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

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Refraction through a glass Refraction through a glass block: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 = fastest diamond = 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?

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RefractionRefraction 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

TASKWhich of these would you expect to refract more?

a) Light through Perspexb) 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

www.A6training.co.uk

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

http://www.teachertube.com/viewVideo.php?video_id=214724

What have you learnt?

• Do you know which of these light travels faster througha) 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?