Glossary
Absorbance (A)
A quantitative measure of how much light a sample absorbs at a specific wavelength, directly proportional to concentration. It is a unitless quantity.
Example:
If a solution has an absorbance of 0.8, it means a significant portion of the incident light was absorbed by the sample.
Absorption
The process by which a substance takes in light energy at specific wavelengths, leading to a decrease in the intensity of the transmitted light.
Example:
A plant leaf appears green because it undergoes strong absorption of red and blue light wavelengths for photosynthesis.
Beer-Lambert Law
A fundamental law in spectrophotometry stating that the absorbance of a solution is directly proportional to its concentration and the path length of the light through the solution.
Example:
Using the Beer-Lambert Law, a scientist can determine the unknown concentration of a protein solution by measuring its absorbance.
Binding Energy
The minimum energy required to remove an electron from an atom or ion, often referring to the valence electron for ionization.
Example:
A high binding energy indicates that an electron is strongly attracted to the nucleus and requires a significant amount of energy to be removed.
Complementary Colors
Pairs of colors located opposite each other on the color wheel; a substance absorbs its complementary color most strongly.
Example:
Since a solution appears blue, it will have the highest absorption of orange light, which is its complementary color.
Concentration (c)
The amount of solute dissolved in a given volume of solvent, typically expressed in moles per liter (mol/L or M).
Example:
By measuring the absorbance of a solution, we can use the Beer-Lambert Law to calculate its unknown concentration.
Cuvette
A small, transparent, rectangular container used to hold liquid samples for analysis in a spectrophotometer, ensuring a consistent path length for light.
Example:
Before placing it in the spectrophotometer, the solution was carefully poured into a clean cuvette to ensure accurate light transmission.
Detector
The part of a spectrophotometer that measures the intensity of light that has passed through the sample, converting it into an electrical signal.
Example:
The detector converts the transmitted light into an electrical signal, which the spectrophotometer then uses to calculate the sample's absorbance.
Dispersion Device
A component, such as a prism or diffraction grating, within a monochromator that separates white light into its individual wavelengths (colors).
Example:
A diffraction grating serves as the dispersion device inside a spectrophotometer, spreading the incoming white light into a full spectrum.
Entrance Slit
The initial opening in a monochromator that allows a narrow beam of white light to enter before it is dispersed into its constituent wavelengths.
Example:
Light from the lamp first passes through the entrance slit of the spectrophotometer, ensuring a focused beam for wavelength separation.
Exit Slit
The final opening in a monochromator that allows only the precisely selected wavelength of light to pass through to the sample.
Example:
After the light is split, only the specific wavelength chosen for the experiment is allowed to exit through the exit slit and proceed to the sample.
Frequency (v)
The number of wave cycles that pass a fixed point per unit of time, typically measured in Hertz (s⁻¹).
Example:
Red light has a lower frequency than blue light, meaning its waves oscillate less rapidly.
Molar Absorptivity (ε)
A constant that indicates how strongly a specific substance absorbs light at a particular wavelength, unique to each substance and wavelength. Its units are L/mol·cm.
Example:
A high molar absorptivity value for a compound means it is very effective at absorbing light at that specific wavelength, making it easier to detect even at low concentrations.
Monochromator
A component within a spectrophotometer that isolates and selects a specific wavelength (color) of light from a broader spectrum to pass through the sample.
Example:
The monochromator in a spectrophotometer acts like a prism and a filter, allowing only the desired green light to illuminate a red solution.
Path Length (b)
The distance that light travels through the sample in the cuvette, typically measured in centimeters.
Example:
Most standard cuvettes have a path length of 1.0 cm, meaning the light travels exactly one centimeter through the solution.
Planck's constant (h)
A fundamental physical constant that relates the energy of a photon to its frequency, with a value of approximately 6.626 × 10⁻³⁴ J⋅s.
Example:
In the photoelectric effect, the energy of emitted electrons is directly proportional to the frequency of incident light, with Planck's constant as the proportionality factor.
Reflection
The process by which light bounces off the surface of a substance, determining the color we perceive if the substance is opaque.
Example:
A white shirt appears white because it undergoes reflection of nearly all visible wavelengths of light.
Sample
The substance or solution being analyzed in a spectrophotometer, typically contained within a cuvette.
Example:
To determine its concentration, the colored liquid is placed in a cuvette, becoming the sample that the light will pass through.
Spectrophotometry
A quantitative analytical technique used to measure how much light a chemical substance absorbs by shining a light beam through the sample and measuring the intensity of light that passes through.
Example:
A chemist uses spectrophotometry to determine the amount of caffeine in a coffee sample by measuring its light absorption at a specific wavelength.
Speed of light (c)
The constant speed at which all electromagnetic radiation travels in a vacuum, approximately 2.998 × 10⁸ m/s.
Example:
Radio waves and X-rays both travel at the speed of light in a vacuum, despite having very different wavelengths and frequencies.
Wavelength (λ)
The distance between two consecutive crests or troughs of a wave, typically measured in meters or nanometers.
Example:
Different colors of visible light correspond to different wavelengths, with red light having a longer wavelength than violet light.