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.
FAQs on Spectrophotometer Principle
1. What is the fundamental principle of a spectrophotometer?
The fundamental principle of a spectrophotometer is based on spectrophotometry, a technique used to measure how much light a chemical substance absorbs or transmits. The instrument works by passing a beam of light of a specific wavelength through a sample solution. The amount of light that passes through the sample is measured by a detector, and by comparing this to the initial intensity of the light, the instrument calculates the substance's absorbance. This measurement is governed by the Beer-Lambert Law.
2. Explain the Beer-Lambert law, which is the core of spectrophotometry.
The Beer-Lambert law is a combination of two separate principles:
- Beer's Law: This states that the amount of light absorbed by a solution is directly proportional to the concentration of the absorbing substance in it.
- Lambert's Law: This states that the light absorbed is directly proportional to the path length, or the thickness of the solution that the light has to travel through.
Combined, the Beer-Lambert Law (A = εcl) establishes that the absorbance (A) of a solution is directly proportional to both its concentration (c) and the path length (l) of the light through the sample. The term 'ε' represents the molar absorption coefficient, a constant unique to the substance at a specific wavelength.
3. What are the main components of a spectrophotometer and their functions?
A spectrophotometer consists of several key components that work together to measure absorbance:
- Light Source: Provides a stable source of polychromatic light (light of many wavelengths). Common sources are tungsten lamps for visible light and deuterium lamps for UV light.
- Monochromator: This component isolates a specific, narrow wavelength of light from the source. It typically uses a prism or a diffraction grating to split the light and a slit to select the desired wavelength.
- Sample Holder (Cuvette): A transparent container of a specific path length that holds the sample solution. It is designed to not absorb light at the wavelength being measured.
- Detector: Measures the intensity of the light that passes through the sample. It is often a photodiode that converts the detected light into an electrical signal based on the photoelectric effect.
- Digital Display: Processes the signal from the detector and displays the result as an absorbance or transmittance value.
4. What is the primary difference between a spectrophotometer and a spectrometer?
The main difference lies in what they measure. A spectrometer is a device that separates light into its constituent wavelengths and measures the spectrum, but it does not quantify the intensity at each wavelength. A spectrophotometer is a more advanced instrument that includes a spectrometer to select a specific wavelength and also incorporates a photometer to measure the intensity of light absorbed or transmitted by a sample at that chosen wavelength. In short, a spectrometer identifies wavelengths, while a spectrophotometer quantifies light intensity at those wavelengths.
5. What are some common real-world applications of a spectrophotometer?
Spectrophotometers are versatile instruments used across various scientific and industrial fields. Key applications include:
- Biochemistry: To determine the concentration of proteins, DNA, and other biomolecules.
- Chemical Analysis: For quantitative analysis of coloured or colourless compounds in a solution.
- Pharmaceutical Industry: To verify the composition and purity of drugs.
- Environmental Science: Used in water quality testing to detect pollutants.
- Clinical Diagnostics: To analyse blood and tissue samples for disease diagnosis.
- Food & Beverage Industry: For quality control, such as measuring colour and consistency.
6. Why does the Beer-Lambert law sometimes fail at high concentrations?
The Beer-Lambert law assumes that the absorbing particles are independent of each other. This holds true for dilute solutions. However, at high concentrations, several factors cause deviations:
- Intermolecular Interactions: Solute molecules become so close that they interact with each other, which can alter their ability to absorb light (changing the molar absorptivity, ε).
- Chemical Changes: High concentrations can lead to chemical reactions like association or dissociation, changing the nature of the absorbing species.
- Refractive Index Changes: Very concentrated solutions can have a significantly different refractive index than the solvent, affecting the amount of light that reaches the detector.
Because of these effects, the linear relationship between absorbance and concentration breaks down, leading to inaccurate measurements.
7. How does a monochromator work to select just one wavelength of light?
A monochromator isolates a specific wavelength using a two-step process. First, it uses a dispersing element, which is typically either a prism or a diffraction grating. This element takes the incoming polychromatic light from the source and separates it into its constituent wavelengths, much like how a prism creates a rainbow. Second, this dispersed spectrum of light is projected onto an exit slit. By precisely rotating the prism or grating, only a very narrow band of the spectrum is allowed to pass through the slit and continue on to the sample. The rest of the light is blocked.
8. What is the impact on absorbance readings if the sample cuvette has fingerprints or is scratched?
Fingerprints, dirt, or scratches on a cuvette's surface significantly compromise the accuracy of spectrophotometer readings. These imperfections scatter the incident light, preventing a portion of it from travelling directly through the sample to the detector. The instrument cannot distinguish between light that was absorbed by the sample and light that was scattered by the flawed cuvette surface. It interprets this loss of light as absorption by the sample, resulting in a falsely high absorbance reading and an overestimation of the sample's concentration.
9. How does a spectrophotometer differ from a simpler instrument like a colorimeter?
The key difference lies in the precision of wavelength selection. A colorimeter is a simpler device that uses a set of coloured filters to isolate a broad range of wavelengths (e.g., all red light). It is less precise and generally used for routine analyses where high accuracy is not critical. In contrast, a spectrophotometer uses a sophisticated monochromator (with a prism or grating) to select a very narrow, specific wavelength of light. This allows for much higher accuracy, sensitivity, and the ability to analyse the entire absorption spectrum of a substance, not just its colour.
10. In what way does the photoelectric effect contribute to a spectrophotometer's function?
The photoelectric effect is the core principle behind the spectrophotometer's detector, which is the component that actually measures light. The detector is typically a photodiode or photomultiplier tube. When photons of light that have passed through the sample strike the surface of the detector, they transfer their energy to electrons. This energy ejects the electrons from the detector's material, creating a small but measurable electrical current. The magnitude of this current is directly proportional to the intensity of the light hitting the detector. The instrument's electronics then measure this current to quantify the transmitted light.
