1. Living Under the Sun: Challenges

2. Interactions of light and matter
Emission Transmission
Absorption Reflection or Scattering

3. Light is an electromagnetic wave*.

4. Wave Characteristics
Frequency (ν): number of waves that pass a point in a given period of time
Total energy is proportional to amplitude and frequency of waves.
Because speed of light (c) is a constant (3 x 108 m/s),wavelength and frequency of electromagnetic waves are inversely proportional:
E = hν; c = νλ

5. Color
White light is composed of all colors which can be separated into a rainbow, or a spectrum, by passing the light through a prism.
Each color light has a different wavelength, and, therefore, frequency.
Spectrum and spectroscopy

6. Amplitude & Wavelength

7. Are there other “Colors”?

8. The Electromagnetic Spectrum
Visible light comprises only a small fraction of all the wavelengths of light – called the electromagnetic spectrum.
Short wavelength (high frequency) light has high energy.
- Gamma ray light has the highest energy.
Long wavelength (low frequency) light has low energy.
- Radiowave light has the lowest energy.

9. Electromagnetic Spectrum

10. Each chemical element produces its own unique set of spectral lines.
Oxygen spectrum
Neon spectrum

11. Emission and absorption spectra are inversely related.
Spectra of Mercury

12. Light can induce electron transitions.
To transition to a higher energy state, the electron must absorb energy equal to the energy difference between the final and initial states.
Electrons in high energy states are unstable. They will transition to lower energy states and emit light.

13. Light induces electron transitions in molecules.
absorbs UV light
“colorless”
Visible light does not have enough energy to move electrons to next energy level.
absorbs visible light
“colored”
Energy levels are closer in “conjugated” system. Visible light is energetic enough to induce transition!

14. Light induces other transitions in molecules.
Energy can also be absorbed and emitted in other “modes” including vibration and rotation. OH

15. Light can induce photochemical reactions.
Photolysis
O2(g)  2 O(g)
O3(g)  O2(g) + O(g)
Conformational change
11 cis retinal to all trans retinal

16. Visible
Greenhouse gases!
Ozone!

17. Sunburn, vitamin D

18. UV Absorbing Molecules
UV – specific mutagen, non-specific oxidative damage
Photochemical reactions
Skin Pigments – absorb and dissipate energy

19. Photochromic Pigments!
How about those beads?
Photochromic Pigments!
Higher energy form
Lower energy form

20. Sunscreen Ingredients
Organic compounds absorb light and dissipate energy as heat.
Octyl methoxycinnamate
Homosalate
Inorganic compounds, e.g. ZnO and TiO2, reflect or scatter light.