Question

Explain why it is necessary to select only one wavelength to pass through the sample (i.e. why don’t we just shine white light into the sample and measure the amount of light that passes through?)

Answer 1

We use electromagnetic spectrum to analyse the chemical samples. Visible light (white light) is an electromagnetic spectrum which our naked eye can see. It has wavelengh of 260 nm to 760 nm (400–790 terahertz). The white light consists of VIBGYOR (violet, indigo, blue, green, yellow, orange, red). each colour will have specific wavelength and speific freuqency.

During the spectroscopic analysis a sample needs to be irradiated with electromagnetic radiation. As the radiation passes through the sample the absorbed radiation will determine the concentration or type of the sample.

Each sample will absorb specific wavelenth or frequency of the electromagnetic radiation. That is the characteristic feature of the sample.

For example take UV-Visible spectroscopy.

If we take methyl halides as an example, these methyl bromide or iodide will absorb radiation ~ 255 nm.

Hence it is necessary to select the wavelength to pass through the sample.