Problem 5 - A Complete Resource Book in Chemistry for JEE Main

Question

The blue colour of water in the sea is due to (a) scattering of blue light by water molecules (b) reflection of blue sky by sea water (c) refraction of blue light by the impurities in sea water (d) adsorption of other colours, except the blue colour by water molecules

Step-by-Step Solution

Answer

(a) scattering of blue light by water molecules

Step 1: Understand the physical phenomenon

To solve this problem, we need to understand why water appears blue. Water absorbs colors in the red part of the light spectrum and scatters and reflects colors in the blue part, which influences the color we observe.

Step 2: Examine each answer option

Let's evaluate each option based on our understanding of light interaction with water: (a) Scattering of blue light by water molecules is indeed a reason why water appears blue. This scattering is similar to Rayleigh scattering which makes the sky blue. (b) Reflection of blue sky by sea water also contributes to the appearance. However, since the color persists under different sky conditions, this is not the primary reason. (c) Refraction of blue light by impurities doesn't cause the general blue color since refraction deals with bending of light, not color changes. (d) Adsorption of other colors except blue would imply that only blue is left, but primarily water absorbs red, and less of blue is absorbed, which is not the main cause of the blue color.

Step 3: Identify the correct answer

The key to the blue color of sea water is the scattering of light. Although both options (a) and (b) have a degree of truth, the scattering of blue light by water molecules is scientifically recognized as the predominant factor for the blue color of clear water bodies.

Step 4: Confirm the most accurate choice

Option (a) matches the scientific explanation accurately. The primary reason water can appear blue is due to its absorption and refraction characteristics, where more red light is absorbed and blue light is scattered.

Key Concepts

Light Scattering

Light scattering is a fascinating phenomenon that occurs when light rays encounter small particles or molecules in a medium. This interaction causes the light to spread in different directions. In the case of sea water, when sunlight penetrates the water, it's made up of a spectrum of colors. However, as it interacts with water molecules, some colors are scattered more than others.

This scattering process is what makes certain colors more prominent. Among all the colors in sunlight, blue light is scattered the most in water, which gives the sea its characteristic blue hue. This is similar to what happens in the sky, where blue light is scattered more by air molecules, causing the sky to appear blue to our eyes.

In summary, light scattering plays a crucial role in determining the colors we observe in nature, from the blue of the sky to the blue of the ocean. It occurs due to the way light behaves when it meets the millions of small water molecules in the sea.

Rayleigh Scattering

Rayleigh scattering is a specific type of light scattering that helps explain why the sky looks blue and why large bodies of water can appear blue too. Named after Lord Rayleigh, this scattering process occurs when the particles causing the scattering are much smaller than the wavelength of the light. Light from the sun comes in a variety of wavelengths, corresponding to different colors.

When sunlight enters the Earth's atmosphere, it collides with air molecules and gets scattered. Blue light, which travels in shorter, smaller waves, is scattered in all directions by these tiny particles. This effect is less pronounced with larger water molecules in the ocean, but still contributes to the blue appearance of water. The water molecules preferentially scatter shorter wavelengths, such as blue, more than they scatter longer wavelengths like red and orange.

This scattering process is not only essential for understanding the color of the sky but also helps explain the color of the ocean. Due to Rayleigh scattering, both water bodies and the atmosphere have that mesmerizing blue tint.

Light Absorption

Light absorption plays a vital role when it comes to understanding the color of water in seas and oceans. While scattering affects the color we see, absorption helps us understand why certain colors are more prominent and others are not. Water naturally absorbs colors from the red end of the light spectrum much more effectively than those at the blue end.

When sunlight enters the sea, the water molecules start absorbing the light. Red, orange, and yellow light are absorbed quite rapidly, which is why they do not reach far depths and contribute less to the color we perceive. Blue light, on the other hand, is absorbed the least. Because it penetrates deeper and is scattered, it is primarily what we see reflected back from the ocean.

The combination of absorption and scattering explains why the sea is blue. While red light gets absorbed, blue light manages to reflect back and scatter, creating the vibrant blue color typical of clear, deep water.