To form a clear image, light must be focused on the retina, which detects the light and sends signals through the optic nerve to the visual centre of the brain. To achieve the focusing, most refraction of light happens at the cornea, with the ciliary muscles adjusting the lens for fine-tuning.
When the lens is relaxed, the eye is focused at infinity - i.e. parallel light rays from distant objects will be in focus.
For a near object, the ciliary muscles have to work harder the closer the object is, to adjust the lens to manage diverging rays. Most people have a near point of about 30cm, but it gets further away as we age, due to natural longsightedness, known as presbyopia.
This image shows the rays being refracted coming from a far object (parallel rays) and a near object (divergent rays).
Adjusting the lens (and constricting the pupil) in response to a near object is called accommodation - the lens needs to converge the rays more strongly for a near object.
This is a reflex coordinated by the second cranial nerve (optic nerve) as the afferent arc (sending a sensory signal to the brain) and the third cranial nerve (oculomotor nerve) as the efferent arc (receiving signals back from the brain and enacting a motor response). This is shown in the diagram below.
For an object closer than the near point, the eye will need the help of lenses to focus the divergent light rays sufficiently. Any sort of prolonged focus closer than infinity will be tiring, but more so the closer the object is.
That is why convex lenses are used for near-eye displays, like in a virtual reality headset. The same type of lenses are used for long-sightedness, when the eye cannot focus diverging light rays from near objects sufficiently to form a focused image on the retina.
This image shows a convex lens focusing parallel rays to a point, and also a diverging or concave lens (which is less relevant here, but is used to correct short-sightedness).
Ideally, you would have the eyes completely relaxed and use lenses to provide an image in focus in this state. This is not practically possible, but the virtual image that the person sees can appear further in front of them, well beyond the natural near point.
The image below shows one example of this.
The image taken from an interesting paper by Xia et al describing a novel approach for a mixed reality headset that is adjustable for those with refractive errors (i.e. who wear glasses or contact lenses).
The middle lens is the main focusing lens, with the one nearer the eye being adjustable. At the far right you can see the virtual image, which is where the wearer will experience the display being; comfortably beyond their near point.
So in summary, the eye cannot be tricked into relaxing unless you’re displaying an image it can perceive to be far away, or at least as far away as possible. While in the example of the VR headset above, this creates a virtual image further away from the eye, it is optics rather than trickery!
