A water tank filled with water appears shallower than its actual depth when viewed from above. Which phenomenon of light is mainly responsible for this apparent shallowing?

Introduction / Context:
When you look into a pool, pond, or water tank from above, the bottom often appears closer to the surface than it really is. This is a classic demonstration of refraction of light at a boundary between two media, water and air. Understanding this effect is important for explaining apparent depth, design of optical instruments, and everyday visual illusions involving transparent materials.

Given Data / Assumptions:
• The tank is filled with water and observed from the air above. • Light rays travel from water (denser medium) into air (rarer medium) and then to the observer's eye. • The observer views the bottom from a position nearly above the tank, not from the side. • The water surface is relatively flat and calm.

Concept / Approach:
Refraction is the bending of light as it passes from one medium to another with different optical density. When light travels from a denser medium (water) to a rarer medium (air), it bends away from the normal to the interface. The human eye traces back the refracted rays in a straight line, assuming they have travelled in a straight path. Because the rays have bent away from the normal, the eye perceives the origin of the rays at a point higher than the actual bottom, causing the bottom to appear shallower. This phenomenon is known as apparent depth due to refraction.

Step-by-Step Solution:
Step 1: Light rays from a point on the bottom of the tank travel upward through water towards the surface. Step 2: At the water air boundary, the rays pass from denser water to rarer air. Step 3: According to refraction rules, when light passes from denser to rarer medium, it bends away from the normal. Step 4: The observer in air receives these refracted rays and traces them backward in straight lines. Step 5: The backward extensions meet at a point that is closer to the surface than the true bottom, giving an apparent depth less than the actual depth. Step 6: Therefore, the bottom appears raised and the tank looks shallower due to refraction.

Verification / Alternative check:
If you insert a stick into water at an angle, it seems bent or broken at the surface. This is also due to refraction. Similarly, a coin at the bottom of a cup may become visible when water is added, again illustrating refraction effects. In all these cases, light rays bend at the interface, and the brain misinterprets their origin. No significant reflection, diffraction, or scattering is needed to explain the apparent shallowing of the tank.

Why Other Options Are Wrong:
Option a (Reflection): Reflection involves light bouncing back from a surface. While some reflection occurs at the water surface, it does not cause the bottom to appear shallower. Option c (Diffraction): Diffraction is bending of waves around obstacles or through small openings and is not responsible for apparent depth changes. Option d (Total internal reflection): Total internal reflection occurs when light is completely reflected inside a denser medium beyond a critical angle, as in optical fibers. It does not primarily explain the shallow appearance in normal viewing conditions. Option e (Scattering): Scattering can make water appear coloured or hazy but does not by itself create the apparent shift in depth.

Common Pitfalls:
Students sometimes confuse refraction with reflection because both occur at surfaces. A helpful clue is that apparent depth and bending of submerged objects are classic refraction examples. Another misconception is to think diffraction applies whenever light bends, but diffraction specifically involves obstacles and slits of size comparable to wavelength. For transparent media boundaries, always think of refraction first.

Final Answer:
A water tank appears shallower from above mainly due to Refraction of light at the water air interface.