Question

Why is monochromatic light used in spectrophotometry?

Answer 1

Monochromatic means "same color". Basically monochromatic is any light which is purely of a single color. In modern terms it is a light of same wavelength.

Many lights are a combination of colors. Sunlight is a combination of seven colors which means that there are seven different light waves with different wavelengths all mixed together to give preception of single color.

Sunlight is a mixture of lights ranging from ~750-450nm in wavelengths (in visible spectrum). If we take a single light wave with wavelength suppose 450nm we will only see a single color, pure violet color. Similarly if we have a light beam of wavelength 570nm we will see pure yellow color. This yellow will not be a mixture of red and green as used in modern display technologies.

This light which has same wavelength will show only a single color and this light will be monochromatic.

Answer 2

heya mate here is your answer
Sometimes the monochromatic light is directed at a sample and the reflected or transmitted light is measured. ... An absorption spectrophotometermeasures the absorption of light by a sample as a function of wavelength.
spectrophotometry
Spectrophotometry is a method to measure how much a chemical substance absorbs light by measuring the intensity of light as a beam of light passes through sample solution. The basic principle is that each compound absorbs or transmits light over a certain range of wavelength.
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