You may remember briefly learning about refraction in physics at school – light moves more slowly in denser materials and therefore light appears to bend when it moves through glass or water. You will likely have been taught that this is why the deep end of a swimming pool is deeper than it looks and if you happen to be a fan of a certain rock band from the 1970s, you may well have read some articles about how one of their most famous album covers doesn’t accurately reflect the movement of light through a prism.
Beyond this, however, refraction seems to be quite a niche concept that one briefly studies and learns for an examination before subsequently forgetting it only to never hear of it again. It is, however, a fascinating phenomenon which has more of an impact in people’s everyday lives than they might suspect at first glance, and it is a factor in some of the technical modelling that Pager Power undertakes for its assessments.
What is Refraction?
Before we continue, let us have a reminder of what refraction is. Refraction is the change in speed of waves as they move from one medium (“material”) to another which causes them to change direction.
Figure 1: An illustration of how light bends when it undergoes refraction – the diagrams are not to scale.
When waves travel from a light medium to a dense medium, they bend towards the normal to the boundary between the two media, and when waves travel from a dense medium to a light medium, they bend away from the normal to the boundary. The angles between the incident ray (which hits the boundary) and the refracted ray (which then leaves from the boundary) are related according to Snell’s law:
nIsin(I) = nRsin(R)
Where nI is the refractive index of the medium which the wave travels through before reaching the boundary, nR is the refractive index of the medium through which the wave travels after crossing the boundary, I is the angle of the incident ray to the normal and R is the angle of the refracted ray to the normal.
Generally speaking, denser media have higher refractive indices, and so when passing to a denser medium, nR is greater than nI and so therefore to maintain the equality, sin(R) is less than sin(I) and so R is less than I – in other words, the refracted ray has a lower angle to the normal, and when moving to a denser medium the light has bent towards the normal.
Atmospheric Refraction of Light and Radio Waves
One application of refraction which seeps into everyday life and isn’t immediately obvious is the refraction of light by the atmosphere. It allows sunlight to bend from below the horizon and as a result the days appear to be slightly longer as the sun is still visible even when technically, it is below the horizon. It means that on the equinoxes, the day is about 12 hours 7 minutes long at the Equator, 12 hours 14 minutes long in London and even longer again as one moves towards both poles. There are actually eight days per year during which both the poles can see the sun, even though for one of the poles the sun is actually below the horizon. The effect is also particularly beneficial for communities living a small distance either side of the Arctic Circle who get to enjoy 24 hours of daylight in the middle of Summer but do not have to endure 24 hours of darkness in the middle of Winter.
Figure 2: Another result of atmospheric refraction is that the increase in bending of light waves as one approaches the horizon causes the Sun to appear to flatten at sunrise and sunset.
How does it work then? The atmosphere is comparatively thick at the surface of Earth but thins quite rapidly as one moves away from it. The result is that as Sunlight moves through the atmosphere at an angle, it bends downwards towards the normal of a hypothetical boundary between more and less optically dense space.
The result of this is that one sees slightly more daylight than one might initially expect. For developers who are building projects which include solar PV installations it means that the installations will generate more energy but that they will also produce more glare!
There is a similar atmospheric refraction effect for radio waves which causes transmitted radio waves to bend back towards Earth, and the result of this is that radio waves can propagate about 33% further than they would be able to without such refraction effects. It should of course be noted that this is only considering visibility of the waves over the horizon and that additional power should be supplied to the transmitter to capitalise on such an increase in visibility.
About Pager Power
Pager Power accounts for all of these effects in the detailed technical modelling which underpins its assessments.
For more information on Glint and Glare Assessments, which account for the atmospheric refraction of sunlight, click here.
For more information on how we can help assess the impact of wind farms on telecommunications and radar, our modelling of which account for the atmospheric refraction of radio waves, click here.
References
Figure 1 has been produced by the author.
Figure 2: Alvesgaspar. Sunset at Porto Covo, west coast of Portugal, 2007. CC-BY-SA 3.0. Last accessed on 17th July 2023.Available at Wikimedia Commons: https://commons.wikimedia.org/wiki/File:Sunset_2007-1.jpg
