The Nature of Light

Photochemical reactions involve interactions of light with matter. They either induce chemical changes, or they induce luminescence and conversions of electronic and other forms of energy into heat. For practical purposes, the photochemical reactions that are discussed here are limited to those that take place in the presence of light that ranges from ultra-violet to infra-red. Light, independent of the source, travels in vacuum with a velocity equal to 2.9979 x 10¹⁰ cm/s.

But that is only true in a vacuum. Within any medium, the light moves with a velocity of:

where e is the dielectric constant of the medium and m is the magnetic permeability.

Fig. 10.1 The instantaneous electric, E_y and magnetic H_x field strength vectors of a light wave as a function of position along the axis of propagation (from Calvert and Pitts [82])

Our concept of light is that it also consists of packets of energy that travel in waves. In each packet, there is a range of energies. These energies cannot be represented by one wavelength, but rather by a whole spectrum of wavelengths. The energy of each particular wavelength in the wave-packet is a discrete unit, a quantum. Electromagnetic radiation is described in terms of a transverse plane wave involving associated electric and magnetic fields. Experimental data suggest that the electric vector E and magnetic vector H which describe the respective field strengths are aligned in planes at right angles to one another, with both planes perpendicular to the direction of propagation of the wave. This was illustrated by Calvin and Pitts [82], as shown in Fig. 10.1. A convenient model for the variation of the field strength as a function of time K and distance x along the axis of propagation is one that can be described in Cartesian coordinates by the sinusoidal functions in the following equations:

In these equations, Ey is the electric field strength vector lying in the xy plane and increasing along the y-axis, Hz is the magnetic field strength vector lying in the xz-plane and increasing along the z axis, A is the amplitude of the electric vector (the intensity of the wave is proportional to A2), e is the dielectric constant, and m is the magnetic permeability of the medium through which the light wave is transported. In a vacuum, e = m and they are approximately unity in air. The length of the wave, that is, the distance between adjacent maxima in the vectors measured at any instant along the direction of wave propagation (the x axis) is l, while v is the frequency or number of complete cycles of vector position change per second. The relationship between l and v is:

C/v = λ

where C is the velocity of the radiation. The frequency v is independent of the medium through which the radiation travels. Wavelength l and velocity C, on the other hand, depend on e and m of the medium. Ordinary light is not polarized. It consists of many electromagnetic vectors that are undulating in fixed, though randomly oriented with respect to each other, planes. When the light is polarized in a plane, it is believed that all the waves have their electric vectors oriented in the same direction. When the light is polarized elliptically, then it is believed that two plane waves of equal wavelength and frequency and with identical directions of propagation have the electric vectors perpendicular to one another and out of phase, as shown in Fig. 10.1. The degree of polarization of light p is usually expressed by the equation

where I|| and I⊥ represent the intensities of light transmitted by an analyzer parallel and perpendicular to the plane of maximum polarization of ht incident light. The above-described model is incomplete and even called naive by some. Mathematically, however, it can successfully account for many observations concerning light, and this theory has been used successfully to describe many phenomena associated with optics.

اخر الاخبار

اخبار العتبة العباسية المقدسة

بمشاركة 72 متسابقًا.. المجمع العلمي ينظم مسابقة قرآنية في بغداد

قسم الصناعات يباشر تصنيع أبواب خشبية لمكتبة ودار مخطوطات العتبة العباسية المقدسة

شعبة فاطمة بنت أسد تنظم زيارة إلى الراقدات في مدينة الإمام الحسين (عليه السلام) الطبية

قسم الإعلام يختتم الدورة الثانية الخاصة بالتصوير الفيديوي لمنتسبيه

المزيد

الآخبار الصحية

نصائح علمية لتقوية دماغك وتقليل التدهور العقلي

هرمون الذكورة يفاقم عدوى خطيرة تهدد حياة الرجال

سبب رئيسي للإصابة بسرطان القولون

بعد سن الأربعين.. هل أنت من الفئات المعرضة لخطر الغلوكوما؟

المزيد

الآخبار التكنلوجية

حرق الأمونيا.. كيف يساعد محطات الكهرباء في اليابان على إزالة الكربون؟

الشمس تقذف نتوءا عملاقا باتجاه قطبها الشمالي

كيف يخدع النعناع جسمك ليشعر بالبرودة؟.. صور عالية الدقة تكشف السر لأول مرة

ما الدور الذي يلعبه الـ AI في "حروب أميركا"؟

المزيد

مواضيع ذات صلة

The Electron Transfer Process

Energy Transfer Process

Interaction of Light with Organic Molecules

Electricity-Conducting Polymers

Immobilized Reagents

Immobilized Nonenzymatic Catalysts

Immobilized Catalysts

Special Gels for Drug Release

Support Materials Other Than Polystyrene

Materials Based on Polystyrene

Polymer Supports for Reagents, Catalysts, and Drug Release

Degradation of Polymeric Materials by Ionizing Radiation