Chapter 5: Light and Matter

Question 1

What is wavelength? What units of measure does it have?

Answer 1

Wavelength (𝛌) is the distance between corresponding points on two adjacent waves. It is measured in cm/m.
E.g. Radio carrier waves have very long wavelengths, and X-rays have very short wavelengths.

Question 2

Define frequency? What are its units? What is a wavenumber?

Answer 2

Frequency (v) is the number of waves that pass a given point in 1 second. It is measured in cycles per second (Hertz).
Wavenumber is the inverse of the wavelength measured in centimeters. One wavenumber is 1cm-1.

Question 3

How is the relationship between wavelength and frequency expressed?

Answer 3

Wavelength and frequency are expressed in the equation: c=𝛌v. In other words, the speed of light = wavelength times frequency. As frequency increases, wavelength decreases. As frequency decreases, wavelength increases.

Question 4

How is the relationship between the energy of a photon and its frequency expressed?

Answer 4

A photon is a packet of energy. A photon and its frequency are expressed in the equation: E=hv. In other words, energy is equal to the product of Planck’s constant and the frequency. It is also expressed as E=hc/𝛌.
As the frequency of light increases, its energy increases. As the wavelength increases, the energy decreases.

Question 5

Rank the following regions of the electromagnetic spectrum in the order of decreasing energy: infrared, radio, visible, X-ray.

Answer 5

X-ray, visible, infrared, radio.

Question 6

What happens to the molecules of a substance when X-rays strike it? Why are X-rays called “ionizing radiation”?

Answer 6

When X-rays strike matter, they pass through most matter but are reflected by dense matter such as bones. X-rays are called ionizing radiation because they have the capability to remove electrons from the atoms and molecules that they pass through.

Question 7

Why is visible light spectroscopy always measured at the same time as UV/visible light spectroscopy? What happens to molecules when light in these regions strikes them?

Answer 7

Uv and visible light both have the same effect on matter, which is why they are measured at the same time during spectroscopy. When a substance is exposed to UV/visible radiation, it will absorb certain photons of particular energy.

Question 8

What happens to molecules when light in the infrared region strikes them?

Answer 8

When light in the infrared region strikes matter, there is not enough energy to promote electrons. Instead, each bond between atoms will vibrate (like two weights connected by a spring). The frequency of vibration depends on the mass of each atom and strength of the bond.

Question 9

What is the most common type of detector used in UV/visible spectrophotometry?

Answer 9

In UV/visible spectrophotometry, a UV/visible spectrophotometer is used.

Question 10

What is the most common type of detector used in infrared spectrophotometry?

Answer 10

In infrared spectrophotometry, an infrared spectrophotometer is used.

Question 11

What type of light interactions with molecules give rise to molecular fluorescence?

Answer 11

When a substance absorbs energy and emits it in the form of visible light, this is fluorescence. Certain materials are fluorescent and thus give off this property.

Question 12

What does the unit “Hertz” measure?

Answer 12

Hertz is a measure of frequency, in cycles per second.

Question 13

The adsorption of light by molecules is said to be quantized. What does this mean?

Answer 13

Quantization occurs when an atom or molecule absorbs light energy and valence electrons are promoted to higher energy levels. This is because the atom or molecule can only absorb the exact amount of energy that corresponds to the difference in energy between the occupied and unoccupied energy level.

Question 14

What is a monochromator? How is it used in spectroscopy?

Answer 14

A monochromator is a device that selects one particular wavelength (or small packet of wavelengths) to be exposed to the sample. It is a prism or grating that is rotated to expose the sample to steadily increasing/decreasing wavelengths of light through the entire spectrum.

Question 15

If one had two fibers that appeared to be the same colour to the naked eye, what spectroscopic technique would you use to determine if they were?

Answer 15

UV/visible spectroscopy is widely used in the analysis of textile fibers. A microspectrophotometer can be used to compare the colours of two fibers.

Question 16

Briefly describe the purpose of the Michelson Interferometer in FTIR.

Answer 16

Fourier Transform IR Spectrophotometry is used to be a modern solution to the moving parts and degradation of sensitivity that occurs in classical IR spectrophotometry. The Michelson Interferometer is an apparatus including a moving mirror that is used to change the path length of one of the beams.

Question 17

What is diffuse reflectance? On what type of samples is it used?

Answer 17

Diffuse reflectance is one of the methods for obtaining reflectance spectra in IR spectrometry. It uses a set of mirrors that direct the IR source light at the sample at an oblique angle to the surface of the material, which then absorbs some of the light. The light that is not absorbed is reflected off the surface, and reaches the detector. It is used on powders and rough surface solids.

Question 18

What is a diamond cell? When is it used?

Answer 18

In IR spectroscopy, a diamond cell is two tiny diamond chips that flattens out the sample and makes it easier to obtain a spectrum. The light from the source is focused on the diamond windows to get a high-quality spectrum. It can be used when the sample is too small to be made into a KBr pellet.

Question 19

What spectroscopic technique would you use to identify a pure sample of an illicit drug?

Answer 19

I would use Infrared spectroscopy to identify a pure sample of an illicit drug. Every substance absorbs radiation in the IR region of the electromagnetic spectrum. IR spectroscopy can be indicative of the type of atoms, molecules, bonds and chemical properties of substances, which is very useful in determining if a sample of a drug is pure.

Question 20

All spectrophotometric detectors measure the amount of light that passes through a sample (or reflects off its surface). How can we determine what light is absorbed by the sample?

Answer 20

An instrument that measures absorbance would be used to measure what light is absorbed by the sample. Also, subtracting the amount of light reflected from the sample from the amount of light admitted onto the sample would result in the amount of light absorbed.