Is this Grass Blue or Green?

My thanks to Peter Gibbs for another fascinating email. You can see our exchange below. If you have your own different interpretation, do please get in touch.

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Glad you liked the spiders web picture. Looking back I remembered I had photographed the same effect over a grazed field at about the same time last year.

But the other thing we spotted yesterday was more exciting and really quite weird. We were walking out in the Cotswolds near the village of Chedworth. The path at one point passed through a long, narrow wood running along the contour of a fairly steep slope on the north side of a river valley. There were open water meadows to the south so the sun was shining into and through the wood (it was early afternoon so the sun was to the SSW). The northern side of the wood was bordered by an open grass-covered field which continued up the slope. You could see it though the trees. Along the boundary where the wood met the fields was a distinctive but hazy zone of powder blue colour, looking like bluebells. At first glance it seemed like there may have been some sort of fence made of blue netting. But then I realised it was the grass! I went up to take a closer look and the effect almost entirely disappeared! Have a look at the photos and you’ll see what I mean.

I thought it might be some sort of diffraction effect caused by the low incident angle of sunlight falling on the scales on the blades of grass. You’ll notice the blades are ‘combed’ down the slope. What’s strange though is that the light coming through the wood is of course very dappled, and yet the blue effect seems relatively uniform. also on one of the close-up pictures notice that there’s a small tree leaf in the bottom centre of the frame which looks unnaturally blue. So perhaps it’s the blue of the sky being reflected by shiny blades of grass, with the shadow of the wood allowing it to be seen, while the effect is washed out further up the slope where the grass is directly lit by the sun.

I’ve never seen anything like it before. Will be interested to hear what you make of it!

blue and green grass close up

Part of my reply:

How wonderful! Thanks so much for sending this my way and with such great pics.

My initial thought is that you are right: blue sky reflection. I think it must be a rare but lovely example of the blue shadows that are easier to spot on snow or other very pale ground surfaces.

As you say, the angle of the grass must be important and offer the necessary angle of incidence. The bluest patch does appear to be offering a reflecting angle straight up to the sky and I suspect that if the light from directly above was shielded we might see the effect disappear.

I also suspect a contrasting optical effect with the green of the leaves and brown of the ground – as I’m sure you know there is a psychological aspect to this, as with all colours we perceive. The effect seems noticeably stronger when the grass hue is juxtaposed with both the leaves and ground. I have tried to crudely screen these out and the effect appears a little diminished. But then again, zooming right in, it is clear that the camera (not subject to these psychological whims or illusions, but possibly subject to some automatic colour calibrations) definitely sees blue there – see attached pixel image.

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