Chemistry / Unit 3: Electrons and the Periodic Table
3.1 Light and Energy
Notes
Light is an electromagnetic wave that also comes in packets of energy called photons.
- Wavelength ($\lambda$) is the distance between wave peaks; frequency ($\nu$) is waves per second, in hertz (Hz $= \text{s}^{-1}$).
- Speed of light: $c = \lambda \nu$, with $c = 3.00 \times 10^8$ m/s.
- Energy of a photon: $E = h\nu$, with Planck's constant $h = 6.626 \times 10^{-34}$ J·s.
- Longer wavelength means lower frequency and lower energy. Red light has less energy than violet; radio waves less than X-rays.
- Example: $\lambda = 500$ nm $= 5.00 \times 10^{-7}$ m gives $\displaystyle \nu = \frac{3.00 \times 10^8}{5.00 \times 10^{-7}} = 6.00 \times 10^{14}$ Hz.
Practice
What is the wavelength of light with a frequency of $1.00 \times 10^{15}$ Hz?
$\displaystyle \lambda = \frac{c}{\nu} = \frac{3.00 \times 10^8}{1.00 \times 10^{15}} = 3.00 \times 10^{-7}$ m (300 nm).
What is the energy of a photon with a frequency of $6.00 \times 10^{14}$ Hz?
$E = h\nu = (6.626 \times 10^{-34})(6.00 \times 10^{14}) = 3.98 \times 10^{-19}$ J.
Which type of light has the highest energy per photon?
Ultraviolet has the shortest wavelength and highest frequency of these, so the most energy.
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