Problem 3 - Arihant AIEEE Physics

Question

The rising and setting of sun appear red because of : (a) refraction (b) reflection (c) diffraction (d) scattering

Step-by-Step Solution

Answer

The red appearance is due to scattering.

Step 1: Understanding the Phenomenon

The rising and setting of the sun appear red due to atmospheric effects. To determine which option is correct, it's important to understand the basic concepts involved in the interaction of sunlight with the atmosphere.

Step 2: Defining Atmospheric Scattering

The atmosphere contains various particles and gases that cause sunlight to scatter. The phenomenon of light scattering explains the color changes we observe in the sky at different times of the day.

Step 3: Why Scattering Causes Red Sunsets

During sunrise and sunset, the sun is at a low angle in the sky, causing sunlight to pass through a thicker layer of the atmosphere. Blue and violet light scatter out of the direct path of sunlight due to shorter wavelengths, leaving red and orange hues, which have longer wavelengths, to reach our eyes, making the sky appear red.

Step 4: Eliminating Other Options

Refraction involves bending of light, reflection involves bouncing back of light, and diffraction involves bending and spreading of light around obstacles. None of these explains why only red hues are visible during sunrise and sunset, making scattering the best explanation for the red color seen.

Key Concepts

Atmospheric Scattering

Atmospheric scattering is a fascinating physical phenomenon that occurs when sunlight interacts with atmospheric particles and gases, influencing the colors we see in the sky. Light waves are scattered in all directions by the small molecules present in the Earth's atmosphere. This scattering is primarily responsible for the blue color of the sky during the day. When sunlight enters the atmosphere, it collides with air molecules and water droplets, causing it to scatter. This effect is more pronounced for shorter wavelengths of light, such as blue and violet.

• Rayleigh scattering is a type of scattering that affects shorter wavelengths more dramatically.
• This scattering is selective, meaning it occurs more for blue light as compared to red light.
• The scattered light that we see when looking away from the sun is predominantly blue.

These scattered blue light wavelengths dominate, giving the sky its signature daytime color. However, during sunrise and sunset, a different scattering effect explains the sun's red appearance.

Sunrise and Sunset

At both sunrise and sunset, the sun appears larger and redder in color, a characteristic phenomenon explained by atmospheric scattering. As the sun is low on the horizon, it passes through a much thicker layer of atmosphere compared to when it is higher in the sky at midday. This journey through the atmosphere causes more scattering of the sunlight.

• During these times, the path of sunlight is oblique.
• It travels through more atmospheric particles, intensifying the scattering effect.
• Shorter wavelengths like blue and violet scatter out of the direct line of sight, while longer wavelengths, such as red and orange, dominate.

This atmospheric condition results in the beautiful red and orange hues that are typically visible during these periods.

Wavelength of Light

Understanding how light's wavelength affects its behavior in our atmosphere is crucial to grasping why different colors appear in the sky at different times. Light consists of electromagnetic waves, and each color of light has a specific wavelength corresponding to how long the wave is.

• Violet and blue light have shorter wavelengths, around 400-500 nanometers, which makes them scatter more.
• Red and orange light have longer wavelengths, around 600-700 nanometers.
• The difference in wavelength causes colors to scatter differently in the atmosphere.

Light with shorter wavelengths (blue and violet) cannot travel as far through the atmosphere and is scattered away, whereas longer wavelengths (red and orange) travel farther and remain visible during sunrise and sunset.

Sunlight Interaction with Atmosphere

As sunlight enters Earth's atmosphere, it interacts with numerous small particles and gases that make up the air, leading to various optical phenomena such as scattering, reflection, and refraction. Scattering takes precedence over other processes in determining the sky's color and how sunlight appears at different times of the day.

• During the day, direct sunlight is scattered predominantly.
• In the context of sunrise and sunset, sunlight traveling through more atmosphere leads to more substantial scattering of shorter wavelengths.
• The direct path of sunlight is left with predominantly longer wavelengths, giving the sky a red tint.

Light refraction and reflection also occur but are much less significant than scattering in influencing the change of color during sunrise and sunset. This interaction between sunlight and the Earth's atmosphere causes the natural and captivating color displays we observe daily.