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Science Focus 8 Light and Optical Systems Topic 1 Topic 1 – What Is Light?

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Science Focus 8 Light and Optical Systems Topic 1 Topic 1 – What Is Light? Science Focus 8 Light and Optical Systems Topic 1 Topic 1 – What is Light? * Have you ever seen a fire burning? Ever wonder why you can see it? Turn to page 176 of your textbook and read the introductory paragraph to find out more. Light is a source of energy that you can see. ​ Natural Light Sources would be things that come from nature like the Sun or fire. ​ Energy that radiates out, or spreads out from a source in all directions and does not ​ require a conductor or something to travel through is known as radiation or radiant ​ ​ energy. ​ Man made sources of light (such as light bulbs) are known as artificial light sources. ​ ​ The First Basic Principle of Light – “Light is a Form of Energy” When light reaches a surface, it can be absorbed and transformed into other types of energy. ● When absorbed by a surface, light can turn into heat or thermal energy. ​ ​ ​ ​ ​ ● When absorbed by a solar cell, light is turned into electricity. ​ ​ ​ ​ ● When absorbed by a tree, light is turned into chemical energy. ​ ​ ​ ​ ​ * Turn to page 177 and 178 of your textbook to read more about light intensity. Sources of Light * Turn to page 179 of your textbook and read “Sources of Light”. Living today, we are lucky to have so many different sources of light. Can you imagine studying next to a candle?! Unfortunately, all this light that’s available to us can also be a burden. Light pollution occurs when sources of light interfere with our view of the sky ​ and stars at night. That’s why sometimes you’re not able to see the stars at night even when it’s clear. Because of this, most observatories are out in the country and usually at high altitudes (see Figure 3.3 on page 179). Incandescent Sources An object can be heated to such a high temperature that it emits visible light. Such an object is called an incandescent source of light. ​ ​ EXAMPLE: the thin filament wire in a light bulb gets white hot and gives off energy in ​ the form of mostly heat (about 95%), but also light Electrical Energy Thermal Energy Visible Light Energy * Turn to page 180 and read “Incandescent Sources”. Science Focus 8 Light and Optical Systems Topic 1 Fluorescent Sources ​ ​ High energy invisible ultraviolet light is absorbed by particles which then emit this energy as light. Ultraviolet Light Energy Energy Absorbed by Particles Visible Light Energy In a fluorescent light bulb, electrical energy causes mercury vapor inside the light bulb to give off ultraviolet energy which is absorbed by the phosphor particles which coat the light bulb. The phosphor particles then glow, which gives off the visible light. This glow is known as fluorescence . ​ ​ Florescent tubes use less energy, last longer and waste less energy to heat than incandescent light bulbs, but they are more expensive to manufacture and the phosphor coating and mercury vapor are both toxic, which make disposal more difficult. * Turn to page 181 and read “Fluorescent Sources”. Phosphorescent Sources A phosphorescent source of light is similar to a fluorescent source. Light energy is absorbed by certain particles that can store this energy for a while. The stored energy is later released as visible light. The main difference between fluorescent sources and ​ phosphorescent sources of light is that fluorescent light is released immediately while phosphorescent light takes longer to emit. * Turn to page 182 and read “Phosphorescent Sources”. Chemiluminescent Sources Light can also result from the energy released in chemical reactions. The chemical reaction produces energetic particles that give off visible light energy. Chemical Energy Visible Light Energy * Turn to page 183 and read “Chemiluminescent Sources”. Bioluminescent Sources Light produced by living organisms through chemical reactions is known as bioluminescence. Examples may include some very deep ocean creatures, fungi and ​ fireflies. * Turn to page 183 and read “Bioluminescent Sources”. All objects that emit light are called luminous, while the ones that do not emit light are ​ ​ called non-luminous. ​ ​ Science Focus 8 Light and Optical Systems Topic 1 The Cost of Lighting Electricity is charge for in kilowatt hours. A kilowatt hour is one thousand watts of electrical power operating for one hour. The symbol for watt is W and the symbol for ​ kilowatt hour kW•h. In other words if you left a 100 watt light bulb on for 10 hours, it would use 1000 watts of energy or 1 kW•h worth of energy. QUESTION 1: ​ If a 60 watt light bulb was left on for twenty hours, how much energy would it use? If electricity sells for 9¢/kW•h how much would this cost? 1. Convert 60 watts into kilowatts by dividing it by 1000 = 60W 1000W = 0.06kW 2. Calculate the number of kilowatt hours by multiplying the power (in kW) by the number of hours = 0.06kW x 24 h = 1.44 kW•h 3. Calculate the cost of leaving the light on for 24 hours by multiplying the number of kilowatt hours by the cost per kilowatt hour: 1.44kW•h X 9¢/kW•h = 12.96 cents Cost (in cents) = Amount of energy (in kW•h) × Unit price (in ¢/kW•h) The Ray Model of Light – “Light Travels in Straight Lines” A ray is a straight line which has a definite starting point, but no ending point. It ​ represents the path of a beam of light. The ray model can be used to predict where ​ shadows will form, and how large they will be. * Turn to page 184 and 185 of your textbook and read “The Ray Model of Light”. Science Focus 8 Light and Optical Systems Topic 1 The ray model can help us understand what happens when a beam of light hits different ​ surfaces. When a beam of light passes straight through a substance, we call that substance transparent. Transparent = TRANSMITTED ​ ​ QUESTION 2: ​ Provide TWO examples of transparent substances. window glass, water, air, lenses ​ When a substance allows light to pass through, but not actual images, we call this substance translucent. The translucent substance bends the light rays and scatters ​ ​ them. Translucent = SCATTERED ​ QUESTION 3: ​ Provide ONE example of a translucent substance. paper, wax paper ​ When a substance or materials completely blocks the light from passing through, the substance is called opaque. Opaque = Blocked/Reflected ​ ​ ​ QUESTION 4: ​ Provide THREE examples of opaque materials. wood, brick, desk, wall ​ QUESTION 5: ​ Turn to page 187 of your textbook and complete the following questions from Topic 1 Review: p. 187 # 1-6 ​ .
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