EL2

EL2

1. Spectroscopy, Absorption Spectra and Emission Spectra

   • SPECTROSCOPY: Many spectroscopic techniques exist but all are based on the same scientific principle - under certain conditions, a substance can absorb or emit EM radiation in a characteristic way. There are different types of EM radiation and by analysing the EM radiation, you can identify a substance or find information about it, such as its structure and the way atoms are held together.
   • ABSORPTION SPECTRA: Glowing stars emit all of the light frequencies between ultraviolet and infrared parts of the EM spectrum. The Sun emits mainly visible light, whereas stars hotter than the Sun emit mainly UV radiation. Outside a star's surface (the photosphere), is a region called the chromosphere that contains ions, atoms and small molecules. These particles absorb some of the emitted radiation so the light analysed from stars is missing certain frequencies. The absorption lines appear black as these are the missing frequencies of light - they've been absorbed by the particles in the chromosphere.
     • EMISSION SPECTRA: When the atoms, molecules, and ions in the chromosphere absorb energy, they are raised from their ground state (their lowest energy state) to higher energy states called excited states. The particles then become unstable and can lose their extra energy by emitting EM radiation. The resulting emission spectra can also be detected. Emission spectra appear as coloured lines on a black background - corresponding to the frequencies emitted.
   • CONTINUOUS AND ATOMIC SPECTRA: White light contains all the visible wavelengths and its spectrum is normally continuous, like a rainbow. However, light seen from stars is not continuous. It consists of lines, corresponding to the absorption or emission of specific frequencies of light - atomic spectra.
     • THE ATOMIC SPECTRA OF HYDROGEN ATOMS: The Sun's chromosphere consists mainly of hydrogen and helium atoms and so hydrogen atoms dominate the chromosphere's emission spectrum, but helium emission lines can also be seen. Hydrogen atoms also have a characteristic emission spectrum in the UV region of the EM spectrum. The hydrogen emission spectrum in visible light is known as the Balmer series. The hydrogen emission spectrum in UV light is known as the Lyman series.

2. Light and Electrons

   • Spectroscopy = the study of how light and matter interact. In order to understand spectroscopy, you need to know about the behaviour of light.
     • BOHR'S THEORY AND WAVE-PARTICLE DUALITY: Chemists use 2 models to describe the behaviour of light - the wave theory and the particle theory. Neither fully explains all the properties of light, but some are explained by the wave model and others by the particle model.
   • THE WAVE THEORY OF LIGHT: Light is one form of EM radiation. Like all EM radiation, it behaves like a wave with a characteristic wavelength λ and frequency v. A wave of light travels the distance between 2 points in a certain time - it doesn't matter what kinda of light it is, the time is always the same. The speed the wave moves, the speed of light c, is the same for all kinds of light and EM radiation. It has a value of 3.00 x 10^8 m/s when the light is travelling in a vacuum.
     • Different colours of light have different wavelengths. Waves can also have different frequencies. For example, both the waves in the image below travel at the same speed, so they both travel the same distance per second but wave B has twice the frequency of wave A.