How does one understand the elements of the Pure Octad in Sunlight?

Scientist in modern day believes "Energy and particles". (weakons = the weak nuclear force, gluon= the strong nuclear force, photon = the electromagnetic force, graviton = the force of gravity.)

In early days they believes "4 Maha Buta"- Pure elements (are abstractions) and "The Rupa Kalapa" is the smallest reality consists "Suddashtaka"(Pure Octed) as attributes.

According to Abhidhamma

The material elements never occur in isolation but in groups or clusters called kalaapas. A kalaapa can contain from eight to thirteen material elements. There is no cluster of matter without at least eight elements, the four primary elements and four secondary elements — namely color, taste, smell, and nutriment. A unit containing only these is called a Pure Octad.

Material phenomena arise through four causes: kamma, consciousness(chita), heat(Utu), and nutriment(Ahara).

There has long been — and still is — a common tendency to create a "Buddhist" metaphysics in which the experience of emptiness, the Unconditioned, the Dharma-body, Buddha-nature, rigpa, etc.

The Buddha taught that clinging to views is one of the four forms of clinging that tie the mind to the processes of suffering. He thus recommended that his followers relinquish their clinging, not only to views in their full-blown form as specific positions, but also in their rudimentary form as the categories & relationships that the mind reads into experience. This is a point he makes in the following discourse Mulapariyaya Sutta: The Root Sequence,

The Blessed One said: "There is the case, monks, where an uninstructed run-of-the-mill person — who has no regard for noble ones, is not well-versed or disciplined in their Dhamma; who has no regard for men of integrity, is not well-versed or disciplined in their Dhamma — perceives earth as earth. Perceiving earth as earth, he conceives [things] about earth, he conceives [things] in earth, he conceives [things] coming out of earth, he conceives earth as 'mine,' he delights in earth. Why is that? Because he has not comprehended it, I tell you.

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From a Vipassana point of view, is this problem occurring because the mind is getting stuck at the concept of Sunlight?

Practical Vipassanā Meditation Exercises -Mahāsī Sayādaw -

The practice of Vipassanā or Insight Meditation is the effort made by the meditator to understand correctly the nature of the psycho-physical phenomena taking place in his own body. Physical phenomena are the things or objects which one clearly perceives around one. The whole of one’s body that one clearly perceives constitutes a group of material qualities (rūpa). Psychical or mental phenomena are acts of consciousness or awareness (nāma). These (nāma-rūpa) are clearly perceived to be happening whenever they are seen, heard, smelt, tasted, touched, or thought of. We must make ourselves aware of them by observing them and noting thus: ‘Seeing, seeing’, ‘hearing, hearing,’ ‘smelling smelling,’ ‘tasting, tasting,’ ‘touching, touching,’ or thinking, thinking.’

If you'll forgive an uneducated guess of mine...

Imagine you're in a forest, after a rain storm: and then the sun comes out and shines. When that happens, then the air you breathe, the forest warmed by the sunlight, would be heavy with moisture.

Perhaps that's how there are other elements (for example moisture) in sunlight: in the real world, the elements are mixed together.

In the mental world (of ideas) you separate/isolate components/elements, you say, "this is sunlight, whereas that is moisture". But that ideal view (I'm using the word "ideal" as an antonym for "real") would be like asking, "where is the aapo in the tejo?"

Similarly in the real world, even if you're in a desert, there's some moisture, although only an infinitesimal amount.

The Octad for sunlight:
1. Pathavi/earth: solidity is present in photons existing in particle form (the other is wave form)
2. Aapo/water: adhesion is present since light emitts "packets" of photons
3. Tejo/fire: in the form of heat
4. Vaayo/wind: motion is present in photons' traveling speed
5. Vanna/color: present in the light spectrum
6. Gandha/smell: a little tricky but we'll get there fairly soon according to this link
7. Rasa/taste: similar to 6.
8. Oja/nutriment: without sunlight, we wouldn't have sat here chatting and earth would've been a dead planet

Buddhism focuses on spiritual development, so the Buddhist analysis of rūpa focuses on sense objects, the sense organs and the body. Non-living things are made up of temperature-born pure-octad groups or temperature-born sound groups. The relevance of non-living things in the Buddhist analysis of rūpa is that non-living things are sense objects.

I would say that sunlight itself does not impact the sense directly, only indirectly. When sunlight bounces off a speck of dust or a wall, then we “perceive” the sunlight through the eye sense. When sunlight makes our skin warm or a rock warm, we “perceive” sunlight through the tactile sense.

The speck of dust, the wall, the skin and the rock… all of these are “non-living things” and are temperature-born pure-octad groups (the eight rūpa that you listed above). They are all to be understood as sense objects.

Using fire-element (tejo) as one example, some sense objects have greater intensity of the fire-element and will be “warmer” to the tactile sense than other sense objects with less intensity of the fire-element, but all sense objects include the fire-element rūpa. The other seven rūpa in the temperature-born pure-octad group can be seen in a similar way... always present, but with different intensities.

According to Vasubandhu's Abhidharma-kosa:

Visible matter (rupa) is twofold.

1. Visible matter is color and shape. Color is fourfold: blue, red, yellow, white; other colors proceed from out of these four colors. Shape is eightfold: long, short, square, round, high, low, even, uneven.

Or twentyfold.

2. Or there are some twenty types: the four primary colors, the eight shapes, and eight more colors: cloud, smoke, dust, mist, shade, heat, light, darkness. Some make a color of the firmament [which appears like a wall of lapiz-lazuli]; this would give us the number twenty-one.

   "Even" signifies "of even shape;" "uneven" is the opposite; mist is the vapor which rises from the ground and from water; "heat" is the light of the sun; "light" is the light of the moon, the stars, fire, grasses and gems; "shade"—arisen from an obstacle to light—is where forms still remain visible; "darkness" is the opposite. The other terms do not call for any explanation.

3. Visible matter (rupa) can be color without being shape: blue, red, yellow, white, shade, heat, light, darkness.

   There can be shapes without there being color: that part of the long, of the short, etc., which constitutes bodily action.

   There can be color and shape at one and the same time: all the other categories of visible matter.

   Some other Masters maintain that only heat and light are exclusively color; for blue, red, etc., present themselves to the sight under the aspect of long, short, etc.

From this you can see that according to Abhidharma of Vaibhasika school which is what Mahayana follows, sunlight can be pure color without shape. As Jayarava used to be fond of repeating (and as Thanissaro Bhikkhu is equally fond of hammering in), the perspective of Buddhism is that of phenomenology and not of metaphysics. From phenomenological standpoint there is absolutely no problem with having a perception of color without a perception of shape (nor taste, smell, nutriment, solidity, adhesion, and motion).