Problem 188 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

A dye absorbs the wavelength corresponding to blue colour of light. The observed colour will be (a) blue (b) orange (c) green (d) red

Step-by-Step Solution

Answer

(b) orange

Step 1: Understand Absorption and Observation

When a substance absorbs a specific wavelength or color of light, it is effectively removing that color from the spectrum of light that passes through or reflects from it. The color we actually observe is the complementary color of the absorbed light.

Step 2: Identify the Complementary Color

In this problem, the dye absorbs blue light. Since blue is being absorbed, it will not be seen. Instead, its complementary color will be observed. The complementary color to blue on the color wheel is orange.

Step 3: Conclude Based on Complementary Colors

Since the absorbed color is blue, and its complementary color is orange, the observed color of the dye will be orange.

Key Concepts

Complementary Colors

In chemistry, complementary colors are key to understanding the observed color of substances. Complementary colors are pairs of colors which, when combined, cancel each other out, producing a grayscale color like white or black. This occurs because the two colors contain all the wavelengths necessary to produce a complete spectrum of visible light.
To visualize this, imagine the color wheel. It is a circular diagram of colors arranged by their chromatic relationship. On this wheel, every color has a direct opposite. These opposites are known as complementary colors.

• For example, red is complementary to green.
• Blue is opposite to orange.
• Yellow complements purple.

When a dye absorbs a color, it is that color which we do not see. Instead, the color observed is the complementary one. This principle plays a crucial role in identifying the perceived color of materials.

Light Absorption

Light absorption is a vital interaction between light and matter. It occurs when atoms, molecules, or ions in a substance absorb specific wavelengths of light. When this happens, these wavelengths are no longer available to the light that is transmitted or reflected, altering the color we perceive.
During light absorption, the energy from the absorbed light is usually converted into heat or used to drive chemical reactions. The specific wavelengths absorbed depend on the substance’s molecular structure. These structures have specific energy levels, and light with energy corresponding to these levels will be absorbed.

Here are some quick pointers to catch on:

• When a dye absorbs blue light, it does not reflect this color, so you will not see blue.
• Absorption typically occurs within specific ranges of the light spectrum called absorption bands.
• The wavelengths not absorbed are reflected or transmitted, and these combinations define the color you see.

Thus, understanding the absorption helps in predicting the appearance of chemical substances under different lighting conditions.

Observed Color in Chemistry

The observed color in chemistry refers to the colors that substances appear to our eyes based on what wavelengths of light they absorb or reflect. As light strikes a material, some wavelengths are absorbed while others are reflected. The observed color is determined by the wavelengths we perceive after this interaction.
For example, if a substance absorbs blue light, the color we observe will be its complement—orange. This is because the blue wavelengths are missing, and the mix of remaining wavelengths leads to the appearance of orange.

• To determine the observed color, start by identifying the absorbed color.
• Find the complementary color to the absorbed wavelength.
• The complementary color is typically what appears to the human eye.

Knowledge of observed color is not only fascinating in academic contexts but also has practical applications, like creating dyes and pigments in various industries.