 This is the diagram of entire electromagnetic spectrum. Electromagnetic spectrum here includes all the wavelengths that we encounter. For example, the common waves that we normally use is again here, the radio waves. Radio waves are long waves and the microwaves are a little bit shorter here. And we have the visible light, that is the light that we use to see each other and we can see things in this range. And we have on the other hand, on the extreme right here, the hard X-rays and gamma rays. These are the rays that we don't see every day. However, they have a lot of energy and we can feel the effects. The same way here, radio waves are long waves. A wave is, it goes up and down, wave travels like this. The distance between the two peaks is the wavelength. The wavelength here is 10 to the 3, which is 1000 meters long. And on this side, on the hard X-rays and gamma rays, you can see the wavelength is only 10 to the minus 12, which is very, very short. And this also implies that the frequency, the frequency with which the wave goes up and down, like this, up and down, that is, again, the frequency is very, very low here, which is only 10 to the 6, which is kind of a million times it goes up and down in a second. And on this side, where the X-rays are, you can see it is 10 to the 20, which is 1 followed by 20 zeros, so the frequency is very high. Obviously, what we can say from this is, as the frequency increases here, the wavelength is decreasing, or the shorter the waves, obviously the higher the frequency. And what we can say about energy is that these waves, which are very, very short here, or in other words, very high frequency, tend to have very high energy content here. You can see energy content is measured, energy of one photon is measured here in electron volts. This is a million electron volts here for X-rays and gamma rays, whereas here it is 10 to the minus 9, which is 1 over a billion, which is very, very less. So what we learn from this is, these long waves, the radio waves, et cetera, don't have that much energy, whereas the X-rays and gamma rays, which have high frequency and short waves, wavelength, they are very high energy content. And that's the reason they have very high energy content, and that's the reason they can penetrate through our bodies and so on and so forth.