# How Many Photons Reach the Retina?

• kingwinner
In summary, the question being discussed is how many photons are emitted by a typical incandescent light bulb per second, and how many of those photons are incident at the image point on the retina at a distance of 10 km. The relevant factors are the distance, the diameter of the pupil, and the surface area of the eye. Experiments have shown that the human eye can detect between 1 and 5 photons under ideal conditions, but in theory, only one photon is needed to activate the biochemical cascade responsible for vision.

#### kingwinner

How many photons?

1) A typical incandescent light bulb emits 3x10^18 visible-light photons per second. Your eye, when it's fully dark adapted, can barely see the light from an incandescent light bulb 10 km away. How many photons per second are incident at the image point on your retina? The diameter of a dark-adapted pupil is 6mm.

Some relevant formulas I found from the textbook are E=hf, c=(lambda)(frequency), lambda=h/(mv)
But none of the above formulas seem to apply by any means to this weird question. Why does the distance 10km matter? Why does the diameter of an eye matter? How can I even use them?

I am really lost and confused. I hope that someone would be nice enough to help me out. Thank you so much!

Imagine that the light bulb is sending out all those photons in all directions equally. What fraction would make it into your eye at a distance of 10 km? One way to figure it out is to think of a huge 10 km radius sphere surrounding the light bulb--compared to the surface area of that sphere, how big is the surface area of your pupil?

Doc Al said:
Imagine that the light bulb is sending out all those photons in all directions equally. What fraction would make it into your eye at a distance of 10 km? One way to figure it out is to think of a huge 10 km radius sphere surrounding the light bulb--compared to the surface area of that sphere, how big is the surface area of your pupil?

But what is the surface area of the eye? Should it be considered full sphere, a semi-sphere (half sphere), or something else?

The photons have to go through the pupil. That's the relevant area, a disk 6mm in diameter.

Thanks for your help! I got it!

I was curious as to the answer. I used to do research in biophysics/vision. Under really good experimantal conditions, the human eye can detect between 1 and 5 photons, an amazing sensitivity when you consider the dynamic range of the system.

Hi all,
discussion is goin on nicely...
I had que. whether this number of photons that our eye can detect is per unit area or on the whole surface of eye...?

agtee said:
Hi all,
discussion is goin on nicely...
I had que. whether this number of photons that our eye can detect is per unit area or on the whole surface of eye...?

Most of these experiments I recall were done in dark adapted volunteers using all of the eye. But in theory, all you need is capture of one photon by one rhodopsin photopigment molecule which sets into motion a biochemical cascade with gain measured in the millions.

## 1. How do photons reach the retina?

Photons reach the retina through the process of light entering the eye and passing through the pupil, lens, and vitreous humor to reach the retina. The retina contains specialized cells called photoreceptors that detect the light and convert it into electrical signals that are then sent to the brain for interpretation.

## 2. How many photons reach the retina per second?

The number of photons that reach the retina per second varies depending on factors such as the intensity and wavelength of the light source. On average, it is estimated that about 10^17 photons reach the retina per second.

## 3. Does the number of photons reaching the retina affect vision?

Yes, the number of photons reaching the retina can affect vision. If there are too few photons, the image will appear dim and may be difficult to see. If there are too many photons, the image may appear too bright and cause discomfort or even damage to the retina.

## 4. Can the number of photons reaching the retina be measured?

Yes, there are techniques and instruments that can measure the number of photons reaching the retina. One common method is using a photometer, which measures the intensity of light and can be used to calculate the number of photons reaching the retina.

## 5. Are all photons that reach the retina detected by photoreceptors?

No, not all photons that reach the retina are detected by photoreceptors. Some photons may be scattered or absorbed by other structures in the eye before reaching the retina. Additionally, some photoreceptors may not be sensitive to certain wavelengths of light, so those photons would not be detected.