Learning session 1: Properties of light In the following lesson plan, descriptions of classroom activities appear in normal text, material spoken by the teacher is italicised, and explanatory notes for the teacher are shown against a yellow background, as is this paragraph. ● Major point 1.1: Light is intangible ● Major point 1.2: Light travels in straight lines ● Major point 1.3: Light travels very quickly, but not infinitely so ● Major point 1.4: Light interacts with matter by being absorbed, transmitted, or reflected. The teacher is found by the students on their arrival shining a torch or laser pointer around the room with a puzzled expression. After a time, someone will ask why. I’m trying to figure out how something I do over here makes a bright spot appear over there. Any ideas? [See note 1.1] This is an appeal by the teacher for help, engaging the students. A typical response from a student would be "It's just a beam of light from the torch", accompanied by a look of incredulity, but most will soon realise that there is more to light than meets the eye. A student will typically respond with something like: "It's a beam of light from the torch!" What's a beam of light? I can't feel anything coming out of the torch, and I can't grab hold of it. I can only see a spot of light appear over there. But when I turn the torch on and off here it appears or disappears over there at the same time. It will show up on a wall, but not on a window, but sometimes when I'm pointing it at a window the spot turns up somewhere else completely! I can't get it to go around corners and it won't go through my hand. If I point the torch through this cellophane, the spot on the wall is the colour of the cellophane! How does it know? What's going on here? Discussion The teacher should encourage a general discussion of the properties of light that such a simple experiment demonstrates, and these should be listed. Entries would include that somehow an action at one place has an effect at a distance, that that can be blocked, that the effect depends on the type of substance involved. One idea that should not be raised at this stage if possible is the idea that light actually travels from place to place. While it certainly does, this conclusion can be reached on essentially logical grounds and the process of doing this is instructive for the students. If it should arise, no great harm will have been done however. Development of a list of properties of light from the Major points above : speed, light can travel very fast; It reflects off some objects; Light travels through some objects; light can disperse and make a rainbow; light can cause interference with other light sources; Refraction light can bend as it goes through some objects. I think we need to investigate this further and I have some resources we could use, but before we start we should record our questions and include any predictions we may have. learning session one - properties of light.docx | Page 1 Here the students should be encouraged to adopt elements of the scientific method: hypothesis formation, experimental design, experimentation, result recording, and analysis. Resources provided should include a torch and possibly also a low power laser pointer (with a warning given about not looking into the beam), mirrors, glass, paper; cardboard, coloured cellophane, string, stopwatch, scales, and an empty box with a lid; while other materials from the classroom may also be used. Experimentation The teacher should move around the room and discuss the experiments and students predictions, focusing student attention on observation and interpretation. "What do you think will happen?" "Why do you think this is happening?" "Would you get the same result if ..?" "Do you get the same result when you repeat the experiment?" "Does the experiment lead you to new questions?" Students should be led to verify that a light beam illuminates all points along its path, and so perhaps something exists within that beam. That this path always seems to be a straight line can also be raised and suggested as a subject for experiment. One method would be to run a string from the torch to the illuminated spot, which, only when pulled tight, is completely illuminated. Another would be to try to pass the beam through three cardboard plates with holes in them and to find that this was only possible when the holes were in a straight line. Attempts to trap light in a box and weigh it can be encouraged, including comparing the weight of a box containing a torch when it is on, and when it is off. Attempts can be made to see if there is a delay between turning a torch on and the appearance of the distant illuminated spot using a stopwatch. These of course will "fail", but are instructive. The types of interaction of light with matter can be explored: reflection, transmission, possibly with colour changes, and absorption. Let's share our findings. What can we conclude from these results and do we have any new questions? The teacher should let each student/group present their findings. They should share their questions and predictions and which resources they used. They could talk about the experimental method they chose and why. Were there any other factors that affected their results? They would also include their results and conclusions and any new questions that arose. The new questions should be documented and kept. I noticed that some of you tried to measure how fast light was with the stopwatch, but no-one seemed to have any luck. What could this mean? Discussion [Two main hypotheses should arise, but these may need to be prompted: that there is indeed something produced by a light source that travels in a straight line to its destination (Empedocles), or that something happens instantaneously at all points between the source and destination, causing light to arise from the space there (Aristotle). Logic should lead to the conclusion that something must be produced and travel, after all, if nothing did, how would some parts of space 'know' to become part of a light beam? Either light is travelling, or something else even more bizarre is! This idea that light travels leads to two further concepts: if something is travelling, it must be something quite different from matter as it has no perceptible mass and is not affected by gravity since it travels in a straight line and we don't find pools of light on the floor; and further, that if it travels at all, it must have a speed (Empedocles again), yet it couldn't be measured so it must be very quick indeed. These last ideas are presented to the students a little later. learning session one - properties of light.docx | Page 2 Lots of scientists have tried to measure the speed of light incluiding Galileo, Rømer and even Einstein. Finally in 1975 it was measured to be around 300 000 km per second – that is about a billion kilometre per hour! In space distances are so great the unit of distance is the light year – how far light travels in a year….can you work this out? Calculation of how far light can travel in a year The accepted figure for the speed of light in a vacuum is c = 299,792,458 m / s. This distance travelled in a year is therefore: 299,792,458 x 60 x 60 x 24 x 365.25 = 9,460,730,472,580,800 m That may seem like a stupidly large number of no use to anybody, but it is really very useful, especially to astronomers. That's because stars and galaxies are so far apart that the "light-year" makes a handy unit. The light from our Sun, our closest star takes 8 minutes to reach us….just think 7 minutes ago the sun could have died and we won’t know for another minute. Don’t worry the sun is not ‘projected’ to die for another 100 million years and that is a completely new Science unit. Light is weird as we only ever see it taking the shortest distance from point A to B – from this we say that light travels in straight lines. We did see it bounce off some substances, pass through some, sometimes changing colour along the way, and get completely absorbed by others. During this unit we will explore these curious properties of light further but for now you may want to choose an activity to be done in a group or independently. You can do more than one if you like. Here are some ideas, but if you want to try something else, let me know. Whenever students are asked to research information on the computer you need to make sure they are equipped with the necessary skills. The information at this link is be very useful and student-friendly in this regard: http://www.kidscomputerlab.org/index.php/research-skills/ Free choice activity, possibilities including: ● Given that light travels at 299,792,458 meters per second, calculate how many kilometres light can travel in an hour, a day, a week, and a month. How far could a beam of light have travelled since you were born? Would a person with a huge telescope on a planet orbiting around Van Maanen's star, one of our close neighbours only 14 light-years away, have been able to see you on the day you were born? There are 1000 m in a kilometre. There are 60 seconds in a minute; 60 minutes in an hour; 24 hours in a day, and use 365.25 as the number of days in a year. The last part prompts the student to realise that in the cosmic sense, light is slow. ● Research how Galileo began to measure the speed of light? Look carefully at the scientific method he used and refine/improve upon it. Present a poster of how you would have done his experiment back in the day. There is abundant information at wikipedia, http://en.wikipedia.org/wiki/Galileo_Galilei , but another good source is http://www.sciencekids.co.nz/sciencefacts/scientists/galileogalilei.html and http://galileo.phys.virginia.edu/classes/109N/lectures/spedlite.html learning session one - properties of light.docx | Page 3 Using the resources given earlier – come up with a question that you wish to do an experiment upon; write a hypothesis and then do an experiment to test your hypothesis – remember to work like a real scientist and record all of your ideas, thoughts and results. Resources Laser pointers, torches, mirrors, coloured cellophane glass, paper, cardboard, string, stopwatch, scales, empty boxes, other common classroom resources, computer access, library. References http://en.wikipedia.org/wiki/Speed_of_light http://www.colorado.edu/physics/2000/waves_particles/lightspeed_evidence.html http://en.wikipedia.org/wiki/Fizeau-Foucault_apparatus learning session one - properties of light.docx | Page 4