Why You Must Never Look at an Eclipse with the Naked Eye
– Even When 90% of the Sun Is Hidden, the Light That Remains Is Just as Strong
During an eclipse, the sky grows a little darker, and the Sun feels less dazzling to look at. But the thin sliver of Sun that remains shines with exactly the same strength as the ordinary Sun. The lens in your eye focuses that light onto a single spot, and the retina feels no pain even when it is being damaged.
Today there is a partial solar eclipse. It is midday, but it has gone a little dim outside, as if evening were coming on.
You look up. The Sun, which you can normally never look at for long, somehow seems possible to gaze at. You even have a pair of dark sunglasses in your hand.
"It's this dark, so a quick look should be fine." That thought is exactly when the danger is greatest.
There are only two reasons it is dangerous
The Moon hides only the Sun's "area". The part left in a thin crescent shape shines with exactly the same brightness as the ordinary Sun.
Like a magnifying glass, the eye gathers sunlight onto a tiny spot on the retina. The retina has no pain nerves, so you cannot tell when it is damaged.
When these two combine, you get a mismatch: the scene feels dim, yet strong light keeps striking the back of your eye. Let's take them one at a time.
Why does the partly hidden Sun feel "darker"?
Move the slider in Figure 1 to cover the Sun. The upper bar shows the total amount of light reaching the ground, and it gets shorter the more the Sun is covered. This is why the sky seems to darken.
The lower bar, however, does not change at all. The Moon is just a "lid" in front of the Sun; it does not weaken the light of the part that remains.
The Sun's image on the retina works the same way. A round, full-Moon-like image simply turns into a thin crescent image. The strength of light on the crescent is no different from before. What burns is not the "surface" of the retina but each individual "point" the light falls on.
Worse still, when the sky darkens, your pupil opens wide to take in more light. The glare drops and the Sun becomes easier to stare at, yet more light actually gets in. An eclipse creates just the conditions that tempt you to look at the Sun for longer than usual.
How far can the eye concentrate light?
Near the front of the eye sits a lens called the crystalline lens. Look at Figure 2. Sunlight entering through the pupil is bent by this lens and forms a tiny image of the Sun on the retina at the back of the eye.
The pupil is about 3 mm across, and the Sun's image on the retina is about 0.16 mm across. The diameter is about 19 times smaller, so the area is smaller by its square, about 350 times. In other words, light about 350 times denser per unit area than sunlight outdoors keeps striking a tiny point on the retina. It is the same thing as the game of burning black paper with a magnifying glass, happening inside your eye.
Retinal cells are thought to be damaged by chemical reactions when they are exposed to strong light, especially bluish light, for a long time. The light itself destroys the cells, before heat can scorch them. And the retina has no pain nerves. You feel nothing while you look, and usually notice only hours to a day later, through symptoms such as "the centre of where I tried to look is blurred" or "straight lines look bent".
The easiest is a "pinhole". Make a small hole in thick paper, stand with your back to the Sun, and project the light onto the ground; the shape of the partly hidden Sun appears. The same thing happens with sunlight through leaves, and on an eclipse day every spot of light on the ground becomes a crescent. For details, see "Why are the spots of light under trees all round?".
So what should we do?
- However much of the Sun is hidden, never look at it directlyEven at 90% hidden, the light that remains is just as strong.
- To watch, use eclipse glasses or project the image onto the groundSunglasses, plastic sheets and black film are no substitute.
- Never point binoculars or a telescope at the Sun without a proper filterThey gather even more light and can damage the eye in an instant.
The retina gives no pain even when it is damaged. If, after watching an eclipse, the centre of what you want to see looks dark or lines look bent, see an eye doctor straight away. Do not use eclipse glasses that have scratches or holes. It is also dangerous to look through binoculars or a camera viewfinder while wearing eclipse glasses.
Summary
Even when an eclipse darkens the sky, the Sun's remaining light is as strong as ever. The lens of the eye gathers that light onto a tiny spot on the retina, making it hundreds of times stronger per unit area than sunlight outdoors. Because the retina gives no pain, you cannot tell on the spot that it is being damaged.
The Moon hides only the Sun's area.
The strength of the light that remains has not dropped at all.
The same Sun–Moon line-up, with the Moon moving into shadow instead, is a lunar eclipse. See also "Why does the Moon turn red instead of vanishing in a total lunar eclipse?". For light that gets through even when it is cloudy, see "Why can you get sunburned on a cloudy day?".
- Poke a round hole 1–2 mm across in the middle of a piece of thick paper with a toothpick.
- Stand with your back to the Sun and project the light through the paper onto a white sheet about 1 metre away. Do not look at the Sun itself.
- The small round spot that appears is the Sun's image. The farther apart the card and white sheet, the larger and dimmer the image.
The image gets dimmer as it grows because the same amount of light is spread over a wider area. Inside the eye it is the opposite: light is gathered into a 0.16 mm spot. On an eclipse day, you can watch the round spot turn into a crescent.
For those who want to know more – terms, formulas, and links to textbooksShows which level each point belongs to, from middle school science to university courses
- Middle schoolCovered in middle school science
- High schoolCovered in high school physics or biology
- High school+Advanced high school material, or textbook sidebars
- UniversityUniversity specialist courses (optics, ophthalmology, photobiology), not taught in high school
- ResearchTopics researchers are still investigating, not taught even at university as settled
Middle schoolTerms: this phenomenon has names
- Convex lens and focal point: a lens that gathers parallel light to a point. The eye's lens and cornea together act as a convex lens.
- Retina: a thin membrane at the back of the eye lined with light-sensing cells. In a camera, it corresponds to the image sensor.
- Solar retinopathy (eclipse retinopathy): a condition in which the centre of the retina is damaged by staring at the Sun. The centre of what you want to see becomes hard to make out.
Middle schoolHigh schoolChecking with a formula: how many times denser does light get on the retina?
We find the size of the Sun's image, then estimate the strength per unit area when the light entering through the pupil is gathered into that tiny image. This is a rough estimate that ignores absorption and scattering inside the eye.
| In symbols | E_ret ＝ E × ( D ÷ d )² 、 d ＝ f × θ |
| In words | Light strength on the retina = outdoor sunlight strength × (pupil diameter ÷ image diameter) squared. Image diameter = focal length of the eye × apparent size of the Sun (angle) |
| Where it comes from | The amount of light passing through the pupil is the same on the image (conservation of light energy). The same amount gathers into a smaller area, so the strength rises by the ratio of areas, that is, by the square of the ratio of diameters. The size of the image is set by how a lens forms images |
| Symbol | Meaning and unit |
| E | Strength of sunlight reaching the ground (watts per square metre) |
| D | Pupil diameter (millimetres) |
| d | Diameter of the Sun's image on the retina (millimetres) |
| f | Focal length of the eye (millimetres) |
| θ | Apparent diameter of the Sun (radians, a unit of angle) |
| Sunlight strength on the ground on a clear day | about 1000 watts per square metre |
| Pupil diameter in bright light | about 3 mm (opens to about 7 mm in the dark) |
| Focal length of the eye | about 17 mm |
| Apparent diameter of the Sun | about 0.53 degrees ＝ about 0.0093 radians |
| Light passed by eclipse glasses | said to be about 1 part in 100,000 |
| Diameter of the Sun's image on the retina | 17 × 0.0093 ≒ 0.16 mm |
| Ratio of pupil diameter to image diameter | 3 ÷ 0.16 ≒ 18.8 |
| Ratio of areas (square of the diameter ratio) | 18.8 × 18.8 ≒ 353 |
| Light strength on the retina | 1000 × 353 ＝ 353000 watts per square metre |
| With eclipse glasses on | 353000 ÷ 100000 ≒ 3.5 watts per square metre |
| With dark sunglasses passing one tenth of the light | 353000 × 0.1 ≒ 35300 watts per square metre |
With a bare eye, light about 350 times as strong as outdoor sunlight strikes one point on the retina. Even with dark sunglasses it is about 35 times, still far stronger than standing in sunlight outdoors. With proper eclipse glasses it drops to a few hundredths of outdoor sunlight. And even in a 99% eclipse, the light striking the image of the remaining crescent stays at this value of 353000.
High schoolHigh school+Damage comes from a chemical reaction of light, not heat
High schoolIn the lens formula, the image of a distant object forms at the focal length, and its size is set by "focal length × apparent angle". The formula for d above is this relation. The energy of a single particle of light (photon) is larger the shorter the wavelength, so blue and violet light are stronger at breaking molecules.
High school+The main cause of solar retinopathy is thought to be photochemical injury by blue light, not a burn from rising temperature. The retina's temperature is said to rise only a few degrees, but strong blue light produces reactive oxygen inside cells and harms the photoreceptors and the pigment layer behind them. So "it isn't hot, so it's fine" does not hold.
UniversityOptical density and eclipse glasses standards
How much a filter weakens light is expressed as optical density (the common logarithm of the transmittance, with a minus sign). Filters for eclipse viewing are governed by the international standard ISO 12312-2, which is said to require visible light transmission to be cut to an optical density of about 5, that is, about 1 part in 100,000. Invisible ultraviolet and infrared must also be reduced within the limits. In ophthalmology, solar retinopathy is treated as damage to the fovea (the central part of the retina with the sharpest vision), and is seen as disruption of the layers on optical coherence tomography (OCT).
📖 Detailed explanation of safe viewing methods and eclipse glasses standards: NASA Science "Eclipse Viewing Safety"
ResearchWhat is still not clearly understood
- How many seconds of looking cause damage? The time at which damage begins varies greatly with pupil size, age, clarity of the lens, and how clear the air is that day, and no safe time has been set for each person.
- How far does it recover? Some people regain their vision within months, while others keep a small central blind spot for life. What separates the two is not yet well understood.
- Are children more easily damaged? A child's lens is clearer and lets more blue light through, so children are thought to be more vulnerable. However, data showing how large the difference is are limited.
In other words, this article too is "an explanation of what is known so far". Precisely because things remain unknown, it matters not to draw your own line of "a little is fine".
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science Year 1 "Properties of light (convex lenses)", Year 3 "How the Moon and Venus look" | The eye's lens focuses light to a point / how eclipses work |
| High school | Physics "Lenses and images", Biology "Visual organs (retina and photoreceptors)" | Formula for image size / role of the retina |
| High school+ | Physics "Photon energy" | Photochemical injury by blue light |
| University | Optics and ophthalmology (optical density, foveal damage) | Eclipse glasses standards and diagnosis of solar retinopathy |
| Research | Photobiology and ophthalmology | Individual differences in safe time, and what separates recovery |
| ― | Links to daily life | How to watch on eclipse day, what to say to children |
- NASA Science "Eclipse Viewing Safety"
- NASA Science "Sun: Facts"
- National Astronomical Observatory of Japan (国立天文台) "Precautions when observing a solar eclipse" (guide to eclipse observation methods)
- ISO 12312-2:2015 "Eye and face protection — Sunglasses and related eyewear — Part 2: Filters for direct observation of the sun"
- Japanese Ophthalmological Society (日本眼科学会) "Eye diseases" (explanations of solar retinopathy and macular diseases)
* This article is a general science explanation. The figures given are rough guides and estimates to help you understand the mechanism. For eclipse viewing, follow the methods advised by the National Astronomical Observatory, local authorities, schools and similar bodies. If you notice anything wrong with your eyes, see an eye doctor rather than judging for yourself.