[Lewis Thomson (OP)]

Robert - @Bikerbob  has been reflecting on this question.

Do humans actually see light or is it only reflected light?

Light from the Sun emits in all directions and all frequencies - white light. Rayleigh theory scattering light bouncing off the molecules in the air is suggested as to why the sky appears to be blue = reflected light.

Sending a beam of light from a torch down a cardboard tube in a totally black room reflects on the opposite wall. The beam of light cannot be seen in the space between the tube end and the opposing wall.

Am I correct in thinking we can only see light that has collided with molecules? Albeit gas molecules in the air, virtually invisible dust in the air, through to everything we see around us?

What do you think? Discuss in the comments below...

[chiralSPO]

Most of the photons we see have interacted with at least one molecule, but that is by no means a requirement for being visible.

I don't recommend doing this, but if you were to look directly into the torch (or a laser!) at close range, some of those photons would reach your eye without interacting with any thing in the air first, and you would see them.

The key to being able to see the light, is that is has to actually go into your eye and get absorbed by one of the light-detecting molecules there.

[Bikerbob]

Hello Chirel SPO,

Thanks for your response. The word 'see' is ambiguous as such.

If the eyelid is closed the retina still reacts to light. As your comment indicates, an eye directly in-line with a light source will react albeit not actually 'seeing' anything. Evidently some blind people still have the ability to register whether it is dark or light.

The electromagnetic radiation has collided with the skin membrane of the eyelid. Could it be said that light is not visible - only when the radiation has collided does it become visible? The collision with matter in that case would be reflectance.

The eyelid is matter therefore the rods and cones of the retina have registered the molecular movement within the skin.

In the example you have given the light source; laser or any heated matter is visible. However could it be said that the electromagnetic radiation discharged as with other sources of radiation, is not visible?

Also: Would it be possible for my original question to be attributed to my name please? At present it has been shown as a question posed by Lewis Thomson.

Thanks you

[Bored chemist]

If you look at a candle flame or a computer screen, you see light that has not been reflected.

[Bikerbob]

Hello Bored Chemist,

Thanks for your response. Would it be considered that the candle flame or computer screen as being the light source though - therefore visible? The radiation that they emit is the 'light energy' and is not visible.

In a similar way to looking down into a long dark tunnel. Nothing to be seen as there is no light. Stand in the tunnel and look outwards and the light (if it is daytime) is visible as it is the source of reflected light.

Thank you

[Origin]

Would it be considered that the candle flame or computer screen as being the light source though - therefore visible?

Yes.

The radiation that they emit is the 'light energy' and is not visible.

That is clearly wrong, since we in fact see the candle.

[Bikerbob]

Hello Origin,

If one has closed eyes there can be no inverted image going through the lens. No vision.

However, if the person is in a totally dark room with their eyes closed, the candle is then lit, the contrast between the darkness and the luminance of the candle flame would be detected. Still no vision though. The person cannot see the candle but can detect there is an origin of light. 

[Colin2B]

Also: Would it be possible for my original question to be attributed to my name please? At present it has been shown as a question posed by Lewis Thomson.

Thanks you

Welcome Bob, glad you could register and join the discussion.
Unfortunately we can’t change the original posting, but the fact that you are here helps people to know who did the posting. I will, however, add a note to the opening post to say you’ve joined us.

I agree that the term see does not have a consistent definition, but the Collins Dictionary says “to get knowledge or an awareness of through the eyes; perceive visually”. That would cover the detection of light passing through the eyelids and being detected by the retina, so no image formed, but definitely visible light.

[Bored chemist]

The radiation that they emit is the 'light energy' and is not visible.

You see it.
So it is visible.
There's really no more to say about that.

[chiralSPO]

Looks like we need to agree on some definitions here!

The cones and rods in our eyes can detect light by absorbing photons. One molecule of rhodopsin absorbs one photon of light, and transmits that signal onward. The history of the photon is irrelevant to its detection (it could have been traveling for 900 million years from another star and not interacted with a single molecule before landing in the eye, or it could have been scattered 20 billion times off of smoke particles since being emitted from a laser 20 meters away)—all that matters is that the photon has the right frequency to be absorbed by the rhodopsin, and that it passes close enough to the rhodopsin that it does (we shall assume that 100% of photons with the right frequency and in the right place will get absorbed, but this is not quite true).

This means we can only visually detect light that goes into our eyes and gets absorbed. (we can also detect light with other instruments, but these also work in roughly the same way, by absorbing the light).

We can see images if enough of our rods an cones are able to detect light emitted from and/or reflected off of our surroundings such that the signals from our rods and cones can be interpreted together in an organized way (the lenses of our eyes focus incoming light onto the corneas such that these images are from organized incoming light.

We can see images of objects if light comes off of the object (again, emitted and/or reflected), and then goes into our eyes, where it is focused by the lenses, absorbed by the rods/cones, and then interpreted by our brains.

Are these definitions acceptable to all?

[Bored chemist]

Hello Origin,

If one has closed eyes there can be no inverted image going through the lens. No vision.

However, if the person is in a totally dark room with their eyes closed, the candle is then lit, the contrast between the darkness and the luminance of the candle flame would be detected. Still no vision though. The person cannot see the candle but can detect there is an origin of light. 

What you see in that case is a very out of focus image of your eyelids.
How was that going to be relevant?

[Origin]

Hello Origin,

If one has closed eyes there can be no inverted image going through the lens. No vision.

However, if the person is in a totally dark room with their eyes closed, the candle is then lit, the contrast between the darkness and the luminance of the candle flame would be detected. Still no vision though. The person cannot see the candle but can detect there is an origin of light.

And then that person could open their eyes and see the light coming directly from the candle flame.  Not sure why you think something so blatantly obvious is not possible...

[Bikerbob]

Hello folks,

The post from ChiralSPO re: definitions is very true. My apologies - have been using the words 'vision' and 'sight' in error. Yes photons are colliding with the eye lids when closed. If photons are purely electromagnetic they could be considered as only energy. If that is the case they cannot be seen by the human eye. As ChiralSPO has stated, it is only when the photon has arrived at the rod or cone does rhodopsin then sends the signal to the brain.

The point of issue is unless the photon has collided with the cone or rod no signal will have been sent to the brain.

The original concept question: 'do we see light or only when the photon has collided with molecules' has been answered. We can only see light if the photons have collided with molecules - reflected photons, and photons that have collided with matter - e.g. within the retina.

Reading information provided from troops that were at the atom bomb testing - they were instructed to face away from the explosion and place their hands over their eyes. There are reports that the intensity of light enabled the bones of the hands to be seen. Their eyes were shut. The suggested conclusion is the intensity of the photons must have been sufficient to make the flesh transluscent. The photons could therefore pass through the flesh, albeit with reduced energy, and agitate the cones and rods to produce the rhodopsin.

The penultimate paragraph of your posting sums it up: -   'We can see images of objects if light comes off of the object (again, emitted and/or reflected), and then goes into our eyes, where it is focused by the lenses, absorbed by the rods/cones, and then interpreted by our brains.'

Thank you -

[Halc]

If photons are purely electromagnetic they could be considered as only energy. If that is the case they cannot be seen by the human eye.

This does not follow. I don't know how you distinguish energy from 'pure' energy, but an eye is very much a device that directly detects electromagnetic energy. They thus detect (are sensitive to) light and not something else.

As to whether an eye 'sees' or not depends on the definition of 'see'. An eye is light sensitive similar to say a leaf, but in contrast to the leaf, the eye is attached to an information processor to complete the act of 'seeing'.

As ChiralSPO has stated, it is only when the photon has arrived at the rod or cone does rhodopsin then sends the signal to the brain.

Cones have photopsins. The rhodopsin (a protein) doesn't directly send signals to the brain. There are more steps involved.

The point of issue is unless the photon has collided with the cone or rod no signal will have been sent to the brain.

OK, with that we agree. This 'point' seems to have little correlation with what was written in prior posts. We're trying to figure out what you're trying to get at.

The original concept question: 'do we see light or only when the photon has collided with molecules' has been answered. We can only see light if the photons have collided with molecules - reflected photons, and photons that have collided with matter - e.g. within the retina.

Retina molecules are matter, so yes, a collision (not a reflection) with those protein molecules seems necessary. Light is necessarily refracted in the human eye before getting that far, but that's not reflection.
Some of you prior posts seem to focus on other molecules like a beam of light being visible only when illuminating something that intercepts it like dust in the air. Yes, light not moving directly towards your eye isn't going to be seen unless deflected like that, which is why we can't see the light from the sun heading out to deep space any more than I can feel a snowball thrown at somebody else.

Reading information provided from troops that were at the atom bomb testing - they were instructed to face away from the explosion and place their hands over their eyes. There are reports that the intensity of light enabled the bones of the hands to be seen.

I can do that with a moderately powerful torch. Yes, flesh (eyelids in particular) is hardly completely opaque to light.

Their eyes were shut. The suggested conclusion is the intensity of the photons must have been sufficient to make the flesh transluscent.

It always was transluscent. Hands block most light, but never all of it. The bright flash from the bomb didn't change that particular property of flesh.

The photons could therefore pass through the flesh, albeit with reduced energy, and agitate the cones and rods to produce the rhodopsin.

Rhodopsin is a protein in rods that is sensitive to light, not a product that is created by light agitating the photoreceptor cells.

The penultimate paragraph of your posting sums it up: -   'We can see images of objects if light comes off of the object (again, emitted and/or reflected), and then goes into our eyes, where it is focused by the lenses, absorbed by the rods/cones, and then interpreted by our brains.'

Yes! Quite simple.

A computer with a camera does the exact same thing with different chemistry. A robot say can navigate a cluttered room without bumping into obstacles, and yet there are those why say it is merely 'processing the image data' and not actually 'seeing'.

Regards, and glad you signed up to continue the discussion. So many questions get asked of the site via email like that and without discourse, it is often impossible to glean exactly what is being asked.

[Bikerbob]

Hello Halc

Thank you for your posting. Following on with your response I have tried to ‘copy paste’ into the ‘reply’ section. Evidently the font reverts back to the native of the website. Therefore I have tried to highlight using a double plus sign.


Quote from: Bikerbob on Today at 09:32:26
If photons are purely electromagnetic they could be considered as only energy. If that is the case they cannot be seen by the human eye.
This does not follow. I don't know how you distinguish energy from 'pure' energy, but an eye is very much a device that directly detects electromagnetic energy. They thus detect (are sensitive to) light and not something else.

++ My understanding is that energy can be defined as shown ‘work done’. Heat energy – increase in temperature; Electrical energy – power to rotate the armature of an electric motor; Chemical energy – breaking of bonds and forming of bonds, for example combustion.
‘Pure energy’ in contrast could be said to be a ‘standalone’ energy. I have read that plasma is thought to be ‘pure energy’. It could be argued that light being electromagnetic can only be ‘pure energy’. There are others who argue that light cannot be pure energy. Who is correct?

As to whether an eye 'sees' or not depends on the definition of 'see'. An eye is light sensitive similar to say a leaf, but in contrast to the leaf, the eye is attached to an information processor to complete the act of 'seeing'.

++ Fully agree – the eye has a receptor that reacts to the intensity of the photons. Evidently the cones and rod ends react to the photons when they collide with the surface. The eye doesn’t ‘see’ anything. It is the brain that interprets the contrast of photon intensity via the optic nerve. The cones are responsive to frequency therefore register colour, whilst the rods provide peripheral information and sensitive to low bombardment in lower levels of light – so I have read.

Quote
As ChiralSPO has stated, it is only when the photon has arrived at the rod or cone does rhodopsin then sends the signal to the brain.
Cones have photopsins. The rhodopsin (a protein) doesn't directly send signals to the brain. There are more steps involved.

++ Evidently photopsins are also proteins. Yes, the optic nerve transports the reactions to the brain, not the protein.
 

Quote
The point of issue is unless the photon has collided with the cone or rod no signal will have been sent to the brain.
OK, with that we agree. This 'point' seems to have little correlation with what was written in prior posts. We're trying to figure out what you're trying to get at.

++ Basically, photons cannot be ‘seen’. They can only produce an increase of activity of the electrons in matter. The term ‘Visible Spectrum’ is mis-leading.

The cones of the human eye can sense reflected electromagnetic radiation and determine shades of colour. (Some cannot – colour blind). The rod receptors cannot determine frequency therefore register contrast only.
The human eye compares intensity of the photons and contrast. A candle flame in a completely dark room will appear bright. In the same room in bright daylight the flame will be the same light intensity but minimal contrast. 

Quote
The original concept question: 'do we see light or only when the photon has collided with molecules' has been answered. We can only see light if the photons have collided with molecules - reflected photons, and photons that have collided with matter - e.g. within the retina.
Retina molecules are matter, so yes, a collision (not a reflection) with those protein molecules seems necessary. Light is necessarily refracted in the human eye before getting that far, but that's not reflection.
Some of you prior posts seem to focus on other molecules like a beam of light being visible only when illuminating something that intercepts it like dust in the air. Yes, light not moving directly towards your eye isn't going to be seen unless deflected like that, which is why we can't see the light from the sun heading out to deep space any more than I can feel a snowball thrown at somebody else.

++ That is exactly the point I was trying to have confirmed. Light cannot be ‘seen’ – it is not ‘visible’ unless it directly hits the retina, or has collided with matter, therefore reflected.


++ A very genuine ‘Thank You’ for taking the trouble of both reading, and providing useful comment. It’s very much appreciated. Would you have any objections to me messaging you? I can explain why I posted the question if you wanted to know.

[Origin]

‘Pure energy’ in contrast could be said to be a ‘standalone’ energy. I have read that plasma is thought to be ‘pure energy’. It could be argued that light being electromagnetic can only be ‘pure energy’. There are others who argue that light cannot be pure energy. Who is correct?

Neither plasma nor photons are pure energy.  Energy is not something that can be contained in a bucket so to speak.  Energy is a property.  Plasma is a phase of matter where the atoms have so much energy that the electron are stripped away.  The plasma is made up of these atomic nuclei and free electrons so it is clearly not pure energy.
Photons are 'particles' (in the excitation of a quantum field sense).  Photons carry energy, but are not 'pure energy'.  Photons have spin and momentum, which are properties of particles not energy.

[Bikerbob]

Hello folks,
Thanks for your interest - much appreciated.
You are correct in stating I have a muddled mind. It has taken 7 decades to perfect it.

It appears that I have not set my questions or comments in a clear and succinct manner. My question: Can the human eye actually see light? The background - photons are just electromagnetic waves therefore not possible to see. It is only when the photons collides with molecules albeit gas, liquid or solid and bounce off (reflected) can the human eye see photons with a specific frequency range.
If photons arrive directly from their origin and onto an eyelid or surface of the eye can they be detected by the cones and rod receptors in the retina. The optic nerve transmit the information into the brain. The cone receptors require high levels of light intensity to enable the colour frequency received. The rods are able to register much lower levels of light. However they cannot determine frequency differences, therefore not able to register colour. There are two main proteins in the retina.

I have not stated ‘the eye cannot see a candle’. Please re-read the words I actually posted.

I actually stated that rods cannot register frequency, only contrast. Contrast is intensity of the photon bombardment. More intensity registers lighter, less intensity = darker = contrast.

It is not my theory that plasma could be said to be pure energy. I have read several website written by people who I presume know about their subject. One such website about astronomy discussed the enormous amount of natural plasma in the universe. He stated it was pure natural energy. Not my words.

 I hope that has clarified my question with the background to my understanding.

[Origin]

My question: Can the human eye actually see light?  The background - photons are just electromagnetic waves therefore not possible to see.

Photons can be seen when they enter your eye and only when they enter your eye.

It is only when the photons collides with molecules albeit gas, liquid or solid and bounce off (reflected) can the human eye see photons with a specific frequency range.

Incorrect, the light does not need to be reflected, you can see light directly from the light source.  A light source of a specific frequency can be seen directly.

If photons arrive directly from their origin and onto an eyelid or surface of the eye can they be detected by the cones and rod receptors in the retina.

Then why did you just say the light needs to be reflected?

It is not my theory that plasma could be said to be pure energy. I have read several website written by people who I presume know about their subject. One such website about astronomy discussed the enormous amount of natural plasma in the universe. He stated it was pure natural energy. Not my words.

The person or persons that said that are wrong.  Wiki is generally a pretty good source for information.  The best sources are university websites that generally end with a '.edu'.  Youtube has some very good video courses from different universities.  If it is not from a university and just some guy, then it may or may not be accurate

[Colin2B]

Hi Bob
Some of what I say will overlap with others, but it’s important to stress certain items.

My question: Can the human eye actually see light? The background - photons are just electromagnetic waves therefore not possible to see.

Saying that photons are just electromagnetic waves misses the point. To simplify, photons (also called quanta) are only a measure of a certain amount of energy in the wave. It is the electromagnetic wave that does the interacting with the chemicals in the rods & cones. It is a mistake to think of photons as little bullets, they are more like brief bursts of electromagnetic waves, in this case visible light.

It is only when the photons collides with molecules albeit gas, liquid or solid and bounce off (reflected) can the human eye see photons with a specific frequency range.

As has been pointed out, the eye can detect light from either a direct source eg candle, or reflected light eg from a wall.

If photons arrive directly from their origin and onto an eyelid or surface of the eye can they be detected by the cones and rod receptors in the retina.

I am going to modify this to say “If photons arrive at the retina they be detected by the cones and rod receptors in the retina”.
It doesn’t matter how they get there. How the light passes through the eyelid is a little complicated, but is irrelevant to this discussion.

Contrast is intensity of the photon bombardment. More intensity registers lighter, less intensity = darker = contrast.

No, contrast is not intensity. Contrast is the difference in intensity between different areas.
If we have 2 areas, one bright white the other black we say there is a high contrast. If the 2 areas are light grey and dark grey, we say there is low contrast.

It is not my theory that plasma could be said to be pure energy. I have read several website written by people who I presume know about their subject.

As has been said, this is wrong, it is sometimes said about light. There is no such thing as pure energy.