Sources, Detection and Uses

Transcript

0:00Introduction

Hello everyone! Let me start with a small puzzle from your kitchen. Put some food on a dry ceramic plate, and run the microwave oven for a minute. The food gets hot, but the waves do not heat the dry plate. Same waves, same oven, so why the difference? Keep that question in mind. Today we walk through the electromagnetic spectrum, from radio waves to gamma rays: what each region is used for, where it comes from, and what our atmosphere does to it. These uses come up often in JEE, so let's begin.

0:35Radio waves and microwaves

Here is the whole spectrum on one strip. Frequency, the Greek letter nu, increases to the right, in hertz, and wavelength, lambda, increases to the left. On the far left are the longest waves, the radio waves, which carry radio broadcasts. For A M radio, the band is five hundred and thirty to one thousand seven hundred and ten kilohertz, where k H z means kilohertz. For F M, it is eighty eight to one hundred and eight megahertz, M H z. Okay? Television broadcasting, written T V, uses fifty four to eight hundred and ninety megahertz. And your mobile phone? It also uses radio waves, in the ultra-high frequency band, written UHF on screen. Clear? Now, just to the right of radio come the microwaves. Radar uses microwaves, not the longer radio waves. Let me ask you this: why do you think that is? Their short wavelength, yes, exactly! Short waves give a narrow beam, aimed in one definite direction. So microwaves are used in radar, in satellite communication, and in speed guns, exactly as the screen says. Got it? And now, our kitchen puzzle. The frequency of a microwave oven matches a resonance of water molecules. So the water in the food absorbs the energy and shares it with the food as heat. The dry plate has no water, so the waves do not heat it, and that solves our puzzle. With me?

2:03Infrared and visible light

Moving right, we reach infrared. Infrared waves are also called heat waves, because most materials absorb them readily and warm up. That is why physiotherapy lamps use infrared, simple as that. Infrared is also used in remote controls, in night vision, and to take photographs through haze and fog. Remember the remote control; we will need it soon. Right? Then comes the narrow visible band. This is what gives us vision. Visible light is emitted when electrons in atoms jump between energy levels. Clear?

2:41Ultraviolet, X-rays and gamma rays

Beyond violet comes ultraviolet, written U V for short. Ultraviolet kills germs, so it is used in water purifiers, and to sterilise surgical instruments. Okay? It is also used in LASIK eye surgery, and to detect forged documents. And one small fact worth remembering: ordinary glass absorbs ultraviolet. Right? Further right, we have the X-rays. In medicine they are used to find fractures, and to treat some cancers. They are also used to study crystal structure. Finally, at the far right, gamma rays, with the highest frequency. They are used in radiotherapy, to destroy cancer cells. With me?

3:26Where they come from, and how we detect them

Now, sources and detectors, from the table in the previous concept. Radio waves are produced by charges oscillating in aerials. Microwaves are made by a klystron or a magnetron. Okay? Infrared comes from vibrating atoms and molecules. We detect it with a thermopile or a bolometer. Right? X-rays come from an X-ray tube, and gamma rays from the radioactive decay of nuclei. Both are detected by photographic film or by a Geiger tube. Clear?

3:59What the atmosphere lets through

Now, what does our atmosphere do to these waves? It is transparent to visible light and to most radio waves. These are called the optical window and the radio window. Okay? So let us understand the greenhouse effect. Visible sunlight passes through the air and warms the ground. The warm ground then radiates infrared. So which of the two does the air hold back? The infrared, that is correct! Water vapour and carbon dioxide absorb the infrared from the ground, and send part of it back. This is the greenhouse effect, and it keeps the Earth warm. Got it? The Sun is a strong source of ultraviolet, but the ozone layer absorbs most of it. And X-rays and gamma rays are absorbed high up in the atmosphere. Clear? Next, the ionosphere, high up in the atmosphere. It reflects short-wave radio back down, and that makes long-distance broadcasting possible. So tell me, for a link to a satellite, would short-wave radio work? No, it would be reflected back by the ionosphere, and never reach the satellite. Microwaves pass through the ionosphere, so satellite links use microwaves. With me?

5:15Example 1: radar, TV remote, sterilising, crystal structure

Time for the examples. Name the part of the spectrum used, in part a, in radar; in part b, in TV remote controls; in part c, to sterilise surgical instruments; and in part d, to study crystal structure. Pause the video and try it first. Done? Okay, let's check. Radar uses microwaves, because of the narrow beam. Now for part b, television signals are radio waves, so the remote also sends radio waves, right? No, no, wait, that is the trap! The broadcast comes as radio waves, but the remote control in your hand uses infrared. We saw it earlier, okay? In part c, ultraviolet kills germs, so ultraviolet sterilises the instruments. And in part d, crystal structure is studied with X-rays. All four done in seconds. Easy, right?

6:09Example 2: photon energy in eV and its region, 4 kW source

Now a harder one, JEE level. We have a source of power four kilowatts, the k W on screen, and it emits ten to the power twenty photons per second. Find the energy of each photon in electron volts, written e V, and the region of the spectrum. Planck's constant, h, is given as six point six times ten to the power minus thirty four joule second. Pause the video and try it first. Done? Okay, let's check. Four kilowatts is a lot of power. So these must be gamma rays, right? No, no, careful, that is the trap! The region depends on the energy of each photon, not on the total power. Four kilowatts means four thousand joules every second, and these four thousand joules are shared by ten to the power twenty photons. So capital E, the energy of one photon, is four thousand over ten to the power twenty, giving four times ten to the power minus seventeen joule. Okay? Now change to electron volts. One electron volt is one point six times ten to the power minus nineteen joule. So divide: four times ten to the power minus seventeen, over one point six times ten to the power minus nineteen, gives two hundred and fifty electron volts. Right? For the region, we need the wavelength. Use the shortcut from the previous concept: lambda in nanometres equals one thousand two hundred and forty over E in electron volts. One thousand two hundred and forty over two hundred and fifty gives about five nanometres; the wavy equals sign means about, and n m means nanometre. We never needed h separately; the shortcut already has h and c inside it. Clear? In the table of regions, ultraviolet runs from four hundred nanometres down to one nanometre. So, by this table, five nanometres is ultraviolet, close to the soft X-ray border. The borders between regions are only conventional, so some charts, like our spectrum strip, put it among the soft X-rays. Either way, it is not gamma rays at all. Always find the energy of one photon first, got it?

8:20Quick revision: uses from radio waves to gamma rays

So, quick revision. Long waves: radio for broadcasting, and microwaves for radar and ovens. In the middle: infrared, the heat waves, then visible light, then ultraviolet, which kills germs. Short waves: X-rays for fractures, and gamma rays to destroy cancer cells. And the atmosphere lets visible light and most radio waves through, while the ionosphere reflects short-wave radio. Now go and try the practice problems on this page. All the best!