Scattering of Light

Light is a type of energy that causes our eyes to experience vision and allows us to perceive various objects in our environment. The light ray could be self-light or reflected light from the object. Luminous objects are those that produce their own light. Sun, bulb, tube light, glow worms, etc. Objects that reflect light from other sources are known as non-luminous objects. They don’t produce their own light. Moon, tree, table, and painting, for example. Light can behave as a ray (reflection), a wave (interference), or a diffraction particle (photoelectric effect). Before discussing the scattering of light we’ll have some related concept 

What is Refraction of Light?

The change in direction of a wave travelling through one medium to another is known as refraction.

One of the most well-known occurrences is light refraction, although other waves, such as sound and water waves, can also refract. Because of refraction, we can employ optical equipment such as magnifying glasses, lenses, and prisms. Light refraction allows us to focus light on our retina.

Atmospheric Refraction: Atmospheric refraction is the refraction of light by different layers of the atmosphere. Atmospheric refraction is the bending of light rays as they move through layers of the earth’s atmosphere with different optical densities. Varying gases and dust particles have different optical densities. Light from a star is refracted by the atmosphere at different altitudes due to changing optical densities of air. When an object sends light rays into the atmosphere, they pass through multiple air layers of varied densities and are refracted by the atmosphere.

Some facts related to Atmospheric Refraction are:

• Apparent Star Position: It’s caused by the refraction of starlight in the atmosphere. Different temperatures and densities exist in different layers of the atmosphere. As a result, we have a wide range of media to choose from. As a light source, a faraway star is used. When starlight enters the earth’s atmosphere, it undergoes continual refraction and bends towards the normal as its refractive index changes from rare to thick. As a result, the apparent position of the star deviates from its true position. The star appears to be larger than it is.

Bending of star’s light

• Twinkling of Star: It’s also due to atmospheric refraction. A solitary point of light comes from a faraway star. The apparent position of the star changes when the beam of starlight deviates from its trajectory due to the physical condition of the earth’s atmosphere. As a result, the amount of light that enters our eyes changes, being bright at times and dull at others. The “Star Twinkling Effect” is the name for this phenomenon.

Scattering of Light

A portion of the light is absorbed by the medium’s particles when it goes from one medium to another, such as air or a glass of water, followed by subsequent radiation in a specific direction. This phenomenon is known as light scattering. 

The intensity of scattered light is influenced by particle size and light wavelength. Shorter wavelengths and higher frequencies scatter more due to the waviness of the line and its interaction with a particle. A line is more likely to collide with a particle if it is wavy. Longer wavelengths, on the other hand, have a lower frequency and are more straight, which means they have a smaller likelihood of colliding with a particle. In the afternoon, the bending of multicolored light may be seen due to refraction and total internal reflection of light. The wavelength of sunlight generates different colors in different orientations. Rayleigh’s scattering theory explains the red color of the sun in the morning and the blue color of the sky.

Different forms of Scattering of light

Light dispersion takes place in many forms that are discussed below as:

• Elastic Scattering: When the energy of the incident and scattered beams of light is the same.
• Inelastic Scattering: When the energy of the incident beam of light and the dispersed beam of light differs. Inelastic scattering is further classified into four types:
  • Rayleigh Scattering: When radiation (light) interacts with molecules and particles in the atmosphere that have a smaller diameter than the wavelength of the incoming radiation, Rayleigh scattering occurs. Longer wavelengths scatter more readily when compared to shorter wavelengths. Small particles, such as NO2 and O2, scatter light with shorter wavelengths (like blue and violet visible light). Red light, which has a longer wavelength, scatters more in the atmosphere than blue light. Incoming sunlight travels a larger distance through the atmosphere at sunrise and dusk. Due to the longer route dispersing the short (blue) wavelengths, we only see the longer (red and orange) wavelengths of light.
  • Mie Scattering: When the wavelength of electromagnetic radiation is similar to the size of air particles, Mie scattering occurs. Mie scattering affects photons in the near-ultraviolet to mid-infrared regions of the spectrum. Mie scattering occurs largely in the lower atmosphere when the sky is overcast, where bigger particles are more frequent. Mie scattering is mostly caused by pollen, dust, and pollution. For example, Mie Scattering makes the clouds appear white.
  • Tyndall Effect: A variety of tiny particles make up the earth’s atmosphere. Smoke, small water droplets, suspended dust particles, and air molecules are examples of these particles. The path of a light beam becomes visible when it collides with such little particles. After being diffusely reflected by these particles, the light reaches us. The Tyndall effect is caused by colloidal particles dispersing light. The phenomenon occurs when a fine beam of sunlight enters a smoke-filled room through a small hole. The particles become visible as a result of light scattering. When sunlight penetrates through a dense forest canopy, the Tyndall effect is noticeable. Light is scattered by little water droplets in the mist. The size of the scattering particles determines the hue of the dispersed light. Very small particles scatter shorter wavelength light, while larger particles scatter longer wavelength light. The dispersed light may appear white if the scattering particles are large enough.
  • Raman Effect: Raman scattering is the scattering of photons at higher energy levels by stimulating molecules. The incident particle’s kinetic energy is either lost or acquired, with Stokes and anti-Stokes components, because the photons are inelastically scattered.

Factors affecting Scattering of Light

• Size of the particles: The color of the scattered light depends upon the size of the particles.
  • Tiny particles scatter light of a shorter wavelength.
  • Large particles scatter light of a longer wavelength.
• The wavelength of the ray: Scattering is inversely proportional to the wavelength.

Scattering ∝ 1/λ

where λ denotes the wavelength of the ray. Higher the wavelength, the lesser the scattering. Lesser the wavelength, the higher the scattering.

Practical Applications of Scattering of Light

• Blue color of the sky: Blue color has a shorter wavelength compared to red color. Since we know that, scattering ∝ 1/λ. Hence, the blue color gets scattered most by tiny minute particles in the atmosphere during daytime. The atmosphere has the presence of various gases such as Nitrogen (N₂) and Oxygen (O₂). These gas molecules are very small in size and form a colloidal (Gas-in-Gas solution). Small-sized particles scatter rays of shorter wavelength and blue color being of shorter wavelength gets scattered more strongly and gives a blue appearance to the sky. The space appears dark to astronauts, as there is no atmosphere. Without the atmosphere tiny particles aren’t there to scatter light, hence giving a dark appearance.
• Why danger signals are of red color? The wavelength of red color is longer when compared to other colors of the spectrum (seven colors formed due to refraction through a prism). As rays of longer wavelength are least scattered by dust and smoke particles, red color reaches far away distances and would help danger signals to reach faster and to more distant places. All other colors scatter away during the night, and the red color reaches our eyes.
• Reddish appearance of sun during sunrise and sunset: During sunrise and sunset, the rays have to travel a longer distance through the layers of the atmosphere because they are very close to the horizon. Therefore, all other colors except the red color scatter away and the red color remains. Most of the red light, which is the least scattered by the particles, enters our eyes. Hence, the sun and the sky appear red. At noon, the sun appears white as less of the blue light gets scattered.

Sample Problems

Problem 1: What color does the clear sky appear to be during the day? Give an explanation.

Solution:

Blue is the colour of the sky throughout the day. This is because of the size of air molecules and other fine particles in the atmosphere is smaller than the wavelength of visible light. Due to this, these particles scatter light rays of shorter wavelengths at the blue end more efficiently than light rays of longer wavelengths at the red end. That is why the scattered blue light gives us the impression of a blue sky when it enters our eyes.

Problem 2: What is meant by scattering of light? 

Solution:

Light scattering is the spreading of light in different random directions. Light scatters when it encounters various types of suspended particles along its path .The colour of scattered light is determined by the size of scattering particles in the environment.

• The larger dust and water droplets in the atmosphere scatter light with longer wavelengths, giving the dispersed light a white appearance.
• The very small particles in the environment, such as air molecules, scatter the blue light contained in the white sunlight.

Problem 3: Why does the sun appear reddish early in the morning?

Solution:

At daybreak, the sun rises near the earth’s horizon (early in the morning). Light from the sun near the horizon must travel through vast layers of air and a great distance via the earth’s atmosphere before reaching our sight. The particles in the atmosphere scatter most of the blue light rays with shorter wavelengths near the horizon. As a result, we are exposed to red light with longer wavelengths. As a result, the sun takes on a crimson colour.

Problem 4: Why red colour is used to make a danger signal or sign?

Solution:

When red collides with small fog and smoke particles, it scatters the most since it has the longest wavelength (visible spectrum). As a result, we can see the red colour clearly even from a great distance

Problem 5: On a foggy day, why does the driver use orange lights instead of white lights?

Solution:

When a driver uses white light while driving in fog, the tiny droplets of water scatter a lot of blue light. This diffused blue light reduces visibility and makes driving a challenge. Orange light does not scatter due to its larger wavelength, allowing the motorist to see clearly.