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# Refraction of Light

This lesson aligns with NGSS PS4.B

Introduction
Have you ever noticed that when you place a straw in your cold drink, it seems to appear bent? Similarly, picture a scenario where you and your friend are hanging out by a lake. Your friend steps into the water. As you watch, you notice something strange; his feet look like they are separated from his legs. Of course, you know his feet did not come off. So, how can you explain the reason behind this curious phenomena? Does the straw genuinely bend when it’s in water? The answer has to do with refraction. In this article, we will learn about the refraction of light, refractive index, optical illusion, and how colors are separated during refraction.

Refraction of Light
Refraction is the phenomenon that occurs when light travels from one transparent medium into another and changes its speed and direction due to the change in density. To understand this better, consider light as a ray or a straight line that travels through air, glass, water, or any other transparent substance. When it enters a new medium at an angle other than perpendicular, it alters its path.

The key factor that drives refraction is the change in the speed of light as it moves from one medium to another. In a vacuum, light travels incredibly fast at a speed of 300,000,000 m/s, but it travels more slowly through matter. For instance, it travels slower through denser substances like water and glass compared to air. This change in speed is responsible for the bending of the light ray.

• When light enters into a medium where the speed of light is slower, the light bends away from the boundary between the media.
• When light enters a medium where the speed of light is faster, the light bends toward the boundary.

Refractive Index
The degree to which light rays bend when they move from one substance to another is referred to as the "Refractive Index." This is symbolized by the letter 'n'. It can be calculated using the formula:
n = c/v

Here, 'c' stands for the speed of light of a particular wavelength in the air, and 'v' represents the speed of light in the given medium.

The refractive index is influenced by the following aspects:
a. The type of material the light is passing through. b. The physical conditions of that material. c. The specific color, or the wavelength of the light being used.

Optical Illusion
Typically, when you glance at something, the light that reflects off it follows a straight path from the object to your eyes. This is how our brain interprets light as traveling in a straight line. However, when you observe something underwater, the light reflecting off the object does not follow a straight path. Instead, it bends the phenomenon we call refraction. The following figure shows how refraction creates an optical illusion.

Refraction and Color Separation
You already know that white light is a blend of all the visible colors. These colors correspond to various wavelengths. When white light goes through refraction the amount of light that bends depends on its wavelength. Light waves with short wavelengths refract more than light waves with long wavelengths. This difference allows white light to split into different colors during refraction, as shown in the figure. This separation of colors during refraction is the reason behind the appearance of rainbows. Rainbows are created when sunlight is refracted by tiny water droplets.

When light travels through a prism, it is refracted twice__ first when it enters the prism and then when it leaves the prism.

Real-Life Examples of Refraction
Mirages
The shimmering appearance of water on a hot road is an example of a mirage, caused by refraction. Hot air near the road's surface has a lower density than the cooler air above, causing light rays to curve and create the illusion of a water surface.

Broken Pencil in Water
The classic experiment of partially immersing a pencil in a glass of water shows refraction. The pencil appears bent at the water's surface due to the difference in refractive indices between air and water.

Summary
• The change in direction or bending of a light wave while passing from one transparent medium to another; caused by the change in the wave’s speed is known as refraction.
• The extent of bending of light rays from one medium to another can be calculated by refraction index.
• Light waves with short wavelengths refract more than light waves with long wavelengths.
• Rainbows are a spectacular display of refraction and reflection. Sunlight refracts as it enters raindrops and is internally reflected, creating the colorful arc we see in the sky.

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