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What Is Refraction? What Is Refractive Index?

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pencil in a glass of water, refraction of light

Refraction is one of the most frequently observed phenomena; without it, our world as we see it now would be a haze.

Refraction is the bending of light as it passes from one transparent medium to another due to the density difference between the two mediums. This bending of light makes it possible to have prisms, rainbows, and optical instruments such as lenses and magnifying glasses. Our eyes also rely on refraction to focus light into our retina.

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How Does It Work?

Light will refract or bend when it travels at an angle into a medium with a different optical density, known as its refractive index. The change in speed causes the change in direction, so if a medium causes the traveling light to slow down or speed up, the light will bend more.

The degree of refraction will be more noticeable if the light enters the medium at a greater angle.

The angles are measured by drawing an imaginary dotted line known as the normal 90 degrees to the surface of the two mediums. If light enters a medium with a lower refractive index, the light will bend away from the normal line and speed up. If it enters a medium with a higher refractive index, it bends towards the normal line and slows down.

drinking straw in a glass of water, refraction
Photo Credit: congerdesign, Pixabay

What Is the Refractive Index?

The refractive index is dimensionless and describes how fast light travels through a medium and is also known as the index of refraction. The refractive index is the ratio of the speed of light in a medium(v) to the speed of light in a vacuum(c). If the refractive index of a medium is represented by n, the following formula can be used to calculate it:    n=c/v

If a light ray travels from a medium with a higher refractive index to another with a lower refractive index, it bends toward the normal; otherwise, it bends away from the normal.

Laws of Refraction

Two laws govern refraction:
  • The point of incidence, the incident ray, the refracted ray, and the normal line all belong to the same plane.
  • Snell’s Law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction (the angle between the refracted ray and the normal) is constant. The constant is affected by the two media as well as the wavelength of the incident light.

What Are the Different Types of Refraction?

  • Refraction from rarer to denser medium: The relative refractive index is greater than one. When the angle of incidence exceeds the angle of refraction, the refracted ray shifts towards the normal. This category includes the refraction of light in water from the air.
  • Refraction from denser to rarer medium: The refractive index is less than one. The angle of incidence is less than the angle of refraction, indicating that the refracted ray deviates from the normal. This category includes refraction from glass to the air.

Refraction in Spectrum

Refraction in the spectrum describes how white light travels through a prism, causing each color to refract at a different angle. As a result, a prism can be used to depict the VIBGYOR colors in the spectrum. Light is made up of waves, which explains why these colors appear when they pass through the prism.

Because the wavelengths of red light and violet light differ, their refractive indexes differ, with red light having a longer wavelength than violet light. The refraction of light in the spectrum demonstrates that white light is composed of the colors of the Rainbow.

It's easy to remember this way:
  • V: Violet
  • I: Indigo
  • B: Blue
  • G: Green
  • Y: Yellow
  • O: Orange
  • R: Red
cocktail drink, an example of refraction
Image By: stevepb, Pixabay

Types of Refraction in our Daily Lives

  • Rainbows form after a shower because of sunlight refraction by the surfaces of water droplets.
  • Stars twinkle due to the refraction of their light.
  • When straight light strikes the bottom of a swimming pool and bends at the water surface, it refracts and gives the pool a shallower appearance.
  • Mirages are optical illusions caused by the refraction of light.
  • Because of light refraction, when white light passes through a glass prism, it splits into seven color components.
  • When the sun rises, its light refracts through the Earth’s atmosphere and reaches our eyes before it crosses the horizon. Similarly, light rays from the sun are refracted and reach us after sunset. As a result, the sunrise is earlier, and the sunset is later.

Applications of Refraction

Refraction is useful in both optics and technology. However, it is primarily determined by the type of spherical lens used, whether convex or concave. A convex lens, also known as a converging lens, is the lens used to manufacture magnifying glasses. It’s thicker in the center and thinner around the edges. A concave lens, also known as a diverging lens, is thinner in the center and thicker at the edges. Light is refracted outwards as it enters a concave lens and then again as it exits it. 

  • Lenses are transparent objects that use refraction to focus light. Cameras and binoculars both make extensive use of lenses.
  • Various telescopes are used to study distant objects in the universe. Microscopes are commonly used in science to analyze tiny objects that would otherwise be invisible to the naked eye. A lens is used for refraction to create magnified images of distant or small objects.
  • An important application of refraction is optical fibers, which are used for low-energy communication.

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Frequently Asked Questions (FAQs)

What is the difference between reflection and refraction of light?

Light reflection occurs when light bounces off a medium. The angle of incidence equals the angle of reflection if the medium has a smooth surface. Light refraction is the change in the direction of light as it travels from one medium to another.

How do you define light?

Light is a type of energy that allows us to see. It comes from a source and bounces off objects our eyes perceive, and our brain processes this signal, allowing us to see.

When is the refraction of light not possible?

Refraction of light is not possible when the incident light is perpendicular to the boundary.

What is an example of light refraction?

A rainbow is an example of refraction because the sun’s rays bend through raindrops, resulting in the rainbow. If you’ve ever noticed that the water’s surface appears shallower than it actually is, it is due to light reflection as it travels from water to air before entering our eyes.

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Refraction is the bending of light as it travels from two mediums of different densities. The refractive index describes how fast the light travels between the two mediums. Refraction has been applied to both optics and technology, making it possible to make optical instruments such as microscopes, telescopes, and lenses.

It is seen in our everyday life, and the reason we can physically see it is also thanks to refraction. Otherwise, it would be a blur. The twinkling stars, rainbows, mirages, and optical illusions are all due to the phenomena of refraction.

Featured Image Credit: Billion Photos, Shutterstock

About the Author Robert Sparks

Robert’s obsession with all things optical started early in life, when his optician father would bring home prototypes for Robert to play with. Nowadays, Robert is dedicated to helping others find the right optics for their needs. His hobbies include astronomy, astrophysics, and model building. Originally from Newark, NJ, he resides in Santa Fe, New Mexico, where the nighttime skies are filled with glittering stars.