The eye as a refracting system

Light from an object enters the eye and is bent, or refracted, towards the retina. The retina is the light-sensitive surface at the back of the eye. The eye forms a real, inverted image there: light rays actually meet at the image, which is upside down relative to the object.

For the image to be sharp, rays from each point on the object must converge at the corresponding point on the retina. If they meet in front of or behind the retina, they spread out over an area of the retina rather than meeting at one point, so the image is blurred.

Refraction and light entry

The cornea, the transparent front surface of the eye, provides much of the refraction. The crystalline lens refracts light further and provides adjustable focusing, allowing the eye to focus objects at different distances on the retina.

The iris controls the size of the pupil, the opening through which light enters the eye. A larger pupil admits more light; a smaller pupil admits less. The pupil regulates the amount of light entering, while the cornea and lens refract it.

Accommodation

Accommodation is the change in the lens’s shape and optical power that focuses objects at different distances on the retina. Optical power describes how strongly the lens converges light.

Light arriving from a nearby object spreads out more before entering the eye than light arriving from a distant object. To focus the nearby object on the retina, the lens becomes more curved and its optical power increases. For a distant object, the lens becomes flatter and its optical power decreases. In each case, the adjustment brings rays from each object point together on the retina.

The eye in a ray diagram

A simplified ray diagram represents the eye’s refracting system as a single converging lens, with the retina as the image plane. To locate the image of one object point, trace two rays from that point:

1. A ray through the centre of the simplified thin lens continues approximately undeviated.
2. A ray parallel to the principal axis is refracted through the far focal point, on the opposite side of the lens.

The rays meet at the image point. When the eye is focused on the object, that point lies on the retina. Repeating this for other object points shows how the inverted image is formed. The simplified lens represents the combined refraction of the eye, not just the crystalline lens.

Spectral sensitivity of the eye

The eye acts as a photodetector: it responds to light entering it. Its sensitivity, however, depends on the light’s wavelength. Two beams with equal incident radiant power but different wavelengths do not necessarily produce the same visual response. One may appear brighter because the eye is more sensitive to its wavelength.

A spectral response curve shows how sensitivity varies with wavelength across the visible spectrum. Wavelength is shown on the horizontal axis and sensitivity on the vertical axis. Where the curve is higher, light produces a greater response for the same incident power; where it is lower, the response is weaker.

Cones and colour sensitivity

In brighter light, vision is mainly controlled by cones. Different types of cone have different wavelength-dependent sensitivities. Light at a particular wavelength stimulates these cone types by different amounts; their combined responses give the eye its sensitivity to colour.

Rods and sensitivity in low light

Rods are more sensitive at low light levels and become more influential as illumination decreases. Their spectral response differs from that of cones, so the eye’s overall spectral sensitivity changes as vision becomes rod-dominated.

As a result, the relative brightness of wavelengths can change between bright and dim conditions. For example, red light can appear dimmer relative to blue-green light when rods dominate. A spectral response curve therefore depends on the light level and the photoreceptors contributing to vision.

Spatial resolution of the eye

Spatial resolution is the ability to distinguish two nearby object points as separate rather than seeing them as one. It can be expressed as the smallest angle between the directions of the two points at the eye for which they can still be resolved. A smaller minimum angle means better spatial resolution: the eye can distinguish points that appear closer together.

Retinal sampling and signal convergence

The eye forms an image on the retina, where light is detected by photoreceptors. To distinguish two nearby points, the visual system must retain enough information about where their light falls on the retina. Resolution therefore depends both on the spacing of the photoreceptors that sample the image and on how their signals are combined in nerve pathways.

In cone-rich regions, especially the fovea, cones are closely spaced and their signals have comparatively little convergence onto shared nerve pathways. This helps preserve information about the positions of nearby points, giving high spatial resolution.

Many rods can contribute signals to a shared nerve pathway. Combining their signals improves sensitivity to weak light, but it makes it harder to identify which rod received the light. Consequently, rod-dominated vision has lower spatial resolution than cone-dominated vision.