If we observe a pebble in a pool, the pebble would appear to be

Question

Prepp Team · Accepted Answer

Understanding Why a Pebble in a Pool Appears Nearer

When you look at a pebble lying at the bottom of a pool of water, it often appears to be closer to the surface than its actual depth. This phenomenon is a common observation and is explained by the principles of optics, specifically the refraction of light.

What is Refraction of Light?

Refraction is the bending of light as it passes from one transparent medium to another, for example, from water to air. This bending occurs because light travels at different speeds in different mediums. When light moves from a denser medium (like water) to a less dense medium (like air), it bends away from the normal (an imaginary line perpendicular to the surface at the point where the light hits).

Applying Refraction to the Pebble in a Pool

Consider the light rays originating from the pebble at the bottom of the pool. These rays travel upwards through the water and then exit into the air above the surface. As the light rays pass from water to air, they bend away from the normal.

Our brain interprets vision by assuming that light rays travel in straight lines. When we observe the bent light rays from the pebble that enter our eyes, our brain traces these rays backward in a straight line. Because the rays have bent away from the normal at the water surface, the backward-traced lines appear to originate from a point higher than the actual position of the pebble.

Apparent Depth vs. Real Depth

The actual depth of the pebble is its real depth in the water. The depth at which the pebble appears to be located due to refraction is called the apparent depth. Because the light bends away from the normal when exiting water into air, the apparent depth is always less than the real depth.

Concept	Explanation
Real Depth	The actual distance of the object (pebble) from the surface.
Apparent Depth	The perceived distance of the object (pebble) from the surface due to refraction.
Relationship	Apparent Depth < Real Depth (when viewed from above)

Analyzing the Options

Let's look at the given options based on our understanding:

deeper in the water than it really is: This is incorrect. Refraction causes the object to appear shallower, not deeper.
at the same depth at which it is: This would only happen if there were no refraction, which is not the case when light passes from water to air.
nearer to the surface than it really is: This aligns with our explanation. The apparent depth is less than the real depth, meaning it appears closer to the surface.
invisible: The pebble is visible, although its perceived position is altered.

Therefore, due to the refraction of light as it travels from water to air, the pebble appears to be nearer to the surface than its actual depth.