Core Physics – Energy L/O :- To be able to explain why waves refract “Does light always travel in a straight line?” Exam Date -

Quick Quiz - What do you remember from last lesson? Draw and label a wave What is the law of reflection? What is the equation for working out the speed of a wave? How does a coal power station turn the turbine to spin the generator? What is the equations for working out efficiency? What are the 2 different types of wave? Give an example How do you work out the pay-back time for insulation? What are the pros and cons of wind energy? Where are step-up transformers used? How does increasing the volume of a sound affect the wave?

Ever wondered why your legs look weird when you look at them in a swimming pool? This is because light bends when it goes into different mediums e.g. from water to air

Archer Fish The Archer fish is a predator that shoots jets of water at insects near the surface of the water, e.g. on a leaf The Archer fish allows for the refraction of light at the surface of the water when aiming at the prey image of prey prey location The fish does not aim at the refracted image it sees, but at a location where it knows the prey to be

Refraction

19/10/2015 Refraction through a glass block: Wave slows down and bends towards the normal due to entering a more dense medium Wave speeds up and bends away from the normal due to entering a less dense medium Wave slows down but is not bent, due to entering along the normal

The speed of light waves depends on the material they are travelling through If light waves enter a different material (e.g. travel from glass into air) the speed changes, causing the light to bend or refract glass air air = fastestdiamond = slowestglass = slower

But why does slowing down cause it to bend? It is similar to why a car bends when there is mud on the road The car will travel more slowly through the mud because there is more friction If it enters the mud at an angle then the front tyres hit the mud at different times First to hit the mud is tyre 1, and will move more slowly than tyre 2. This causes the car to turn towards the normal When the car leaves the mud for the road, tyre 1 hits the road before tyre 2 and this causes the car to turn away from the normal Tyre 1 Tyre 2 Normal What would happen if the car hit the mud straight on?

19/10/2015

Refraction Refraction is when waves ____ __ or slow down due to travelling in a different _________. A medium is something that waves will travel through. When a pencil is placed in water it looks like this: In this case the light rays are slowed down by the water and are _____, causing the pen to look odd. The two mediums in this example are ______ and _______. Words – speed up, water, air, bent, medium

End TASK Which of these would you expect to refract more? a)Light through Perspex b)Light through glycerol c)Light through water? What is your prediction? Don’t forget to explain why you think this You have 2 minutes to discuss this with the person next to you and you will then be asked to write this down on your own

2 Minute Timer 2:001:591:581:571:561:551:541:531:521:511:501:491:481:471:461:451:441:431:421:411:401:391:381:371:361:351:341:331:321:311:301:291:281:271:261:251:241:231:221:211:201:191:181:171:161:151:141:131:121:111:101:091:081:071:061:051:041:031:021:011:000:590:580:570:560:550:540:530:520:510:500:490:480:470:460:450:440:430:420:410:400:390:380:370:360:350:340:330:320:310:300:290:280:270:260:250:240:230:220:210:200:190:180:170:160:150:140:130:120:110:100:090:080:070:060:050:040:030:020:01End

How could you test your prediction? What will you record? Design a table What type of graph should you use to represent your data? How could you check the reproducibility Can you work out the refractive index? Refractive index = sin i sin r

So how can do we use refraction? A forensic scientist may be sometimes be asked to match pieces of glass, from example from a crime scene and from the clothes of a suspect. To do this they will try to match the refractive index To measure the refractive index of a tiny bit of glass, a forensic scientist immerses the fragment in oil, under a microscope. The scientist then slowly heats and cools the oil. This changes the oils refractive index. When the refractive index of the oil and the glass match, the glass disappears. A computer then coverts the temperature of the oil into a refractive index value

What have you learnt? Do you know which of these light travels faster through a) Water b) Air c) Glass Can you explain why light bends when in goes from air to glass? Can you draw what a light ray would look like going through this block?