Every fact in the three eye lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Simplifications and care points: this is general science education, not medical advice or an eye test; the classroom near-point and blind-spot activities do not check anyone's eyesight. The textbook models the eye with a lens-to-retina distance of 2.00 cm and treats the eye as a single thin lens; real eyes differ. The two OpenStax textbooks give slightly different limits for visible light (380–720 nm and 380–740 nm) and slightly different cone peaks (530/534 nm and 560/564 nm); we use them as approximate values. The near point of 25 cm is the textbook standard for normal vision and changes with age. Never shine lights into eyes and never look at the Sun. Pupils who find it hard to see the board or a book should tell a parent or teacher; an eye-care professional can check.
All sources
[1] How the Eyes Work
National Eye Institute (NIH, USA) · Reliability: High
Type: National health research agency
Light passes through the cornea, the clear front layer, which is shaped like a dome and bends light to help the eye focus. Some light enters through the pupil; the iris (the coloured part of the eye) controls how much light the pupil lets in. The lens works together with the cornea to focus light on the retina, where photoreceptors turn the light into electrical signals. These travel from the retina through the optic nerve to the brain, which turns them into the images you see.
Humans see light between about 380 and 740 nm. Rods are strongly photosensitive, lie towards the edges of the retina, work in dim light and detect only shades of grey; cones are weakly photosensitive, lie near the centre of the retina and give daytime colour vision. The fovea, at the centre back of the eye, has a high density of cones and gives the sharpest vision. Three cone types peak at about 420, 530 and 560 nm. The cornea and lens refract light onto the retina; muscles change the shape of the lens to change its focal length. In bright light the iris contracts and the pupil gets smaller; in dim light the pupil enlarges. In myopia the eyeball is elongated and the focus falls in front of the retina; in hyperopia the eyeball is too short. Humans can distinguish about 2 million colours.
[3] Anatomy and Physiology, 14.1 Sensory Perception
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Visible light lies between about 380 and 720 nm. The photoreceptors are rods and cones; rods contain rhodopsin, which absorbs best at about 498 nm, and work in low light. Three cone opsins absorb best at about 420 nm (blue), 534 nm (green) and 564 nm (red); the brain compares their relative activity to perceive colour. Retinal ganglion cell axons form the optic nerve. In the fovea each cone connects to a single ganglion cell; in the periphery up to 50 receptors converge on one. The blind spot is where the optic nerve leaves the retina and has no photoreceptors.
The cornea provides about two-thirds of the eye's power, because light slows considerably going from air into the cornea; the lens provides the rest. Refractive indices: cornea 1.38, aqueous humour 1.34, lens 1.41 (average), vitreous humour 1.34. The eye forms a real, inverted image on the retina, which the brain interprets the right way up. Thin lens equation P = 1/d_o + 1/d_i with P = 1/f in dioptres (1 D = 1/m), and magnification m = h_i/h_o = −d_i/d_o. With a lens-to-retina distance of 2.00 cm the power is 50.0 D for distant objects and 54.0 D for objects at 25.0 cm, the near point of normal vision; the ciliary muscle makes the lens thicker for close objects (accommodation). The near point increases with age. A hair 1.20 × 10⁻² cm wide at 60.0 cm forms an image 4.00 × 10⁻⁴ cm wide. The pupil and chemical adaptation let the eye work over light intensities from the lowest observable to about 10¹⁰ times greater.
A nearsighted (myopic) eye over-converges light, so rays from distant objects cross in front of the retina; its far point is closer than infinity and it is corrected with a diverging lens of negative power. Example: far point 30.0 cm, spectacles 1.50 cm from the eye, so the lens must form an image at −28.5 cm: P = 1/∞ + 1/(−0.285 m) ≈ −3.51 D. A farsighted (hyperopic) eye under-converges and is corrected with a converging lens of positive power; example: near point 1.00 m, P ≈ +3.24 D. Laser surgery reshapes the cornea: it is flattened for nearsightedness.
National Eye Institute (NIH, USA) · Reliability: High
Type: National health research agency
Nearsightedness (myopia) happens when the eyeball grows too long from front to back and makes far-away objects look blurry. Farsightedness (hyperopia) happens when the eyeball grows too short and makes nearby objects look blurry. Both can also come from the shape of the cornea or lens. Astigmatism comes from a cornea or lens with a different shape than normal. Presbyopia: with age the lens becomes less flexible, making it hard for middle-aged and older adults to see things up close.
National Eye Institute (NIH, USA) · Reliability: High
Type: National health research agency
The most common type, red-green colour vision deficiency, is genetic: parents pass the genes to children. It is more common in men because the gene is on the X chromosome: men have one X chromosome, so one copy causes it; women have two and would need the gene on both, which is rare. Blue-yellow and complete colour vision deficiency affect men and women equally. Colour vision can also change later in life through eye diseases, some medicines, injuries or ageing.
Physics Lecture Demonstrations, University of Colorado Boulder · Reliability: Medium-high
Type: University physics demonstration page
Each student uses a card with an X and an O: with the left eye closed, looking at the X, the O disappears at the right viewing distance. The blind spot is the region where the optic nerve exits the retina, with no photoreceptors; the demonstration shows that the brain actively fills in the missing information.