Refraction at a Plane Surface

Refraction at a Plane Surface

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Reflection happens when light waves hit a surface and bounce back in a different direction. Refraction, on the other hand, is when light waves pass through a medium and change direction due to a difference in speed. When light waves pass from one medium to another, like from air to water or from water to glass, they change speed and bend. This bending is called refraction at a plane surface. It happens because light travels faster in air than in water or glass. For example, if you put a pencil in a glass of water, it looks like it's bending. This is because of refraction. The light waves from the pencil bend when they pass from air to water and again when they pass from water to air. Refraction has a lot of practical uses too. One example is lenses, which use refraction to focus light and create clearer images. Another example is rainbows, which happen when sunlight is refracted by raindrops. So, reflection and refraction are both amazing properties of light that help us understand how it behaves in different situations.

The refraction of light rays at a plane surface

Refraction occurs when an electromagnetic wave, such as light, changes direction as it moves from one medium to another. This happens because the speed of the wave changes when it enters a new medium. At the boundary between the two mediums, the light changes direction at an angle from the normal, which is a line perpendicular to the surface. The amount of change in direction depends on the densities of the two mediums. When light passes from a low-density medium to a high-density medium, it bends towards the normal line, and when it passes from a high-density medium to a low-density medium, it bends away from the normal line. However, when the light passes perpendicular to the boundary, it does not bend. This is shown in figure 1, where on the left side, light passing through the normal does not bend, and on the right side, light passing through a non-perpendicular angle bends towards or away from the normal depending on the densities of the two mediums. Refraction at a plane surface is an important concept in understanding how light behaves when passing through different mediums.

Figure 1. Refraction of light

The speed of light changes depending on the medium it passes through. Light moves slower in high-density mediums because more molecules in the way slow it down. As a result, the light bends towards the normal when it passes from a low-density medium to a high-density medium.Waves travel faster in deeper water and when they reach the shoreline, they change direction and move perpendicular to it.Rainbows happen when light passes through raindrops, and each color bends differently because of its wavelength. The speed of light in a vacuum is constant, but it slows down when it passes through other materials. A high refractive index means a substance is optically dense and slows down light. The refractive index is always greater than 1 because light can't travel faster in any material than it can in a vacuum. The refractive index of air is usually considered to be 1.

The law of 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. The incident and refraction angles are shown below in figure 3.

Figure 3. Refraction and Snell’s law
Figure 3. Refraction and Snell’s law

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:

What is total internal reflection?

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. For total internal reflection to occur, two conditions must be met: The angle of incidence must be greater than the critical angle.The refractive index n1  must be greater than the refractive index n2.

Figure 4. The total internal reflection of light
Figure 4. The total internal reflection of light

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?


We know that the refraction angle must be 90 degrees to have a total internal reflection. Using Snell’s law, we get: 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.

Refraction at a Plane Surface - Key takeaways

Refraction is the change of direction of light when it passes from one medium to another due to its change in speed. Light is reflected or scattered in a different direction when it interacts with a surface. The diffraction angle depends on the densities of the two mediums through which the light rays pass. Total internal reflection occurs when the incident angle is greater than the critical angle, and the refractive index of the incident medium is greater than the refractive index of the second medium.

Refraction at a Plane Surface

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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