If you'd like to know more about the essential role that light plays in photography, then you’ll love our course, A Photographer's Guide to Light. Photons have an electromagnetic field that fluctuates as it moves through space. In this lesson, you’ll find out why that matters to photographers.
Noticing Polarised Reflections
The electromagnetic field has an electric field and a magnetic field that oscillate as the light moves through space. Conventionally, polarised light refers to the polarisation of the electric field.
This wave is polarised because the oscillations are happening in one direction.
If you were to look at it from the front you would see something like the image above. You have your x-axis and your y-axis and the electric field is only oscillating in the y-axis.
Most light sources produce light that is unpolarised. If you imagine this xy-axis, you have fields in all orientations. This is unpolarised light.
How Does Light Get Polarised?
When light reflects off certain surfaces, the polarisation of the field aligns itself to the surface. To your eye, it looks like direct reflection, only about half as bright. If it looks like direct reflection, what makes it special?
Polarised reflections can be blocked with a polarising filter. A polarising filter only lets light through if it's polarised in a certain direction.
Imagine that this string is unpolarised light. It’s oscillating in both the x and the y orientation.
If you put metal bars around the string to restrict the oscillations to the y axis - the vertical direction – then this simulates what happens when light gets polarised by reflecting off a surface.
This little metal wire over the string represents a polarisation filter. In one direction it’ll allow the light to pass right through, but as soon as it’s turned 90 degrees relative to the polarised light, it’s blocked completely.
This is exactly how a polarisation filter works. Light gets polarised by reflecting off objects or transmitting through objects and you want to block that polarised light, so you put a polarising filter on it. Then when you turn that filter 90 degrees relative to the polarised light, the polarised light is blocked. In the real world, this has a range of applications.
Polarisation in the Real World
This photo of a street was taken with no filter (left) and again with a polarising filter (right).
Most of the reflections in the windows are no longer visible in the image made with the filter. The sky changes too, that’s because when the sun shoots down through the atmosphere, the atmosphere reflects the light and some of that light is polarised, so when you block the polarised light you can see more of the blue colour of the atmosphere.
How dramatic the result is depends on a number of things: the angle of the sun, the amount of moisture in the atmosphere, and your angle relative to the sun that you're shooting.
Here's a closeup of a Chinese restaurant on the same street. In the left-hand image you can see there are a lot of reflections in the window and the lighter brick façade is reflecting some of the sky. On the right is the same image but again using a polarising filter. You can see the result is quite striking, the reflections are less noticeable and now we can see a lot more of the detail inside the window, like the plants and in particular those on the far right side of the shot.
In The Studio
This effect isn’t limited to the outside world, you could also take advantage of it in the studio. High gloss black plastic creates mostly polarised reflections and you can use a polarisation filter to kill those reflections and limit glare.
This product photo of a USB battery pack shot was created with one light and a large piece of white paper and has a lot of reflections on it (left-hand image). The right-hand side shows the same image again with a polariser; a huge difference. You can see it wasn’t just the top of the battery showing reflection either, the sides and front edge are much darker with the filter too.
In general, metal objects don't produce polarised reflections, but a lot of other objects do, so learning how to manage polarised reflections can be a really useful tool to have in your kit.
More Photography Resources
About the Authors
David Bode created the video course that includes this lesson. Dave is an expert on video and audio production, and he lives in the upstate NY area. He works as a camera operator, editor, inventor, motion graphics designer, recording engineer, and studio musician.
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