Reflection and refraction are properties of light. The electromagnetic waves that comprise light are refracted when they pass from one medium to another because of their change in speed. Light is also reflected or scattered in a different direction when it interacts with a surface. Let’s look at refraction in more detail.
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Team Refraction at a Plane Surface Teachers
Refraction is the change in the direction of an electromagnetic wave when it passes from one medium to another. When the wave moves into a different medium, the magnitude of its speed also changes and causes the wave to change direction.
At the boundary between two mediums, the rays of light change direction at an angle from the normal (which is the normal direction perpendicular to the surface). This diffraction angle depends on the densities of the two mediums through which the light rays pass.
The above scenarios are illustrated in figure 1. On the left-hand side, you can see that when light passes through the normal, it does not bend. On the right-hand side, you can see that when light passes at any other angle from a high optical density material to a low optical density material, the light bends away from the normal and vice versa.
The speed of light depends on the medium the light passes through. When light passes from a low-density medium to a high-density medium, the rays will slow down. This is because there will be more molecules in a high-density medium and therefore more obstacles that slow down the light. As a result, the light will bend towards the normal and away from the boundary.
Speed and wavelength change during refraction, but the frequency of the wave remains constant.
Reflection of light is the change in the direction of light when the light wave comes in contact with a surface that does not fully absorb the wave’s energy. This results in the reflection of light, which you can see in figure 2.
The refractive index, denoted by n, is a property of a material that measures the amount by which light slows down when it passes through it. We can calculate the refractive index with the following equation:
\[n = \frac{c}{c_s}\]
Here, c is the speed of light in a vacuum, and cs is the speed of light in a material (they are both measured in m/s).
Light travels through a vacuum at a constant speed. However, it slows down when it passes through other substances. A high refractive index value means that the material is optically dense, so light travels through that substance slower.
The refractive index should always be greater than 1 as the speed of light in any substance cannot exceed its speed in a vacuum. The refractive index of air can be assumed to be 1 if it is not given.
The law of refraction is the law that explains refraction at a plane surface.
Snell’s law states that the angle of the refracted ray of light and the incident ray compose the normal of the boundary at the point of refraction.
Since the refraction angle depends on the medium through which the light passes, there is a relation between the angle of incidence, angle of refraction, and the refractive index. This relation is described by Snell’s law below, where n1 and n2 are the refractive indices of the two mediums, θ1 is the angle of incidence, and θ2 is the angle of refraction.
\[n_1 \cdot \sin \theta_1 = n_2 \cdot \sin \theta_2\]
The incident and refraction angles are shown below in figure 3.
A ray of light is directed at an angle of incidence of 45° and passes through glass, exiting at an angle of refraction of 32°. Find the refractive index of glass.
Solution
Assume that the refractive index of air is 1. Using Snell’s law and substituting the given values, we get:
\(n_1 \cdot \sin \theta_1 = n_2 \cdot \sin \theta_2 = n_1 \cdot \frac{\sin\theta_1}{\sin \theta_2} = 1\cdot \frac{\sin(45)}{\sin(32)} = 1.334 \)
When the angle of incidence increases, the angle of refraction increases. When the refraction angle reaches 90°, the light is reflected along the boundary. This angle of incidence that causes total internal reflection is known as critical angle θc, and we can calculate it using the equation below, derived from Snell’s law.
\[\sin \theta_c = \frac{n_2}{n_1}\]
For total internal reflection to occur, two conditions must be met:
A light beam passes from water to air. What is the critical angle between air and water if their refractive indices are 1.55 and 1, respectively?
Solution
We know that the refraction angle must be 90 degrees to have a total internal reflection. Using Snell’s law, we get:
\(n_1 \cdot \sin \theta_1 = n_2 \cdot \sin \theta_2 \rightarrow 1.55 \cdot \sin \theta_c = 1 \cdot \sin(90) \rightarrow \sin \theta_c = \frac{1}{1.55} = 0.65 \Rightarrow \theta = 40.2^\circ\)An example of refraction is a mirror. When a ray of light penetrates the glass, it gets refracted. It then reaches the other end of the mirror and gets reflected. The reflected ray gets refracted again on the outer surface and moves away from the mirror.
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What is the plane surface formula?
The plane surface formula is n1⋅sin(θ1) = n2⋅sin(θ2), where n1 is the refractive index of the medium that the incident light is coming from, n2 is the refractive index of the opposite medium, θ1 is the angle between the incident light and the normal of the boundary, and θ2 is the angle of refraction.
What is the reflection of light at a plane surface?
The reflection of light at a plane surface is the change in the direction of light when the light wave comes in contact with a surface that does not absorb the wave’s energy. This results in the reflection of light.
Does refraction occur in a plane mirror?
Yes, refraction occurs in a plane mirror.
Can refraction occur in air?
Refraction occurs when light passes through two different mediums. Light passing through air would not cause it to refract as it requires another medium.
Why is there no refraction at 90 degrees?
If the incident light ray is incident at 90°, this means that it is parallel to the normal and it cannot bend away or towards it, so refraction does not occur.
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