Chem Talk Lab 5

In this lab we looked at the colours that make up visible light using LED’s. Light is a form of energy known as electromagnetic radiation. Electromagnetic radiation is the energy that travels through space as waves. For example, X-rays, ultraviolet rays (UV), microwaves, radio waves and visible light. The electromagnetic spectrum is a list of electromagnetic radiation in order of wavelength.

Light is characterized by its wavelength and its energy. The wavelength of light is the distance between two corresponding points of a wave. It could be from crest to crest, or from trough to trough. The units for wavelength is nanometers. (a nanometer = 1*10^-9 meters)

All waves of light travel at the same speed in a vacuum. This speed is called the speed of light. In a vacuum the speed of light is 3.00*10^8 m/s. There is also something called frequency. Frequency is the number of cycles of a wave that occur in a second. The frequency and wavelength of light are related:

  • c = λ * f
  • c = speed of light = 3.00*10^8 m/s
  • f = frequency in waves per second
  • λ = wavelength in meters 

Waves with longer wavelengths have relatively low frequencies. Waves with shorter wavelengths have relatively high frequencies. (as wavelength increases, frequency decreases) The energy of light is related to the frequency and wavelength of the light.

  • E = h*f
  • E = energy in Joules
  • h = Planck’s constant (6.63*10^-34J)
  • f = frequency in s^-1

Light that has a long wavelength has a less energy than light that has a short wavelength. The wavelengths in visible light range from 700nm to 400nm. When the wavelength becomes shorter, the energy increases. The wavelength of light determines if the light has enough energy to interact with the electrons in an atom. 

The valence electrons of the atom take part in the chemical reactions. The electrons absorb energy going from ground state to an excited state. The excited state means the electrons have been raised to a higher energy sub level. Eventually the electrons will fall from the excited state back down, given up energy. When light is emitted from atoms or molecules it is called fluorescence. This process is when the electron absorbs energy, becomes excited then quickly releases energy as visible light when coming back to the ground state. When the glow-in-the-dark tape gave off light, this process was called phosphorescence. With phosphorescence the electron is at the excited state but temporarily gets stuck there in this intermediate state depending on the conditions like the temperature. This results in a delayed emission of light. 

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