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An introduction to its origins and properties

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A form of radiation called electromagnetic because it is associated with changing electric and magnetic fields When light enters our eye, the energy stimulates nerve endings and we see what we call light.

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Light acts as both a wave & a particle A particle of light is called a photon Light can be referred to as a bundle of waves

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2 kinds of light sources: Luminous sources : actually produce the light they give off EX: sun, other stars, lamps, fireflies Non-luminous sources : can be seen only after light reflects off of them EX: moon & people

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1.A fuel such as gasoline or wood is ignited 2.The chemical potential energy stored in the molecules of fuels is converted as burning occurs 3.Usually both light energy and heat energy are given off

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When metals give off light as a result of being heated EX: a welding torch at a temperature of 800 C EX: steel is heated to 2500 C

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1.An ordinary incandescent light bulb converts electrical energy to light energy 2.An electric current is passed through a fine wire filament made of the metal Tungsten 3.As the electrons move, some of the energy is changed to heat 4.Heat warms the filament to incandescence and light energy is given off

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1.Fluorescent lights contain a tube with an electrode at each end 2.Electrons bounce around in the tube and collide with Mercury atoms 3.The collisions cause UV-radiation to be formed 4.The radiation strikes the fluorescent coating on the inside of the tube and light is given off

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Tungsten metal is used because of its high melting point of 3400 C Incandescent lamps are energy wasters, only about 5% of the electrical energy used is turned to light Fluorescent lights change 20% of the energy to light Fluorescent lights contain mercury which is a hazard if you break them inside a house.

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Nuclear reactions can cause light energy Nuclear fission and fusion reactions use the potential energy contained in atoms This is converted to both heat and light

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EX: A Neon Sign Energy can cause an electron of an atom to jump to a higher energy level When it falls back down, it releases a photon of light

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The Law of Conservation of Energy states yes! EX: Solar Cells on satellites circling Earth change light energy to electrical energy EX: Camera exposure meters turn light energy to electrical energy

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Radiation coming from the sun is called Electromagnetic because it is associated with changing electric and magnetic fields that travel through space, transferring energy from one place to another Electromagnetic radiation moves through space at 300,000 km/sec (186,000 miles/sec) or 3.0 X 10 8 m/sec

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By: Brews Ohare

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A periodic repeating disturbance = a wave Measuring successive wave peaks = wavelength Greek letter for Wavelength is lambda

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Light also acts as a particle called a photon A photon is like a packet of light energy When light energy strikes a photo electric cell, it behaves like a stream of particles carrying specific amounts of energy The amount of energy depends on its wavelength

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Shorter wavelengths have more energy! h= Plancks constant = 6.6262 X 10 -20 Joule/sec c = speed of light = 3.0 X 10 8 m/sec lambda = wavelength in meters

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A spectrum is an array of electromagnetic radiation in order of wavelength Just as we sense the wavelength of sound as pitch, we sense the wavelength of light as color Humans can see the visible light spectrum See the next slide for other examples of the spectrum

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A photometer is used Astronomers use it to analyze starlight

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Unlike sound and other waves, light can travel through a vacuum Light can travel through some media Transparent media you can see through like air and glass Translucent media transmits some light like smoke and frosted glass Opaque media transmits no light such as wood, black plastic and metal

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This is called rectilinear propagation If light travelled in a curved path, youd be able to see around a corner The path of light is called a ray A bundle of rays is called a beam

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Most objects you can see are non-luminous, that is you can see them because they reflect light to your eyes Most non-luminous objects have rough surfaces which reveals their texture and shape Some non-luminous objects are shiny and smooth and reflect light in such a way that images are formed = mirrors

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Flat = plane mirror The rest are curved mirrors Parabolic mirrors are specially designed to focus reflected light to a point in front of the mirror Solar cookers In telescopes by astronomers Can also focus radio waves

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Light may bend as it travels from one medium to the next This bending is called refraction

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Now you will perform some investigations to help you understand the properties of light. Materials for investigation 1: meter stick or tape measure, slide projector and graph paper Materials for investigation 2: Pinhole camera supplies or make one: tissue or shoe box, wax paper or oiled paper, black electrical tape or duct tape, aluminum foil, fine pin, candle, matches, rulers, scissors

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Now you will perform some investigations to help you understand the properties of light. Materials for investigation 3: flashlight, white wall or paper, equilateral triangle prism Materials for investigation 4: white cardboard cut to 10-12 cm in diameter, protractor, markers the color of the rainbow, pencil and pin.

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1.Place a slide projector at the back of a completely dark classroom. Project the beam of light toward the front of the room. Adjust the lens so it diverges as much as possible. 2.Stand in the beam of light and face the projector. 3.Use the light meter to measure light intensity at a distance of 1m from the lens 4.Repeat step 3 for distances 2m, 3m, 4m and 5m. 5.Record your results in a data table. 6.Plot a graph with distance on the x-axis & light intensity on the y-axis.

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1.According to your graph, how does distance affect light intensity? 2.How far do you think light can travel before it can no longer be seen? (HINT: extrapolate your graph to find the distance at which the intensity would be zero.)

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The pinhole camera is a simple application of the rectilinear propagation of light. It consists of an opaque box with a pinhole at one end and a translucent screen at the other end. In this investigation, you will use the assumption that light travels in straight lines to predict the nature of the image of a pinhole camera. 1.Size : is the size of the image larger or smaller than the object? 2.Attitude : is the image erect or inverted 3.Kind : is the image real or imaginary (a real image can be caught on screen, an imaginary image cannot.)

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1.Obtain an opaque box such as a shoe box or large tissue box 2.Remove a rectangular section from one end. Replace it with wax paper or oiled paper. 3.Cut a hole 2-3 cm in diameter at the other end. Cover this hole with Aluminum foil. Make a pinhole in the center of the foil with a fine pin. 4.Make sure the rest of the box doesnt leak light.

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1.Put your camera in front of a lighted candle. 2.View the image on the wax paper. 3.Draw a diagram in your journal showing the lighted candle and the image on the wax paper. Draw lines to show how light rays would enter the pinhole camera. 4.Experiment by moving the camera closer or farther away. 5.Describe in your journal the size, attitude and kind of image this camera produces.

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1.Compare your predictions regarding the characteristics of the image with what really happened. 2.Does this investigation support that light travels in straight lines? 3.What happens to the size of the image as you move the camera closer to and farther away from the image?

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Shine a light through an equilateral triangle prism so that it meets one side at an oblique angle. The light should come through onto a white wall or a white piece of paper. Experiment until you can see the colors of the rainbow. Record/Sketch your results.

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1.From a piece of white cardboard, cut a circular piece that is 10-12 cm in diameter. Draw a diameter through the circle, then use a protractor to divide the circle into 18 segments of 20 degrees each. 2.Color the segments in a repeating sequence of red, orange, yellow, green, blue, violet in a counter clockwise fashion. Repeat the sequence 3 times. 3.Push a common pin through the center of the circle and then into an eraser of a pencil 4.Spin the disc rapidly and observe the effect. If possible, do this in bright daylight.

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