2 Light & Electromagnetic Spectrum

EM Radiation Passing an electric current through gas in a gas discharge tube energizes the electrons of the atoms of the gas, and causes them to emit light… WHY??? To understand this, we must first understand light.

EM Radiation Light consists of electromagnetic waves. Electromagnetic radiation includes radio waves, microwaves, infrared waves, visible light, ultraviolet waves, X-rays, and gamma rays.

Waves A wave cycle starts at zero, crest highest value trough lowest value amplitude of a wave is the wave’s height from zero to crest Wavelength is the distance between the same points on two “next door” waves Symbol: Greek letter lambda,  Units: m(meters) or nm(nanometers) crest trough

Waves The number of waves passing a given point per unit of time is the frequency (Greek letter nu,). The SI unit is called the hertz (Hz).

Waves Relationship Between Wavelength & Frequency For waves traveling at the same velocity, the longer the wavelength l, the smaller the frequency n. This means that wavelength and frequency are inversely related to one another. As  ,  … as  , 

Light The color of light depends on its frequency (and wavelength) Red has the longest wavelength, the lowest frequency, and the lowest energy LONG  LOW  LOW E Dec.  Inc.  Inc. E SHORT  HIGH  HIGH E

Waves Energy, Wavelength & Frequency Waves that have a shorter wavelength (and therefore a higher frequency) have higher energy. Waves that have a longer wavelength (and therefore a lower frequency) have lower energy. e.g. waves at a beach and erosion Wavelength and frequency are inversely related Wavelength and energy are inversely related:  , E  , E Frequency and energy are directly related:  , E  , E

Continuous Spectra & Light Sunlight consists of light with a continuous range of wavelengths and frequencies When sunlight passes through a prism, the different frequencies separate into a continuous spectrum of colors.

Light LONG  LOW  LOW E Dec.  Inc.  Inc. E SHORT  HIGH  HIGH E

Atomic Emission So how does light relate to electrons??? atoms absorb energy electrons move into higher energy levels electrons then lose energy by emitting light when they return to lower energy levels.

Atomic Emission Each electron “jump” produces a distinct brightly colored line in the spectrum. This spectrum is called an atomic emission spectrum and each element’s spectrum is unique light emitted by an electron moving to lower energy level has a frequency directly related to the energy change of the electron.

Atomic Emission Absorption of Energy Atom absorbs energy from some outside source (e.g. electricity, heat, etc.) Electron has too much energy to stay in the energy level it is in Electron moves from GROUND STATE to EXCITED STATE + +

Atomic Emission Emission of Energy Electron in an excited state is now UNSTABLE It loses its excess energy and moves back to a lower energy level The greater the “leap”, the more energy given off Each “leap” is equal to a an exact frequency + +

Atomic Emission The greater the energy transition (“leap”), the higher the energy released, the higher the frequency, the shorter the wavelength Color in the “visible region” is dependent on this transition. Energy H