Lesson 8 of 10 / Homeostasis and response
Reading an eye diagram
Which structures guide light, and which detect it?
In this lesson: Identify eye structures in front and horizontal views and connect them to their functions.
About 6 min
The key ideaLight is refracted by the cornea and lens, then detected by receptors in the retina. The optic nerve carries impulses to the brain.
9. Retina: Inner receptor layer. Light is detected here and converted into nerve signals.
Green numbered marker: selected structure. Section layers run from outer sclera to choroid to inner retina. Original schematic; not to scale. The front view makes the transparent cornea visible with a dashed outline when selected.
Explanation
In a front view, the white sclera surrounds the coloured iris and the dark pupil. The pupil is an opening in the iris, not a solid black structure. The transparent cornea covers the front of the eye, including the iris and pupil beneath it.
In a horizontal section, light passes through the cornea, aqueous humour, pupil, lens and vitreous humour before reaching the retina. The cornea provides much of the refraction; the lens adjusts the focus. The aqueous and vitreous humours transmit light and help maintain the shape of the eye.
The retina contains light receptors that convert a light stimulus into nerve signals. The fovea has a high density of cones and gives sharp, detailed vision. At the blind spot, the optic nerve leaves the eye and there are no light receptors. The optic nerve carries impulses, not light, to the brain.
The tough sclera protects the eye. The pigmented choroid absorbs stray light and contains blood vessels supplying eye tissues. The iris controls pupil size. Ciliary muscles and suspensory ligaments alter lens shape for focusing; they do not control the pupil.
Step by step
- 1
Orient the view
Find the cornea at the front and the optic nerve at the back of a section.
- 2
Follow the light path
Trace cornea, aqueous humour, pupil, lens, vitreous humour and retina.
- 3
Match a structure to an action
Separate refraction, controlling light entry, detecting light and carrying impulses.
Worked example
A blind-spot label
A label points to the place where the optic nerve leaves the retina. Explain why an image falling there is not detected.
One way to explain it
This is the blind spot. It has no light receptors, so light falling there is not detected and converted into receptor signals.
Why this answer works
- Use the labelled location to identify the structure.
- Explain using the absence of receptors.
- Do not say the optic nerve is unable to carry impulses.
Is this true? "The pupil is a black lens that sends light down the optic nerve."
The pupil is an opening. Light reaches the retina; nerve impulses then travel along the optic nerve.