1. 12.6 Light and Atomic Spectra
The quantum mechanical model of the atom grew out of the study of light.
Isaac Newton ( ) thought of light as consisting of particles.
By the 1900’s scientists thought of light as a wave.
We now know that light has properties of both waves and particles.

2. 12.6 Light and Atomic Spectra
In the wave model, light is considered to consist of electromagnetic waves that travel at a speed of 3.0 x 108 m/s (186,000 mi/s).
electromagnet radiation – includes radio waves, microwaves, visible light, infrared and ultraviolet light, X-rays, and gamma rays.

3. 12.6 Light and Atomic Spectra
amplitude – the height of the wave from the origin to the crest.
wavelength(λ, lamda) – the distance between the crests
frequency(ν, nu) – the number of wave cycles to pass a given point per unit of time

4. 12.6 Light and Atomic Spectra
The speed of light (c) in a vacuum is constant and is equal to frequency (ν) multiplied by wavelength (λ) Equation: c = ν λ
This equation can be rewritten as follows:
ν = or λ =
Note:
speed of light = c (must be in m/s)
frequency = ν (must be in reciprocal seconds (s-1) or hertz, Hz)
wavelength = λ (must be in m) c λ c ν

5. 12.6 Light and Atomic Spectra
Wavelength and frequency have an inverse relationship.
This means when the value of one increases the value of the other one decreases.
As wavelength increases, frequency decreases
As wavelength decreases, frequency increases

6. 12.6 Light and Atomic Spectra
Light refers to a small part of the electromagnetic spectrum that our eyes can see
Sunlight consists of light with a continuous range of wavelengths and frequencies
When sunlight passes through a prism, the light separates into a spectrum of colors

7. 12.6 Light and Atomic Spectra
Each color in the spectrum (ROYGBIV) blends into the next.
In the visible spectrum, red light has the longest wavelength and lowest frequency; violet light has the shortest wave length and highest frequency.

8. 12.6 Light and Atomic Spectra
Elements emit light when energy is passed through them.
Electrons absorb energy and are excited to higher energy levels; they then release the energy in the form of light when they return to their ground state.
Passing the light emitted by an element through a prism gives the atomic emission spectrum of the element.
The atomic emission spectrum are unique to each element and are a useful tool for identifying elements.

9. 12.6 Example
Calculate the wavelength of the yellow light emitted by a sodium lamp if the frequency of the radiation is 5.10 x 10-14s-1.
Known: c = 3.0 x 108 m/s
ν = 5.10 x 10-14
Solution - Use the equation: λ =
λ = = = 5.88 x 10-7 m c ν
3.00 x 108 m/s
5.10 x 1014 s-1
3.00 x 108 ms-1
5.10 x 1014 s-1

10. 12.6 Concept Practice
11. List the colors of the visible spectrum in order of increasing wavelength.
12. What is meant by the term frequency of a wave? What are the units of frequency? Describe the relationship between frequency and wavelength.

11. 12.6 Practice
13. What is the frequency of radiation whose wavelength is 5.00 x 10-6cm? In what region of the electromagnetic spectrum is this radiation?

12. 12.6 Practice
14. A hydrogen lamp emits several lines in the visible region of the spectrum. One of these lines has a wavelength of 6.56 x10-5 cm. What are the color and frequency of this radiation?