagnostics and treatments Gamma rays10 -11 m > Cancer treatments; food irradiation

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The speed of light All electromagnetic waves move at the speed of light. The speed of light is approximately 3.00 x 10 8 m/s, or 186,000 mi/s. The wave speed equation for light is c = f Speed of light = frequency x wavelength

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Huygens Principle A continuous line of wave crests is called a wave front. The motion of a wave can be determined by treating each point of a wave front as a new wave source. Holt Rinehart & Winston

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CHARACTERISTICS OF VISIBLE LIGHT PART II

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Color Electromagnetic waves with wavelengths between 700 nm and 400 nm fall within the visible spectrum. The wavelength (or frequency) of a light determine the color of the light.

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Color The colors of the visible light spectrum vary continuously in the visible range, but are usually named as follows, from longest to shortest. Red Orange Yellow Green Blue Indigo Violet

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Additive Primary Colors Light from a source such as the sun contains a variety of colors. The additive primary colors of light are red, green and blue. When these colors of light are combined in equal amounts, they produce white light. When they are added together in various proportions, they can produce all the colors of the visible spectrum. Red BlueGreen

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Pigments When an object is illuminated by a light source, it may absorb some colors of light and reflect others. The color of light that an object appears to have is determined by the colors of light that it reflects. The primary pigments, or primary subtractive colors, reflect one color of light and absorb all others. These pigments are cyan, magenta, and yellow. When these three are combined, they filter out all colors, and the mixture is black. Holt Rinehart & Winston

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Additive and subtractive primary colors Red BlueGreen YellowMagenta Cyan

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Polarization The electric and magnetic fields in an electromagnetic wave are always perpendicular to each other and the direction of the wave. The direction of oscillations is usually random. The wave can be made to oscillate in one direction only. This is known as a linearly polarized wave. Source: Wikipedia

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Polarizing Visible Light A polarizing filter polarizes light waves. When light is filtered through two filters aligned at right angles, then the transmitted light wave is sharply reduces. Light that reflects off a flat surface at certain angles can be polarized. This effect is used in polarized sunglasses in order to reduce glare.