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00:00Music
00:29Hello, thank you, and welcome to another Think of a Number.
00:35I want to tax your brains now.
00:36I'm going to show you a picture, and I want somebody to shout out
00:39and tell me exactly what it is.
00:41Are you ready? Go. What is it?
00:43Light bulb.
00:44What kind of light bulb?
00:4510,000 watt.
00:4610,000 watt light bulb.
00:48That's brilliant.
00:52Just having a light snack.
00:54The reason I asked about that light bulb
00:56was because that is what this program is all about.
00:59It's about something we could not do without.
01:03Light.
01:04Oh, can we have a ray of light, please?
01:08Can we have a ray of...
01:09Oh, thank you. Hip-hip. Hooray.
01:12Now, the light's shining on me, and you can see me now,
01:15and you should be able to see our studio audience,
01:17because they're here somewhere.
01:18There you are. It's quite bright, isn't it, that light?
01:21They're all there.
01:22How many have you got brothers and sisters?
01:24Lots of them. All of them, I think.
01:27Good heavens, that's incredible.
01:28Well, there's nothing on this program to do with brothers and sisters,
01:31but we will be talking about relativity later
01:33and the speed of light.
01:35We'll also be reflecting on how light travels in straight lines
01:38but can be bent into a whole rainbow of colours.
01:42We'll be looking at light that you can't see,
01:45and we'll be seeing how light could possibly solve the energy crisis,
01:51because if it doesn't, the future could be very, very black.
01:54Oh, no, the lights have gone out again.
01:57Hang on. What's happening?
02:06Perhaps Father Christmas is coming now, at this very minute.
02:11Do you believe in Father Christmas?
02:12No.
02:14Oh!
02:16They don't believe in Father Christmas.
02:19Surely you'd believe...
02:20That's... I can't believe it.
02:23I think the reason people don't believe in Father Christmas
02:25is because they can't believe that he could possibly visit
02:28every house in the world in one night.
02:30Well, I think he can do it,
02:33and I think the way he does it
02:36is to do with Rudolph's nose.
02:40Because Rudolph, as you know, has got a very special nose.
02:42He picked it himself.
02:45And as it's the only nose of its kind,
02:47he doesn't want the light to escape,
02:49so he chases after it at the same speed,
02:52the speed of light,
02:53dragging the sleigh and Father Christmas with him,
02:56which would explain how they got round the earth.
02:58Because travelling at the speed of light,
03:00they could go round the earth
03:01seven and a half times every single second.
03:04So they'd be able to cover all the land mass on earth
03:06in less than eight hours.
03:08And then by staying on the dark side of the earth,
03:11the night would last 24 hours.
03:13And they could visit every home.
03:15How many homes?
03:16There's about a billion homes in the world.
03:18So they could visit every one if he didn't stay long.
03:21And I believe he doesn't stay long.
03:23At your house, he probably stays
03:25about 86 and a half millionths of a second.
03:30Which explains why you don't see him.
03:32It also explains why he comes down the chimney.
03:34He hasn't got time to open the door.
03:36Talking about time,
03:46I'm opening doors.
03:48There's a clock.
03:50And here is a watch.
03:52Watch.
03:52The watch.
03:53Very good.
03:54Now,
03:54this is my digital watch.
03:56It's called a digital watch
03:57because to move the numbers,
03:59I have to push it with my digit.
04:01You see?
04:02That's all right.
04:03It's because once you can understand
04:05how far the Christmas gets about
04:07by travelling at the speed of light,
04:08you may be able to understand
04:10the theory of relativity
04:12because it's all to do with light.
04:15Let me show you.
04:16Here's a watch that says 12 o'clock.
04:19And there's a clock
04:20that says 11.59 or a minute to 12.
04:23Why?
04:24Well, imagine that that clock
04:26is 11 million miles away
04:29because that's how far
04:32light travels in one minute.
04:34So if that was 11 million miles away,
04:35it would take the light
04:36a minute to get here
04:37so the clock would seem
04:38to be a minute slower.
04:40All right?
04:41You happy so far?
04:42Good, good, good.
04:43That says 11.59.
04:45Now, that clock is going
04:46and so am I.
04:47I'm going in this direction
04:48at the speed of light.
04:50So watch carefully
04:50or you'll miss me.
04:52Ready?
04:52Go!
04:52Go!
04:55I've been travelling one minute.
04:57I've gone 11 million miles.
04:59And that clock is changing.
05:02Let's see what time it says now.
05:0411.59 and 35 seconds.
05:08Now, why is that?
05:10The reason is,
05:11although I've travelled 11 million miles,
05:13light coming from that clock
05:15has taken a lot longer
05:16to come along this line.
05:17It's actually taken
05:18one minute, 25 seconds.
05:20So if you take
05:20one minute, 25 seconds off that,
05:2211.59 and 35 seconds.
05:24I'm going a bit further.
05:25In fact, twice as far.
05:26Here I go.
05:27I've been going two minutes.
05:31And the clock now says
05:3211.59 and 46 seconds.
05:36Because it takes
05:37two minutes and 14 seconds
05:38to get here.
05:39So that clock is going
05:40at a very odd time.
05:43Let's go a lot further.
05:44Here I go.
05:4715 minutes I've been going.
05:48The clock says 11.59
05:50and 58 seconds.
05:52Still two seconds to 12 o'clock.
05:54If I go twice as far,
05:55what does it say now?
05:5711.59 and 59 seconds.
06:01It only went a second
06:02for all that time.
06:03If I go twice as far again,
06:05I've been going an hour,
06:07but this clock will now
06:08only say 11.59 and 59
06:11and a half seconds.
06:15And it will never reach
06:1612 o'clock.
06:18Why?
06:19Well, because that's what happens
06:21when you go at the speed of light.
06:23Everything seems to change.
06:26Time is different
06:27according to where you are.
06:29The only thing that's constant
06:30catches...
06:31Well done.
06:33You've stopped time.
06:34But the only thing that's constant
06:36is the speed of light.
06:39And time always depends
06:40on where you are
06:42and what you're watching.
06:44Well, we're all here
06:45in the television studio.
06:47You're watching me.
06:48And at home,
06:49you're watching your television.
06:50And the signal is coming
06:51from the transmitter
06:52to your television set
06:53at the speed of light.
06:56And the colours
06:57and everything to do with television
06:58is picked out by light.
06:59In fact, you could say
07:00that television is a medium
07:02totally reliant
07:03on light.
07:06Do you want to watch television?
07:09There you are.
07:10Several of them.
07:11All with pictures.
07:12And we've got some more over here.
07:14Bring this one on here.
07:16This is my special one.
07:19And there's another one coming out.
07:21OK.
07:23So there you are.
07:24You've all got a television to watch.
07:25And have you got one to watch at home?
07:28Oh, of course.
07:28You won't be watching.
07:29Yes.
07:30Right.
07:30Some of you, though,
07:31may have a black and white television.
07:33Well, that's all right.
07:34Because we can take the colour
07:36out of this set
07:37and make it black and white.
07:40There we are.
07:41You see,
07:41a colour television camera
07:43is really three cameras in one.
07:45One for red,
07:46one for blue,
07:47and one for green.
07:48So,
07:50I can make that,
07:52by switching one of those cameras in,
07:54I can make the picture
07:55completely red.
07:57Look,
07:58as I'm blushing.
08:00Completely blue,
08:02like the inside of a fridge.
08:03Or completely green,
08:05like a cabbage now.
08:07Or I can mix them all up
08:08and have colour.
08:11And there they are.
08:11Now,
08:14just as you can add colours
08:16one by one
08:16to make a complete colour picture,
08:18you can also take colours away.
08:20And if you take them away
08:20one at a time,
08:21you can make certain colours
08:22disappear.
08:24I'll show you what I mean.
08:25Have we got the people
08:26in T-shirts, please?
08:27Yellow and blue T-shirts.
08:28Can you come out?
08:32Put that there.
08:34Got a few things to do.
08:35Hang on.
08:35Put that there.
08:42Put that there.
08:44Can you hold that
08:45for a second
08:46and we'll look at you first
08:48because you're
08:49in a yellow T-shirt,
08:51aren't you?
08:51So,
08:52if we take the yellow
08:52away from the picture,
08:54your shirt will disappear
08:55and we'll see what's inside you.
08:58And there it's happened,
08:59look.
09:00That's your inside.
09:01Hang on,
09:02what have you had for dinner?
09:03There's a half-eaten kipper there.
09:05You could save that
09:05for tea,
09:06wouldn't you?
09:06All right.
09:07You come out of it.
09:08What that actually is,
09:09you see,
09:10is this card
09:11that's superimposed
09:13on the space
09:14when we've taken
09:14your shirt away.
09:15So,
09:16you come out
09:16and you go in there now.
09:19That's lovely.
09:20What's your name?
09:21I'm sorry.
09:22Russell.
09:22Russell.
09:22And yours is?
09:23Catherine.
09:23Catherine.
09:24I forgot to ask.
09:24I'm sorry.
09:25Okay, Russell.
09:26You stand there.
09:26Now,
09:26you've got a blue shirt
09:27and a blue background here.
09:29So,
09:29if we take blue away,
09:30the whole lot should disappear
09:32and we should be left
09:33with your head.
09:34And I'm holding a plate here
09:36and we might
09:38just get your head on a plate
09:39and there you are.
09:41With your head on a plate.
09:42Can you see it?
09:42Don't bob about.
09:43I'll drop you.
09:44Look straight up at my face.
09:46Look straight up in the sky.
09:47There we are.
09:48Oh, Russell.
09:50Okay.
09:51Fair enough.
09:51There you are.
09:52Your head on a plate.
09:54Thank you very much, Russell.
09:55Do you want to take the tray with you?
09:56And if you go and sit down
09:57and you come in here.
09:59No, hang on.
09:59I haven't finished with you.
10:00You stand there
10:01because you don't have to walk back to your seat.
10:03We can actually put you in your seat.
10:05I want you to stand here
10:06and look in that,
10:07look at the fellow over there.
10:08All right.
10:09In that direction there.
10:10And we should be able to
10:12take away the background
10:13and put the audience in its place.
10:15And there you are
10:15sitting back in your seat.
10:17Isn't that incredible?
10:19Right.
10:19Okay.
10:20Thanks for being walk back to your seat.
10:21Now, can you give him a round of applause
10:22because he's ever so well.
10:28So there you are.
10:30Television relies very much on colour.
10:32And colour is only part of the spectrum.
10:35Just the middle bit.
10:36I've got to map the spectrum.
10:37Here.
10:37Here.
10:37It didn't come out very well.
10:50We need it tightening up at this end.
10:51Oh, that's lovely.
10:52Thank you very much.
10:53There's a fellow behind there.
10:54Now, here's the visible spectrum.
10:57What we get from the sun is white light.
10:59But it really is made up
11:01of all the colours of the rainbow.
11:03And you can see it being split up
11:04when you see a rainbow in the sky.
11:07Now, here,
11:08there's a form of light that we can't see
11:11called ultraviolet light.
11:13And this is the kind of light
11:14that gives you sunburn,
11:15produces vitamin D in the body.
11:17In fact, a lot of it
11:19that comes from the sun
11:20is soaked up by the atmosphere,
11:21which is just as well
11:22because it would burn us all up.
11:25However, ultraviolet light
11:26can be used in other ways.
11:28Here's a white card.
11:30If I shine ultraviolet light onto it,
11:34it illuminates these letters.
11:36The letters have been treated
11:37with fluorescent paint.
11:39And the light is coming
11:39from a fluorescent bulb
11:42with ultraviolet light.
11:44As you see.
11:46I'll switch it on and off.
11:48So, ultraviolet light
11:49can be used in security.
11:51You can make a mark
11:52with fluorescent paint
11:53that can't be seen
11:54on something like a banknote.
11:56Shine ultraviolet light onto it
11:57and you can see it.
11:59Some more light you can't see,
12:00it's about here.
12:02It's called infrared.
12:04And that is the heat source.
12:07You heat a red-hot poker,
12:08it glows.
12:09And that's infrared that's glowing.
12:10In fact, when the red has faded,
12:12infrared's still coming out of it.
12:13Infrared radiation's coming
12:15from our bodies.
12:16Can I sit here?
12:18Are you feeling warm?
12:19Yeah.
12:20I'm roasting.
12:21And that's all infrared.
12:22Now, we've got an infrared camera
12:24over here.
12:25That's that one there.
12:26OK.
12:27And we've got an infrared lamp
12:29up there.
12:30Now, it doesn't look
12:30as though it's on
12:31because infrared is invisible.
12:33But it is on,
12:34shining at us.
12:35And if you look at the monitors now,
12:36you'll see two pictures.
12:38The top one
12:39is the actual television picture
12:40and the bottom one
12:41is an infrared picture.
12:43Now, watch what happens
12:44when we fade the lights out
12:46in the studio.
12:49The lights are going
12:50and we're all in the dark.
12:51And yet, amazingly,
12:53you can still see us
12:54on the screen.
12:56And we're lit by infrared.
12:57I've got a candle here.
12:58Can you hold that?
12:58I've just found it.
13:00We can only just see it,
13:01can't we?
13:01We're nearly in the dark.
13:03Now, watch what happens
13:03when I light the candle.
13:06The match flares up
13:08because the camera
13:09is sensitive to heat.
13:14And you get two great flares.
13:16OK.
13:17Now,
13:17snuff of that.
13:20Oh, light's on.
13:21Thanks very much.
13:22Have you got my glasses?
13:23Yes.
13:23So there we are.
13:24There's just two kinds of light
13:25that we can't see.
13:27But there's a whole spectrum
13:28of light
13:30all coming from the sun.
13:32So let's have a closer look
13:33at the sun.
13:34The sun is 330,000 times
13:46more massive than the earth.
13:48And it's like a gigantic hydrogen bomb
13:50continually exploding.
13:52All the energy on earth
13:53has come originally from the sun.
13:57In a square metre,
13:58there's enough energy
13:59to boil a kettle
14:00every 20 seconds.
14:02If only we could harness
14:04that energy.
14:06So let's see
14:08how we're getting on
14:09in harnessing
14:10the sun.
14:13Quite recently,
14:15somebody decided
14:15to hold
14:16a competition
14:18to build a car
14:20powered by just
14:22one
14:22single
14:23solar cell.
14:26That's a solar cell.
14:28And the heat from the studio
14:29is making the wheels turn around.
14:31So can somebody come out
14:32and could you come out
14:33and help?
14:35I want you to let
14:36this one go first.
14:37Put them over there
14:38and then just take
14:39the shield off.
14:41This one was made
14:42by the post office.
14:45And here it comes.
14:46It might just stick.
14:48That's right.
14:48And there it goes.
14:52Made out of balsa wood,
14:53a small electric motor
14:54and one cell.
14:56Can you send
14:56the next one down?
14:58This was made by Pi.
15:00It was a very successful one.
15:03A fair whizzes along,
15:05doesn't it?
15:05But the winner
15:06of the competition
15:07became the
15:09Rolls-Royce
15:10of solar powered cars.
15:12It was made
15:13by Rolls-Royce.
15:14And here it comes.
15:15And it travelled.
15:18I'll send it back.
15:20It travelled
15:20at about
15:21six miles an hour.
15:24Can you stop it
15:25just before it hits the rail?
15:26That's it.
15:27I'll send the others
15:28back as well.
15:29If you scale
15:30that speed up,
15:31that six miles an hour,
15:33it's equivalent
15:34to about
15:35144 miles an hour.
15:38But that doesn't mean
15:39that they can make
15:40a Rolls-Royce car
15:41solar powered,
15:43travel at 144 miles an hour.
15:45There are massive
15:46problems to overcome.
15:48But people all over
15:48the world
15:49are trying
15:50to harness
15:51that power from the sun.
15:52And there is one machine
15:54that has actually
15:55functioned
15:56and carried a man
15:58in flight.
16:00A solar powered
16:01plane.
16:06This is just a model
16:07of it.
16:09In fact,
16:10it's only one tenth
16:11the size
16:12of the actual plane.
16:13It's called
16:14solar one.
16:16And it's British.
16:20Prop at the front.
16:22And the wings
16:23are covered
16:25in solar panels.
16:27Now,
16:28it's ten times
16:31that big,
16:31the actual plane.
16:32So if I was standing
16:33here...
16:33Hello.
16:35You can sit down
16:36now if you like.
16:37Thanks a lot.
16:38That was smashing.
16:39If I was standing here,
16:41the wingtip would be
16:42the girl on the end
16:43in the yellow.
16:43Could you raise your hand?
16:45The wingtip would be
16:46about there.
16:47And the other wingtip
16:48would be the same distance
16:48in that direction.
16:49Unless, of course,
16:50it crashed,
16:50in which case the wingtips
16:51could be everywhere.
16:52But it hadn't crashed.
16:53It's actually flown.
16:54It's flown to a height
16:55of 80 feet.
16:56It's flown
16:58at about
16:5954 knots.
17:00It doesn't take off
17:01until it's going
17:01about 18 or 20 knots.
17:03And it's travelled
17:04for three quarters
17:05of a mile,
17:06at least.
17:08But every sunny day,
17:09they're trying
17:10to improve
17:11this plane.
17:12And who knows,
17:13in the very near future,
17:15we might get it to fly
17:15quite a distance
17:17and won't take
17:19too long
17:20before they'll be
17:21attempting
17:22to cross the channel.
17:23Let's have a puzzle now.
17:28Who's got a blindfold?
17:29You've got a blindfold.
17:30Could you blindfold me?
17:31Because I'm going
17:31to do this trick
17:32without the aid
17:33of any light whatsoever.
17:35Ah,
17:36have you done this before?
17:37Is it very good at it?
17:39Oh!
17:39Oh, that's lovely.
17:40I can't see a thing.
17:41Now, somebody had
17:41a piece of paper.
17:42Can you give me
17:43the piece of paper?
17:44And I want to ask you,
17:45I'm going to tear
17:45this piece of paper up.
17:46And I want to ask
17:47the people in the audience,
17:48is there anybody
17:49who likes
17:51sunbaity,
17:52who really likes
17:53soaking up
17:53the sun?
17:54Is there anybody?
17:55Yes.
17:56Oh, good.
17:57That's lovely.
17:58Right.
17:58I want you to give,
17:59I'll just tear this paper up.
18:01And I want you to give
18:01that person
18:02a piece of paper.
18:03Okay.
18:06Give them that piece of paper
18:07and a pencil.
18:09Who's got the pen?
18:09Somebody's got the pencils.
18:10Right.
18:11And people who don't
18:13like sunbathing,
18:14are there many of those?
18:16Are there?
18:16Yes.
18:17I can't see a thing.
18:17Right.
18:19Some more pieces of paper.
18:20Give them all
18:21a piece of paper
18:22and a pencil.
18:22And when you've got
18:24a piece of paper each
18:25and a pencil,
18:26I want you to write
18:27a three-figure number.
18:29Somebody's dropped
18:29all the pencils, I think.
18:31A three-figure number.
18:32A number between
18:32a hundred and a thousand
18:34on the paper.
18:36As soon as you've finished,
18:37pass them back
18:38to my helpers here.
18:40And you can put
18:41the pieces of paper
18:41on my hands.
18:42Okay, is that the lot?
18:48Yes.
18:48Oh, it's the lot.
18:49Right.
18:50I'll shut them up.
18:51All right.
18:51Can I take this blindfold off?
18:54Come on, get it off!
18:56Oh, it wasn't a bad job.
18:57Now, I don't know
18:59where these pieces of paper
19:00came from,
19:01but you can always tell
19:03when somebody lights the sun
19:04because they seem
19:05to radiate heat.
19:08So, just by feeling
19:09their piece of paper,
19:10I should be able to tell
19:11which one really liked
19:14the sun.
19:18And I think it's the person
19:19who wrote 106.
19:22Is that right?
19:23Yeah.
19:24That's really amazing,
19:25isn't it?
19:26Actually, it's not.
19:27It's a trick.
19:28And if you'd like to know
19:29how to do the trick,
19:30drop me a line,
19:31Johnny Ball,
19:31think of a number,
19:32and I'll explain
19:33because it's really
19:33nothing to do with this.
19:34But the sun is very important.
19:38And somehow,
19:39we've got to crack
19:39our energy crisis.
19:42And the aim is
19:43to build
19:45solar panels
19:47in space,
19:49send a rocket up
19:50so that we get
19:50the radiation from the sun
19:51before it hits the atmosphere.
19:53These solar panels
19:54could be perhaps
19:55miles square,
19:57always facing the sun,
19:58sunbathing,
19:59basking in the radiant heat,
20:02gathering the energy
20:03and beaming it down to earth.
20:05That's the plan.
20:06If we do that,
20:07we may have at last
20:09conquered our energy problems.
20:11Because if we can do that,
20:13we will never run out of energy.
20:16Goodbye.
20:16applause
20:18applause
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