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

Short answer

The compound will appear red.

Step-by-step solution

Understanding Color Absorption and Appearance

When a compound absorbs light at a certain wavelength, it absorbs most of that color from the visible spectrum. The compound reflects or transmits other colors, which are not absorbed, giving the compound its apparent color. The color directly opposite on the color wheel to the absorbed color is typically seen.

Identifying the Absorbed Color

The problem states that the compound absorbs light in the green wavelength region. In the visible spectrum, green light typically has wavelengths between 495 and 570 nanometers.

Determining the Complementary Color

To find the color that the compound will appear, locate green on a color wheel and identify the complementary (opposite) color. On a traditional color wheel, the complementary color to green is red.

Concluding the Apparent Color

Since the compound absorbs green light and the complementary color to green is red, the compound will appear red to an observer.

Key concepts

Visible Spectrum

The visible spectrum is the portion of the electromagnetic spectrum that is visible to human eyes. It includes all the colors that can be seen in a rainbow, ranging from violet to red. Each color corresponds to a different wavelength of light. These wavelengths are typically in the range of 400 to 700 nanometers (nm).

The colors of the visible spectrum are often remembered by the acronym ROYGBIV, which stands for Red, Orange, Yellow, Green, Blue, Indigo, and Violet. When light passes through a prism, it spreads out into these distinct colors, forming a spectrum.

• **Red**: Long wavelength (~620-750 nm)
• **Green**: Middle wavelength (~495-570 nm)
• **Violet**: Shortest wavelength (~380-450 nm)

Understanding the visible spectrum helps in analyzing how different materials absorb or reflect different wavelengths, resulting in the colors we observe.

Wavelength

Wavelength is a fundamental concept in understanding light and color. It refers to the distance between consecutive peaks of a wave, such as those found in electromagnetic radiation. Light waves vary in length, and this affects the color that we see.

The wavelength of light determines its color:

• **Red Light**: Longer wavelengths (~620-750 nm)
• **Green Light**: Intermediate wavelengths (~495-570 nm)
• **Blue Light**: Shorter wavelengths (~450-495 nm)

Wavelength is crucial for understanding how different materials interact with light. A material will absorb certain wavelengths and reflect others, which is why we perceive different colors. For example, if a substance absorbs green light, it does not reflect it, and the color we observe is largely due to the reflected light that is not absorbed.

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. On the color wheel, complementary colors are opposite each other.

For example:

• Green and Red
• Blue and Orange
• Yellow and Violet

When a compound absorbs one of a pair of complementary colors, it will appear as its opposite. This is because the absorbed color is removed from the light that is reflected from or through the substance, emphasizing the complementary color. This is why a compound absorbing green light appears red.

Color Wheel

A color wheel is a circular diagram that represents the relationships between colors. It visually displays complementary colors directly opposite each other.

Understanding a color wheel can help identify which colors are complementary:

• **Primary Colors**: Red, Blue, Yellow
• **Secondary Colors**: Green, Orange, Violet (created by mixing primary colors)
• **Tertiary Colors**: Intermediate colors formed by mixing primary and secondary hues

In the context of light absorption, identifying the absorbed color on the color wheel helps determine the color perceived. This knowledge aids in predicting how a material will appear when it interacts with light from the visible spectrum.

Light Absorption

Light absorption refers to the process where a material takes in light energy rather than reflecting or transmitting it. Different substances absorb different wavelengths based on their molecular structure.

This process occurs when light energy excites the electrons in a molecule. When a material absorbs specific wavelengths of light, the colors corresponding to those wavelengths are removed from the spectrum of light that is either reflected or transmitted. The remaining light, which is not absorbed, determines the color of the material.

• **Example**: If a material absorbs wavelengths corresponding to green, it appears in the complementary color red.
• **Frequency and Energy**: The absorbed wavelength corresponds to a specific energy level, allowing electrons to move to higher energy states.

Understanding light absorption is key in fields such as chemistry and biology, as it explains why substances have distinct colors when viewed under various lighting conditions.