Underwater optics: Snell's window, colour loss and what fish really see
Course pack: Approach and colour-test log. Coach Willy, our virtual AI coach, can explain each step, set you a practice task and check your understanding.
Who it's for
You've done the basics and want to tighten things up. Aimed at carp, tench, chub, pike.
Where and when
Done on the bank. Best in: all.
What you'll work through
- 1Snell's window: why kneeling beats standing
- 2Colour loss with depth and water clarity
- 3Line refraction: fluorocarbon, nylon and the lakebed
- 4Light, shadow and ripple: fish with the fish's eyes
Step 1: Snell's window: why kneeling beats standing
Alright mate, let's get stuck into underwater optics: Snell's window, colour loss and what fish really see. I'll walk you through what to look for, what to try and how to tell whether it helped. We'll start with snell's window: why kneeling beats standing, then work through a proper practice job at each step. Take it at your own pace and keep a few notes so you've got a useful plan for next time.
By the end of this step
Have a go at: snell's window: why kneeling beats standing.
Willy’s take
Right, this one's a bit of proper physics, but stick with me because it changes how you walk up to every swim. When light goes from air into water it bends. Because of that bending, a fish looking up doesn't see the whole sky.
It sees the world above the surface squashed into a circle right over its head, a cone roughly 97 degrees across. Anglers call it Snell's window.
Outside that circle the underside of the surface acts like a mirror, so the fish mostly sees a reflection of the bottom and the water around it. The interesting bit is the edge of the window. Things standing low on the bank, near the horizon, get squeezed into a thin, blurry band right at the rim.
Things standing tall stick up higher into the window and are much easier to see.
The window also grows with depth. As a rough guide it's a bit over twice as wide as the fish is deep, so a fish at one metre sees a circle a bit over two metres across, and a fish at three metres sees one nearer seven.
Deeper fish see a wider slice of the bank, but it's blurrier because there's more water in the way.
So the practical rule is simple: keep low, keep back, and move slowly. On shallow, clear water a standing angler on the skyline is about as subtle as a lighthouse. Kneel, use the cover behind you and you drop into that squashed, blurry band at the edge of the window.
On the bank
Worked example
Imagine this: you're stalking carp in a clear, metre-deep margin on a bright day. Every time you stand up to look, the fish drift off. A mate fishing the same bank on his knees behind a bush is getting fish to his hookbait.
Your turn: Snell's window: why kneeling beats standing
- 1
Pick one swim and look at where your head sits against the skyline when you stand, kneel and sit
- 2
Approach the margin on your knees with dark, plain clothing and no sudden movement
That's the free sample — this is where it stops
The full step keeps going with the rest of the practice job, common mistakes and fixes, a check-your-understanding quiz and your Approach and colour-test log. Then 3 more steps, each with its own task, and Coach Willy on hand to talk it through.
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