Why is it when trout are rising they cannot see an angler in front of them?
It is not that feeding trout cannot see you, it is that their vertical view limits what they can see in front of them. When a trout is feeding at the surface its vision, much like the human vision, is vertical and not horizontal. It is impossible to see horizontally when you are looking up. When a trout is looking up it cannot see horizontally through the river to where an angler may be standing. The trout is further hampered by the meniscus (Surface tension). Air breathers such as humans do not see the meniscus, we see right through it. However a trout, and a human underwater wearing a face mask, can see the meniscus. It looks like a mirror and you cannot see through a mirror. This makes a human almost disappear to a feeding trout. This concept and the concept that a trout cannot do two things at once, such as feed and be safe makes a smart fly fisher work a pod of rising trout with impunity.
Of course if an angler keeps walking up to a trout, the trout will eventually see them. A trout, and a human underwater, have a circle of vision. If an angler keeps walking up on a feeding trout eventually the angler will enter this circle of vision and spook the trout. However smart anglers use fly rods and distance to keep under the refracted angle of the trout’s eye sight known as Snell’s Window. This is why you often see anglers leaning and bending down while casting to rising trout.
I will tell you, trout who are not actively feeding can definitely see you. The non-feeding trout might not be able to see you through the mirrored meniscus but they can certainly look out horizontally and see an angler under water from the meniscus down. A good rule of thumb to go by is this: When trout are actively feeding you can be in front of them using the current and angle as an advantage to your casting and presentation. Your pinpoint cast will be easier, more accurate and drag free. If you are smart you will not scare them or be seen by them. Another way to say it is: The only time you are in front of a trout is when it is feeding. All other times try and be behind where you think trout may be in a river. Each river has its own different circumstances that you must adapt to when you are strategically casting. Remember there is an exception to every rule in fly fishing.