DESIGN OF COSMETIC SUN SCREENS 99 of matter it is propagated as a wave form. But when it is absorbed it is in the form of discrete packets--photons--whose energy is a function of the wavelength of the wave form. The relationship between energy and wavelength is expressed by the general equation E = • = •(c/x) where E = energy in ergs h = Plank's constant, 6.6242 X 10 -27 erg-second • = frequency, per second X -- wavelength, centimeters, and c = velocity of electromagnetic radiation, 3 X l0 w cm./sec. E3 E• E• E:• GROUND EXCITED STATE STATE AE = E• - E,= hlJ PHOTON -ATOM INTERACTION When atoms are excited by heat or electrical energy their electrons are raised through one or more steps of a possible series of energy levels, E2, Ea, etc., which may be stable, or may revert to a lower level, or even to the ground state with liberation of the absorbed energy. For every drop in energy level, the difference in the energy content of the atom is in the form of monochromatic radiation of frequency such that, for a drop from E•. to E1 AE = E9. -- E• = h•, (2) This radiation forms the emission spectrum of the atoms characteristically shown in an electric arc or in a flame test. When light passes through an absorbing medium, the reverse process takes place: the energy is absorbed and results in raising the energy levels of the electrons in the absorbing medium. This absorption of specific frequencies causes gaps to appear in the spectrum of the original light source which are shown as dark absorption lines at those wavelengths which are absorbed: AE = E• -- Es = h(c/X) (3)
100 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The absorption of energy by molecules is more complex than atomic absorption, especially if the molecules are in the condensed liquid or solid states, or in solution. The total energy content of a molecule is made up of electronic bonding energy and kinetic energy. The latter has both vibrational and rotational components: E = Eboncling d- (E•'ibrational d- Ero,ati .... 1) (4) E TOTAL = EBONDiNG + IEvIBRATIONAL + EROTATIONAL I ENERGY KGAL/GM-MOLE RADIATION BONDING 200 FAR U-V I QQ NEAR U-V 5Q VISIBLE VIBRATIONAL 5 NEAR I-R ROTATIONAL 0..5 FAR I-R Quantitatively, the electronic bonding energies are the largest, and the vibrational energies are greater than the rotational energies. Thus bonding energies are associated with absorption bonds in the ultraviolet and visible regions (200 kcal./mole in the far ultraviolet, 100 kcal./mole in the near ultraviolet, 50 kcal./mole in the visible range. Vibrational changes are of the order of 5 kcal./mole, in the near infrared, and rotational . changes are of the order of 0.5 kcal./mole, in the far infrared. LINE a- BAND ABSORPTION SPEGTRA OF BENZENE 3OO 2OO I00 SOLUTION IN PEN TANE IVAPORI WAVELENGTH
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DESIGN OF COSMETIC SUN SCREENS 99 of matter it is propagated as a wave form. But when it is absorbed it is in the form of discrete packets--photons--whose energy is a function of the wavelength of the wave form. The relationship between energy and wavelength is expressed by the general equation E = • = •(c/x) where E = energy in ergs h = Plank's constant, 6.6242 X 10 -27 erg-second • = frequency, per second X -- wavelength, centimeters, and c = velocity of electromagnetic radiation, 3 X l0 w cm./sec. E3 E• E• E:• GROUND EXCITED STATE STATE AE = E• - E,= hlJ PHOTON -ATOM INTERACTION When atoms are excited by heat or electrical energy their electrons are raised through one or more steps of a possible series of energy levels, E2, Ea, etc., which may be stable, or may revert to a lower level, or even to the ground state with liberation of the absorbed energy. For every drop in energy level, the difference in the energy content of the atom is in the form of monochromatic radiation of frequency such that, for a drop from E•. to E1 AE = E9. -- E• = h•, (2) This radiation forms the emission spectrum of the atoms characteristically shown in an electric arc or in a flame test. When light passes through an absorbing medium, the reverse process takes place: the energy is absorbed and results in raising the energy levels of the electrons in the absorbing medium. This absorption of specific frequencies causes gaps to appear in the spectrum of the original light source which are shown as dark absorption lines at those wavelengths which are absorbed: AE = E• -- Es = h(c/X) (3)
100 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The absorption of energy by molecules is more complex than atomic absorption, especially if the molecules are in the condensed liquid or solid states, or in solution. The total energy content of a molecule is made up of electronic bonding energy and kinetic energy. The latter has both vibrational and rotational components: E = Eboncling d- (E•'ibrational d- Ero,ati .... 1) (4) E TOTAL = EBONDiNG + IEvIBRATIONAL + EROTATIONAL I ENERGY KGAL/GM-MOLE RADIATION BONDING 200 FAR U-V I QQ NEAR U-V 5Q VISIBLE VIBRATIONAL 5 NEAR I-R ROTATIONAL 0..5 FAR I-R Quantitatively, the electronic bonding energies are the largest, and the vibrational energies are greater than the rotational energies. Thus bonding energies are associated with absorption bonds in the ultraviolet and visible regions (200 kcal./mole in the far ultraviolet, 100 kcal./mole in the near ultraviolet, 50 kcal./mole in the visible range. Vibrational changes are of the order of 5 kcal./mole, in the near infrared, and rotational . changes are of the order of 0.5 kcal./mole, in the far infrared. LINE a- BAND ABSORPTION SPEGTRA OF BENZENE 3OO 2OO I00 SOLUTION IN PEN TANE IVAPORI WAVELENGTH

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