Chemistry Unit 04 Notes: Light and the Electromagnetic Spectrum

What is Light?

  • Light is electromagnetic radiation.

  • We see it as color and brightness.

  • Why do we see it? Because of electrons.

    • The negatively charged, least massive subatomic particle.

Electromagnetic Spectrum

  • So light is because of electrons.

  • Do we see all the light that is possible - NOPE!

  • All the ranges of light are a part of the electromagnetic spectrum.

  • EMS - the range of wavelengths and frequencies over which electromagnetic radiation extends.

  • Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma Ray

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  • We see light in the visible range which is only a small portion of the EMS.

  • All light travels at the same speed - the speed of light.

  • It is a constant value equal to about 3\cdot10^8 m/s (that’s 300 million m/s btw).

  • So if all of the light travels at the same speed, what makes it different?

  • Light travels at the same speed, but if you look back at the definition of EMS, you see that you have to take into account the wavelength and frequency of the light.

  • c=\lambda f c is the speed of light (remember that is a constant), \lambda is the wavelength of the light in meters and f is the frequency in Hertz which is 1/sec.

  • If we look at this equation and know that every time our answer for c is 3\cdot10^8 m/s we can see the relationship between the wavelength and the frequency of light.

  • As one of them increases the other one has to decrease. This is an inverse relationship.

  • So we can only see light that has specific wavelengths and frequencies.

What is Light?

  • So we are back to the “what is light” question.

  • We now know it has to do with electrons.

  • We also know there is a vast range of light and only a small portion that we can actually see. We see light with wavelengths of about 390 nm to 700 nm. (There are one billion nm in a meter. That should sound familiar).

  • When electrons are excited, they absorb energy.

  • This makes them jump or leap to a new energy level.

  • What goes up, must come down. That excited electron has to come back to its normal energy level.

  • When it does, it gives off energy in the form of LIGHT!

  • Finally we answer the question! Light is the visible radiation from an excited electron falling back to its normal resting place.

Atomic Spectrum

  • When an electric current is passed through a gas tube, e- are energized and they emit light.

  • When atoms absorb E, e- move into higher E levels.

  • The e- then lose the E by emitting light as they return to the lower E levels.

  • So another nod to PS - LOCOE (Law of Conservation of Energy). That E absorbed by the e- really isn’t lost. It gets converted to light.

  • Light that is emitted by atoms consist of a mixture of only a few specific frequencies.

  • Each frequency corresponds to a particular color.

  • The frequencies of light emitted by an element separates into discrete lines that give the atomic emission spectrum of each element.

  • Each element has its own unique spectrum.

  • We can use this like a fingerprint or DNA for that element to determine if it is present in a sample.

  • You can look at the absorption spectrum or the emission spectrum for an element.

  • When you place the gas tube in the power supply and cut it on, you excite e- to a higher E levels and then they fall back down to their original E levels and give off the light.