Problem 59 - A Complete Resource Book in Chemistry for JEE Main

Question

If a compound absorbs in the wave length region corresponding to green, then it will appear (a) red (b) violet (c) green (d) orange

Step-by-Step Solution

Answer

The compound will appear red.

Step 1: Identify the Visible Spectrum Colors

The visible spectrum consists of the colors red, orange, yellow, green, blue, indigo, and violet. Each color corresponds to a specific range of wavelengths. Green typically corresponds to wavelengths around 495-570 nm.

Step 2: Understand Color Absorption

When a compound absorbs light in a particular region of the visible spectrum, it means it removes that color from the reflected light. The color perceived by our eyes is usually the complementary color of the absorbed light.

Step 3: Determine Complementary Colors

The complementary color of green is red. Therefore, if a compound absorbs light in the green wavelength region, it will appear as its complementary color, which is red.

Key Concepts

Visible Spectrum Colors

The visible spectrum refers to the portion of the electromagnetic spectrum that can be seen by the human eye. It includes a range of colors that are produced when light is dispersed, similar to what happens in a rainbow. The primary colors of the visible spectrum are:

• Red
• Orange
• Yellow
• Green
• Blue
• Indigo
• Violet

Each color corresponds to a specific range of wavelengths measured in nanometers (nm). For instance, red light has longer wavelengths ranging from about 620 to 750 nm.
When a material interacts with visible light, it can affect how we perceive its color due to the absorption of certain wavelengths and the reflection of others.

Complementary Colors

Complementary colors are pairs of colors that, when combined, cancel each other out. This means they produce a grayscale color like white or black, when mixed in the right proportions. On the color wheel, complementary colors are located opposite each other.
For example, blue and orange are complementary, as well as yellow and violet. When it comes to color absorption in compounds, when a compound absorbs a particular color, we often see the complementary color.

• For example, if a compound absorbs green light, it appears red, since red is the complementary color to green.

Understanding complementary colors aids in predicting the appearance of compounds based on the wavelengths of light they absorb.

Wavelength of Light

The wavelength of light is a fundamental property of electromagnetic waves, measured in nanometers (nm). It refers to the distance between successive peaks of a wave and determines the color of light we perceive.
Shorter wavelengths correspond to colors such as violet and blue, while longer wavelengths correspond to colors like red and orange. Specifically:

• Violet light ranges from about 380 to 450 nm.
• Blue light ranges from about 450 to 495 nm.
• Green light ranges from about 495 to 570 nm.
• Yellow light ranges from about 570 to 590 nm.
• Orange light ranges from about 590 to 620 nm.
• Red light ranges from about 620 to 750 nm.

By understanding the wavelength of light, we can predict how substances will interact with light, such as which colors will be absorbed and which will be reflected.