Light and the

Electromagnetic Spectrum

Light Phenomenon• Light can behave like a wave or like a

particle

• A “particle” of light is called a photon

The Electromagnetic Spectrum

The electromagnetic spectrum represents the range of energy from low energy, low frequency radio waves with long wavelengths up to high energy, high frequency gamma waves with small wavelengths.

Frequency Ranges

• The shorter the wavelength, the higher the frequency.

• The longer the wavelength, the lower the frequency.

• Close your eyes for a moment……..

Frequency and Wavelength

• The frequency (f) of a wave is the number of waves to cross a point in 1 second (units are Hertz – cycles/sec or sec-1)

• the wavelength (λ) - the distance from crest to crest on a wave. measured in meters (m), centimeters (cm), and nanometers (nm).

• The product of wavelength and frequency always equals the speed of light.

c = λf• Why does this make sense?• NOTE:

c is a constant value= 3.00 x 108 m/s

• Calculate the wavelength of yellow light emitted from a sodium lamp if the frequency is

5.10 x 1014 Hz (5.10 x 1014 s-1)

List the known info List the unknownc = 3.00 x 108 cm/s wavelength (λ) = ? cmFrequency (f) = 5.10 x 1014 s-1

C = λf λ = c f

λ = 3.00 x 108 cm/s = 5.88 x 10-7 cm

5.10 x 1014 s-1

PROBLEMS

YOUR TURN

• The photoelectric effect – When light shines on metals, electrons (photoelectrons) are ejected (or released) from their surface.

• Planck discovered the relationship between a quantum of energy and the frequency of radiation.

• Einstein called the particles that carried a quantum of energy photons.

Red light will not cause electrons to eject!

Energy of light

• The photoelectric effect has practical applications in photoelectrical cells used for solar powered cars, and solar powered calculators.

Below is a picture of the spectral lines given off by hydrogen. Note there are 3 different frequencies.

Hydrogen-Atom Line-Emission Spectrum

• Current is passed through a glass tube containing Hydrogen.• This current excites the electrons into an excited state.• The excited electrons fall back down to its ground state and gives off energy in the form of colored light • Each element has its own uniqueline-emission spectrum

• The emission spectra makes it possible to identify inaccessible substances. Most of our knowledge of the universe comes from studying the emission spectra of stars.

• Below is the spectra of a few more elements.

Helium

• Neon

• Argon