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Have you ever wondered why a pencil appears bent when partially submerged in water? Or why a straw in a glass of water seems to be disjointed? These fascinating optical illusions result from the mesmerizing phenomenon known as wave refraction. In this article, we embark on a captivating journey to demystify the laws behind wave refraction, focusing on the renowned Snell's law.
By understanding the relationship between the angle at which a wave approaches a boundary, the angle at which it changes direction (refraction), and the materials it encounters, we gain valuable insights into the behaviour of waves. These insights have profound implications in our comprehension of light and sound and in fields such as optics, underwater exploration, and telecommunications.
Join us as we delve into wave refraction, shedding light on its enchanting mechanisms. Let's explore the wonders of Snell's law together as we uncover the secrets of how waves bend and sway, shaping our perception of the world around us.
What is Refraction?
Every wave type, whether transverse or longitudinal, can undergo reflection and refraction.
When a wave moves from one substance to another, it can change its path. This change is called refraction.
Refraction happens when the wave bends as it crosses a boundary. It means that the wave enters the new substance in one direction and comes out in a different direction. The wave will not alter its direction if it crosses the boundary at the normal.
Dispersion of Light
- Refraction upon entering a triangular prism: When white light moves from the air into a triangular glass prism, it bends or refracts due to the change in the density of the medium.
- Refraction upon exiting a triangular prism: As the white light goes through the prism, it refracts again. The different colours within the white light have different wavelengths, resulting in varying refraction angles.
- The occurrence of dispersion: Through these distinct angles of refraction, the white light undergoes a fascinating dispersion process. It splits into the seven colours of the rainbow. Remember, red light has the longest wavelength and undergoes the least refraction, while violet light has the shortest wavelength and experiences the most refraction.
- Monochromatic description: Each light colour can be described as monochromatic, meaning it consists of a single wavelength.
Effect of Density of Material on Direction of Light
When light moves from one substance to another, like air to glass, it changes speed and direction. This change happens because the second substance is denser than the first. Here's how it works:
- Light entering a denser medium: When light enters a thicker material, such as glass, it slows down and bends towards an imaginary line called the normal. The normal is drawn perpendicular to the surface where the light enters.
- The degree of bending: The amount of bending depends on how dense the material is. The thicker the material, the more the light will bend as it enters. Different materials have different densities, so the degree of bending can vary.
In simpler terms, when light encounters a denser material, it slows down and changes direction, bending towards the normal. The extent of this bending is determined by how thick the material is.
When light travels from a medium that is not very dense to one that is denser, like going from air to glass, it bends towards an imaginary line called the normal. This bending is known as refraction.
Here are a couple of things to note:
- Refraction from less dense to more dense medium: When light passes from a less dense medium to a more dense one, the angle of refraction (r) is smaller than the angle of incidence (i). This means that the light bends towards the normal.
- Refraction from more dense to less dense medium: Conversely, when light travels from a more dense medium to a less dense one, like going from glass to air, the angle of refraction (r) is larger than the angle of incidence (i). In this case, the light bends away from the normal.
Snell's Law
Scientists use an equation called Snell's Law to describe this relationship between the angles of incidence and refraction. Snell's Law helps us calculate how much light will bend when it moves from one medium to another.
Here:
"n" is the refractive index of the material
"i" is the angle of incidence of light
"r" is the angle of refraction of light
We can rearrange this equation in the form of the following formula triangle:
Refractive Index
The number which is related to the speed of light in the material is referred to as the refractive index.
Remember that the refractive index is always less than the speed of light in the vacuum.
Refractive Index (n) = 
Refractive Indices of Different Materials
The refractive index is always greater than 1, and different materials have different refractive indices. For instance:
- Refractive index of those objects which have greater optical density is higher. For example, the refractive index of a diamond is 2.4
- Conversely, the refractive index of objects having less optical density is lower. For example, the refractive index for glass is 1.5
Because the refractive index is a ratio, hence it has no unit.
Calculating Refractive Index - Example
A light beam passes from water into a diamond with a refractive index of 2.42. The angle of incidence is 45° concerning the normal. Determine the angle of refraction inside the diamond.
Solution
Step 1: Known quantities:
Refractive index of diamond, n = 2.42
Angle of incidence, i = 45°
Step 2: Applying the formula:
The formula for calculating the refractive index using angles is:
Where:
- n is the refractive index
- i is the angle of incidence
- r is the angle of refraction.
Step 3: Rearranging the formula to solve for the angle of refraction:
Plugging in the values:
Step 4: Calculating the angle of refraction:
Using the inverse sine function, we can find the angle of refraction:
Therefore, the angle of refraction inside the diamond is approximately 25.1°.
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