## Have you ever stood on a boardwalk staring at the sea and somehow witnessed large waves breaking slowly in front of your eyes?

It's a common phenomenon, and it is particularly visible when powerful swells hit the coastline or during stormy days.

You're standing in front of the surf, and the big waves seem to crash very slowly at a distance.

Like in a slow-motion replay, the wave grows in size, starts to bend, and then slowly crumbles forward.

A similar event can occur when you're flying, minutes before landing, and you look down at the coastline from the airplane window.

You can see and recognize waves and white foam from above, but they look like they're stationary and not moving at all.

So, why do large waves seem to break and move very slowly and at the same place, but on another day, smaller waves break quickly?

### Visual Perception

It all has to do with several visual perception variables and how our eyes and brain interpret optical inputs and movement.

In the beach example, in most cases, what happens is that those big waves are crashing further out the back compared to the smaller waves that we often see breaking near the shore.

So, our eyes perceive big waves breaking slowly because they're far away and take longer to cross our line of vision than those close by.

From up above, they seem like they're actually frozen or still, and the interval between waves - wave period - seems super long, if not inexistent.

The first thing we have to understand is that when we're far from land - for instance, inside an airplane at 10,000 feet (1.9 miles or 3 kilometers) - your eyes have no stationary references.

As a result, you can't exactly determine the "real" motion of the waves.

But there's more - relative velocity plays a trick on us.

### Relative Velocity

When we're on terra firma, we estimate the wave velocity with respect to the shoreline.

However, when we're inside an airplane, we tend to get a bigger picture and gauge the velocity of the waves based on the whitewater lines and other wave crests moving at the same speed.

So, because we get that bigger picture and compare the several visible wave lines, there's no apparent movement.

Well, in reality, there is movement.

But our brain is fooled into believing that those waves should be moving faster and can't figure out why.

We have to take into consideration that a centimeter of ocean or land observed from the sky equals dozens of yards or meters of ground or water.

Therefore, it's not hard to imagine why a wave looks like it is stationary, given the fact that, from above, a regular wave travels at an average speed of 10 feet (3 meters) per second.

Our eyes can't detect a 30-meter move from a long distance.

It's also important to stress that while we're flying, we only pay attention to the waves for a short amount of time, resulting in a small displacement of waves.

And when we're about to land - at around 1,000 feet (300 meters) from the ground - the phenomenon might be even more mesmerizing.

The ground is moving fast from left to right (or vice versa), and the waves in the back appear to move slowly.

Why? Let's do simple math.

The whitewater lines take around one second to travel 10 feet; the airplane is flying at 600 miles per hour (965 kilometers per hour), which means that it travels at 880 feet per second.

So, we've got a huge discrepancy between speeds - 10 feet per second (waves) versus 880 feet per second (airplane).

### Motion Is Relative

Science tells us that all motion is relative.

When something is moving, there is no correct answer to how fast it is going or even if it is moving at all.

It all depends on the frame of reference of the observer. The frame of reference is the view of the person or object observing the motion.

The frame of reference may be static or moving.

From a person standing on the beach, the waves are moving.

But from the person's frame of reference on an airplane near the ground, it's the houses and shoreline that appear to be moving.

According to the laws of physics, there's no way to distinguish between an object at rest and an object moving at a constant velocity.

As a result, we can't answer the question: "How fast is something moving?" It's all relative, and the eyes can easily deceive the observer.

### An Ophthalmologic Perspective

We've seen that the eyes and the brain work together to provide us with extraordinary experiences.

So, we've asked for an expert for an ophthalmologic evaluation on this topic.

Fernando Falcão Reis is a professor at the Faculty of Medicine of the University of Porto (FMUP) and head of the ophthalmology department of the São João University Hospital Center (UHCSJ) in Porto, Portugal.

He notes that "motion detection is an essential component of the visual system and depends on the temporal processing of specialized cells of both the retina and cortex."

"The sensitivity to changes in light - and therefore any object - in time is influenced by several variables such as the size of the object, color, the background characteristics, the surroundings, etc."

"Motion is a peculiar form of time variation, in which the time change is associated with a location change."

"In fact, motion is nothing but the change of position of an object through time. Virtually everything we see in our world is in motion."

"If not the object itself - the object could be stationary - it could be the motion/movement of our head or eyes, which results in a motion of the object in the retina."

"The visual system is highly sensitive to changes of position in space."

"As a result, all it's needed to extract the sense of motion is to compare the locations of an object in successive moments."

"So, it's easy to understand that the distance we're from an object, preventing or diminishing the visualization of the object in different locations, is an obvious limitation of the ability to detect movement."

"It is also important to underline that motion perception is not just a simple representation of physical movement."

"A clear example is the motion perception induced by two or more fixed lights that turn on in sequence."

"Another example is the sense of motion of a stopped car after looking at a moving vehicle for a long time," concludes Fernando Falcão Reis.

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