Stop for a bit if you’re drinking water from a bottle or a glass through a straw (hopefully a metal one), and look at the side of your container of choice. The straw appears skewed as if it abruptly bent once it entered the water. What gives?
This phenomenon has captivated minds for centuries and continues to be a popular subject of wonder. Here, we’ll unravel the mystery behind this optical illusion and explore how a simple glass of water can bend light. (Don’t let me stop you, though—finish that glass of water.)
Refraction Facts
Let’s start with the basics. Light is a form of electromagnetic radiation that travels in waves. These waves, in turn, consist of tiny packets of energy called photons. When light encounters a different medium, like air, water, or glass, its speed can change, causing it to refract or bend.
When light travels from one medium to another, like air to water, it changes speed due to the different densities of these materials. In the case of water, which is denser than air, light “slows down” as it enters the liquid. This difference in speed causes the light waves to refract, creating the illusion of objects appearing displaced or distorted when viewed through the interface of two different media.
This difference in optical properties between two media through which light can pass is also considered for specific calculations. STEM students will know it well; it’s often described as a dimensionless ratio between the speed of light in a vacuum and the apparent speed of light through a specific medium, and it’s called a refractive index.
Air, for instance, is commonly set to have a refractive index of 1—that is, light that passes through the air is often assumed to behave just as it would if it had passed through a vacuum (the absence of air) instead. Conversely, water has a slightly higher refractive index at 1.33 (at 20 °C), as water “slows” light down as it passes through compared to light passing through a vacuum.
The Last Straw
Let’s now return to that picturesque glass of water. When you place a glass of water on a table and look at an object through it, like a straw, the light from the object travels through the air, enters the water, and slows down. As a result, the light waves refract, causing the straw to appear displaced from its actual position.
“But hold on,” some of you might say. “Isn’t the speed of light fixed? How does it slow down when it always moves at that speed?“
And to that, yes, it does move at the same speed; nothing in this process of light passing through media slows it down. It always stays the same; the apparent speed difference emerges because the light that passes through media gets absorbed and reemitted countless times by all the atoms within that media. After all, the media is made of countless atoms with electrons that can themselves interact with the light that passes through them. Those interactions take time, and that time builds up over the many, many atoms that stand between the light and its intended exit, hence the apparent “slowing” of light.
In Conclusion
So there you have it—a simple glass of water can create the illusion of bending or breaking objects due to refraction. It’s a captivating example of how light interacts with different materials, and it all boils down to how light interacts with electrons housed within atoms, creating substances with different optical properties. The next time you enjoy a refreshing glass of water, take a moment to appreciate the science. (And yes, once again, finish that glass of water.)
References
- Siegel, E. (2017, July 29). Ask Ethan: Does Light Always Move At The Same Speed? Forbes. https://www.forbes.com/sites/startswithabang/2017/07/29/ask-ethan-does-light-always-move-at-the-same-speed/
- Sutter, P. (2023, July 24). How does light slow down? Space.Com. https://www.space.com/how-does-light-slow-down
