If you look above at a pencil partially submerged in water, the underwater part looks bent or misshapen. Remember that you see any object because light waves are coming off that object and reaching your eye. But those light waves behave differently in different materials. They travel faster or slower, depending on the material.
Pencils or any objects look bent in water due to refraction. Refraction is the change in the direction of light as it passes across the boundary separating two media caused by its change in speed! Let’s learn more about it.
Why does a pencil look bent in water?
Light travels faster through the air than it does through water or plastic. If light rays move through the air and reach a different material at an angle, they’ll change direction slightly. You can see the light beam bent as it enters the plastic. The speed of the light ray decreases as it reaches the plastic block but then speeds up again as it exits the block. This bending of light is called refraction.
We can say that the light is refracted. So how does refraction affect how we see the pencil in water? The pencil moves away from the pencil through water, refracting it as it travels between water and air. It speeds up when exiting the water, bending away from the normal before reaching your eyes.
Your brain assumes that light travels in straight lines. So it looks like the pencil has been bent to reach where you think the light beam has come from. It’s because of refraction that we can focus on things using lenses. As a light ray reaches this lens, the ray is refracted toward the normal. When leaving the lens, it is refracted from the normal because of its shape.
All of the emergent rays are refracted toward each other, focusing at a single point called the focal point. It is called a convex lens, sometimes known as a converging lens, because the light rays converge through the lens. This type of lens in our eyes allows us to focus light from objects onto our retina so we can see a clear image.
An experiment of bending a pencil in water
Let’s learn about the effects of refraction through the concept of the apparent bending of a pencil.
- Take a glass of water and a pencil.
- Place the pencil into the water.
- Look at the pencil from the top.
- Then, look at the pencil from the side.
Notice the changes in its appearance. Your pencil appears to be sharply bent at the surface of the water. Sunlight bends when it passes through one medium to another of a different density. As the nature of light, it first travels through the air and then the water. Usually, water is denser than air. So the light rays bend in the water and make the pencil seem to bend as an illusion. This process of light bending is known as refraction.
Here’s how the bending of light causes the pencil to appear bent:
Light travels through the air and moves straight toward the water’s surface.
- When it reaches the water’s surface, the light encounters a change in the optical density, which causes it to slow down and change direction.
- The change in the direction of light is known as refraction. Light bending occurs because the waves change speed from air to water.
- As the light enters the water, it travels in a straight line but at a different angle compared to its original path in the air.
- When you look at the pencil from above the water’s surface, your brain tries to trace the path of the light rays back to your eye. However, your brain assumes that light travels in straight lines, so it extrapolates the path of the light in a straight line from where it appears to enter the water.
- Since the light bends at the air-water interface, your brain mistakenly perceives the pencil as bent, as it connects the straight line in the water with the actual portion of the pencil above the water.
In reality, the pencil is not physically bent. It’s an optical illusion caused by the refraction of light at the air-water interface. This phenomenon is a classic example used to demonstrate the behavior of light and the principles of refraction.
Laws of reflection
The first law says that the incident light ray is reflected so that the angles of incidence and reflection are equal. The second one says that the incident ray, the reflected ray, and the normal lie in the same plane.
The refracted ray will bend toward the normal if the refractive medium is optically denser than the incident medium. If it is optically rarer, it will bend away from the normal. Note that the light is incident obliquely on the surface! When the incident light is perpendicular to the surface, it does not change direction!
Consider light traveling from some medium 1 to another medium 2, which is optically denser than medium 1. So its speed in medium 2 decreases, and it bends towards normal.
The first law of refraction says that the incident, refracted ray, and the normal at the point of incidence all lie in the same plane. The second law is also called Snell’s law of refraction. It gives us the relation between the incident and refraction angles and the refractive indices of the mediums. The Refractive index of a medium increases or decreases the speed of light.
Mathematically, it is the ratio of the speed of light in the incident medium to the speed of light in the refractive medium. It’s denoted by ‘N,’ and the two numbers in the base are the two mediums.
N21 = V1/V2
We read this as a refractive index of medium two concerning incident medium 1. Also, “V1” and “V2” are light speeds in mediums one and two, respectively.
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