Understanding Light Phenomena: Refraction vs. Scattering

This question asks us to identify which option is NOT an example of the refraction of light. To answer this, we need to understand what refraction is and how it differs from other light phenomena like scattering.

Refraction of light is the bending of light as it passes from one transparent medium to another. This bending occurs because light travels at different speeds in different media. Examples of refraction include how lenses work, why objects submerged in water appear displaced, and the separation of white light into its colours (dispersion), which is a form of refraction.

Analyzing the Options Based on Refraction

Let's examine each option to see if it involves refraction of light:

• Option 1: Image formation by human eye

  The human eye uses a lens (specifically, the cornea and the crystalline lens) to focus light onto the retina. This focusing is achieved through the bending of light rays as they pass through these structures, which are made of different transparent materials with different refractive indices. Therefore, image formation by the human eye is a classic example of refraction.

• Option 2: Formation of rainbow

  Rainbows are formed when sunlight passes through raindrops. As light enters a raindrop, it undergoes refraction and dispersion (separation into colours). It then reflects off the back inner surface of the raindrop and undergoes refraction again as it exits the raindrop. Both refractions are essential for the formation of the rainbow's spectrum. So, rainbow formation involves refraction.

• Option 3: Twinkling of stars

  Stars twinkle because of atmospheric refraction. As starlight travels through the Earth's atmosphere, it passes through layers of air with varying temperatures and densities. These variations cause continuous changes in the refractive index of the air. Consequently, the light rays from the star are refracted by different amounts as they reach our eyes, making the star appear to shift position slightly and fluctuate in brightness, which we perceive as twinkling.

• Option 4: Red colour of setting sun

  The red colour of the setting sun (and the sunrise) is primarily due to the scattering of light by particles in the Earth's atmosphere, not refraction. When the sun is near the horizon, sunlight travels through a much larger thickness of the atmosphere compared to when the sun is overhead. Blue and green light, which have shorter wavelengths, are scattered away more effectively by air molecules and fine particles (this phenomenon is called Rayleigh scattering). Red and orange light, having longer wavelengths, are scattered less and are therefore transmitted through the atmosphere to our eyes, making the sun appear reddish.

Identifying the Non-Refraction Phenomenon

Based on the analysis, the red colour of the setting sun is caused by the scattering of light, whereas the other three options involve refraction of light.

Conclusion on Light Phenomena

Therefore, the phenomenon that is NOT an example of refraction of light among the given options is the red colour of the setting sun.

Revision Table: Light Phenomena Summary

Additional Information on Light Phenomena

Let's delve a little deeper into the concepts of scattering and dispersion.

Scattering of Light

Scattering is the phenomenon where light rays are redirected in various directions when they encounter particles or molecules in a medium. The amount of scattering depends on the wavelength of the light and the size of the scattering particles.

• Rayleigh Scattering: Occurs when light interacts with particles much smaller than its wavelength (like air molecules). It is inversely proportional to the fourth power of the wavelength ($\propto 1/\lambda^4$). This means shorter wavelengths (blue, violet) are scattered much more than longer wavelengths (red, orange). This is why the sky is blue during the day (blue light from the sun is scattered in all directions) and why the sun appears red at sunset/sunrise (blue light is scattered away, leaving red light to pass through).

Dispersion of Light

Dispersion is the phenomenon where white light is separated into its constituent colours (the spectrum) when it passes through a transparent medium like glass or water. This happens because the refractive index of the medium is slightly different for different wavelengths (colours) of light. Shorter wavelengths (like blue and violet) are bent more than longer wavelengths (like red). Dispersion is a specific case involving the wavelength-dependence of refraction.

Understanding these different interactions of light with matter helps explain various optical phenomena we observe around us.