Optics Grows in Meaning
Earlier, Optics was the study of eyesight and vision. When lenses and other visual-aid devices were invented then Optics took on the broader meaning of the study of light, its applications, and its properties when passing through different media. With more advancements and discoveries, the twentieth-century meaning of optics also includes the study of the electromagnetic radiation spectrum, which includes the part invisible to the human eye, like X-rays, ultraviolet, infrared, and microwave radio waves.
Birth of Optics
Optics was first studied and discussed by Abu Yusuf Al-Kindi, an Arab philosopher, mathematician, physician, polymath, and musician in the 7th century. The next notable work on Optics appeared in the 10th century by another Arab mathematician Ibn Sahl, who wrote a treatise on Ptolemy’s Optics. Ibn Sina, another giant of the Islamic Golden Age, supported the views of Ibn Sahl.
However, it was only in the 11th century that Hasan Ibn Haytham, an Arab mathematician, astronomer, and physicist of the Islamic Golden Age, wrote the Book of Optics, where he laid the foundation of modern-day optics. He is considered the Father of Optics. He was the first to figure out that we see something when light reflects from an object and enters the eye, and that image is interpreted by the brain and not the eye. Five centuries before the Renaissance, he was the earliest proponent of the basic concept of science – that hypothesis needs to be aided by experiments that follow confirmable procedures or mathematical evidence.
Qutub-ud-Din Al-Shirazi and his student Kamal-ud-Din Al-Farisi took Haytham’s findings forward in the 13th and 14th centuries, revised his book, and also defined and explained the rainbow for the first time.
Meanwhile, in the West, several mathematicians studied and theorized upon the works of Euclid, Aristotle, and the Arab scholars.
In the 16th and 17th century Johannes Kepler studied the lunar and solar eclipses. Willebrord Snellius found Snell’s Law and Descartes calculated the angle of the rainbow during the same time.
Then in the late 17th century Newton unveiled the spectrum and wrote Opticks, overshadowing anyone else in Optics for half a century.
What Study Optics?
Optics studies the origin and spread of light and the changes it undergoes and produces, and other phenomena associated with it, when passing through different media.
Parts of Optics
Optics has two major branches – physical and geometrical. Physical optics is concerned about the nature and properties of light. Geometrical optics studies the principles that govern the image-forming properties of lenses, mirrors, and other devices using light. Geometrical optics also studies optical data processing, which is the manipulation of the information content of an image formed by coherent optical systems.
Smaller branches of optics are atmospheric, geometrical, and quantum.
Spectrophotometer
The spectrophotometer is an instrument that measures the amount of intensity of light absorbed by the sample solution as a function of wavelength. This technique of measuring the amount of absorbed light through sample solution is known as spectrophotometry. The uses of a spectrophotometer include quantitative analysis of various known compounds in a mixture. It is used in various fields such as chemistry, biochemistry, chemical engineering etc. This instrument is used by scientists also for various purposes.
The spectrophotometer was invented by Arnold J. Beckman in 1940. The instrumentation of the spectrophotometer is given below –
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Working of a spectrophotometer - In a spectrophotometer, commonly used UV radiation sources are hydrogen and deuterium lamps and for visible radiation, a tungsten filament lamp is used. For IR radiation Nernst Glower is used. A monochromator is used to resolve polychromatic radiation into individual wavelengths and differentiate them into very narrow bands. A monochromator includes a collimator, prism or grating and a slit. The sample solution is taken into cuvettes or sample containers. One detector is used to detect the current. Generally, photocells are used as detectors. So, it works on the photoelectric effect. The current which the detector detects is proportional to the light intensity so indirectly it measures the light intensity. The Digital meter displays the reading.
What is a Spectrometer?
It is used as a part of the spectrophotometer. It is a device for detecting and analyzing wavelengths of electromagnetic radiation, generally used for molecular spectroscopy. In a spectrophotometer, it is used to produce the desired range of wavelengths of light. Thus, the spectrometer uses electromagnetic radiation for spectroscopic analysis of sample materials.
A photometer is also used in the spectrophotometer. The photometer detects the amount of intensity of light. It includes a detector or digital display.
Spectrophotometer Principle
The spectrophotometer can be operated in the UV region, Visible spectrum and IR spectrum as well. This instrument is based on photometric techniques. According to photometric technique, when a beam of incident light of intensity I0 , passes through a solution, a part of the incident light is reflected, a part is absorbed and the rest of the light is transmitted. If the part of incident light which got reflected is Ir , the part which got absorbed is Ia and the part which got transmitted is It . Then we can write –
I0 = Ir + Ia + It
In the spectrophotometer, Ir is eliminated because the measurement of It and I0 is enough for the measurement of Ia. So, Ir is kept constant. The relationship between the amount of light absorbed and the concentration of the substance can be established by following two laws-
Beer’s Law
According to this law, the amount of light absorbed is directly proportional to the concentration of solute in the solution under analysis.
Ia c
Where Is = light absorbed
c = concentration of solute in the solution.
Lambert’s Law
According to this law, the amount of light absorbed is directly proportional to the length or thickness of the solution under analysis.
Ia l
Where Is = light absorbed
l = length or thickness of the solution
Thus, in simple words the spectrophotometer is based on the Beer-Lambert Law which states that the amount of light absorbed is directly proportional to the concentration of the solute in the solution and thickness of the solution under analysis.
A cl
Where A = absorbance
c = concentration
l = path length
A = ∈cl
Where ∈ = absorption coefficient
Difference between Spectrometer and Spectrophotometer
By spectrometer, we can measure the wavelength of absorbed light and reflected light while by using a spectrophotometer, we can measure the relative intensity of light absorbed and reflected.
Applications of Spectrophotometer
A spectrophotometer is used to know the concentration of solutes colorless or coloured in a solution.
It is used for the determination of the rate of reaction by measuring the rate of formation and disappearance.
It is used in forensic sciences.
It is used in molecular biology. We can measure the growth of microorganisms like bacteria by spectrophotometer.
UV – spectrophotometer is used in the pharmaceutical industry to determine the composition of the drugs.
It is used in the foods and paints industry.
It is used in water quality checks.
Blood is analyzed by a spectrophotometer.
It is used in the diagnosis of diseases.
It is used in the detection of impurities in organic compounds.
This topic is the first detailed introduction of spectroscopy with students. This is an important topic for students to make their base stronger for higher studies in spectroscopy or chemical sciences.
