Back to P6 Home
P6 E) Refraction
P6 E) Refraction
Different materials have different densities. When a wave enters a new material, the speed of the wave will change. If the new material is denser than the previous material, the wave will slow down. If the new material is less dense than the previous material, the wave will speed up.
The normal is an imaginary line that is perpendicular (at 90°) to the second surface. If a wave hits a new denser material along the normal, the whole of the wave front slows down at the same time and the wave will continue to travel in the same direction at a slower speed. This is shown on the diagram below.
The normal is an imaginary line that is perpendicular (at 90°) to the second surface. If a wave hits a new denser material along the normal, the whole of the wave front slows down at the same time and the wave will continue to travel in the same direction at a slower speed. This is shown on the diagram below.
There are two substances on the above diagram. The blue substance (air) is less dense than the green substance (glass), and this means that the wave will travel faster through the less dense air compared to the denser glass. The wave hits the new material along the normal, which means that the whole wave front slows down at the same time. This means that the wave continues travelling in the same direction at a slower speed. For this case, the wave has not been refracted as the wave is travelling in the same direction.
If the wave hits a new material at an angle, the direction of the wave will change. If the new material is denser, the wave will bend towards the normal. If the new material is less dense, the wave will bend away from the normal. Let’s have a diagram of a wave hitting a new material at an angle.
If the wave hits a new material at an angle, the direction of the wave will change. If the new material is denser, the wave will bend towards the normal. If the new material is less dense, the wave will bend away from the normal. Let’s have a diagram of a wave hitting a new material at an angle.
The substances are the same as before; the blue substance (air) is less dense than the green substance (glass). For the above diagram, the top of the wave hits the denser glass first, which results in this part of the wave slowing down first. This causes the direction of the wave to change towards the normal. The wave has been refracted in this example because the direction of the wave has changed.
The amount of refraction depends on how much the wave slows down, which depends on the densities of the two different materials and the wavelength of the wave. The greater the difference in densities, the greater the refraction will be. Optical density is a measure of how quickly light travels through a material – the greater the value for optical density, the slower light travels through that material.
The amount of refraction depends on how much the wave slows down, which depends on the densities of the two different materials and the wavelength of the wave. The greater the difference in densities, the greater the refraction will be. Optical density is a measure of how quickly light travels through a material – the greater the value for optical density, the slower light travels through that material.
Wavelength, Frequency & Speed
From previous sections, we know that we can work out the speed of a wave by multiplying the frequency by the wavelength.
From previous sections, we know that we can work out the speed of a wave by multiplying the frequency by the wavelength.
The frequency of a wave always remains the same when it travels through different materials (f is the same). When a wave travels through a denser material, the wave will slow down (v decreases). As the frequency of the wave stays the same, a decrease in speed will result in the wavelength also decreasing (λ also decreases).
The opposite will happen when a wave travels through a less dense material; the wave will speed up (v increases), the frequency will remain the same (f) and the wavelength will increase (λ increases).
Ray Diagram
I am now going to go through a ray diagram for the refraction of light going through air into a glass block. The normal is an imaginary line that touches the new material at a right angle (90°). The normal and the two different materials are shown below.
I am now going to go through a ray diagram for the refraction of light going through air into a glass block. The normal is an imaginary line that touches the new material at a right angle (90°). The normal and the two different materials are shown below.
The ray of light travelling through the air is known as the incidence ray. The angle between the incidence ray and the normal is known as the angle of incidence.
The ray of light travelling through the glass block is called the refracted ray. The angle between the refracted ray and the normal is known as the angle of refraction.
The diagram is shown below.
The ray of light travelling through the glass block is called the refracted ray. The angle between the refracted ray and the normal is known as the angle of refraction.
The diagram is shown below.
If the second material is denser than the first material, the refracted ray will bend towards the normal, which means that the angle of refraction is smaller than the angle of incidence; this will happen when the second material has a higher optic density. This is happening on the diagram above.
The opposite will happen if the second material is less dense than the first material; the angle of refraction will be greater than the angle of incidence.
The opposite will happen if the second material is less dense than the first material; the angle of refraction will be greater than the angle of incidence.
Glass Block Example
We can undertake an experiment to investigate refraction by shinning a ray of light at a glass block. We then draw along the lines from the light source to the glass block, and from the glass block out to the air. We then lift the glass block and draw a line going across. We will obtain a diagram like what is shown below.
We can undertake an experiment to investigate refraction by shinning a ray of light at a glass block. We then draw along the lines from the light source to the glass block, and from the glass block out to the air. We then lift the glass block and draw a line going across. We will obtain a diagram like what is shown below.
When light hits the glass block most of the light enters and passes through the glass. Some of the light is reflected away from the glass block and for the reflected ray, the angle of incidence is equal to the angle of reflection.
The glass is denser than air and this causes the wave to slow down. This results in the wave bending towards the normal; the angle of refraction is less than the angle of incidence.
When the light reaches the other side of the glass block, it enters air and is refracted again. The air is less dense than glass, so the wave bends away from the normal; the angle of refraction is greater than the angle of incidence.
The glass is denser than air and this causes the wave to slow down. This results in the wave bending towards the normal; the angle of refraction is less than the angle of incidence.
When the light reaches the other side of the glass block, it enters air and is refracted again. The air is less dense than glass, so the wave bends away from the normal; the angle of refraction is greater than the angle of incidence.
Shinning White Light through a Prism
White light is a spectrum of all of the different visible light colours. Within the spectrum of visible light, red light has the longest wavelength and violet light has the shortest wavelength (also, red light has the lowest frequency and violet light has the highest frequency). When we shine white light through a triangular glass prism, the different parts of the spectrum of visible light will be refracted by different amounts. Red light is slowed down the least by glass and is refracted the least because it has the longest wavelength. Violet light is slowed down the most and is refracted the most because it has the shortest wavelength. The image below shows how the different parts of the spectrum of visible light are refracted as they enter and leave a triangular glass prism.
White light is a spectrum of all of the different visible light colours. Within the spectrum of visible light, red light has the longest wavelength and violet light has the shortest wavelength (also, red light has the lowest frequency and violet light has the highest frequency). When we shine white light through a triangular glass prism, the different parts of the spectrum of visible light will be refracted by different amounts. Red light is slowed down the least by glass and is refracted the least because it has the longest wavelength. Violet light is slowed down the most and is refracted the most because it has the shortest wavelength. The image below shows how the different parts of the spectrum of visible light are refracted as they enter and leave a triangular glass prism.