Final reports from two NTP Developmental and Reproductive Toxicity (DART) studies are now available (DART-05 and DART-06).

We see the transformation of light everywhere: in store windows, in the glare of the sun on water, and of course in the mirror. But we don't really think about the mechanisms and principles of this phenomenon. But these basics are actively used in different spheres of our life. Let's go a little deeper into what light is, how it refracts, and how it is used in life.

To avoid confusing theory, let's understand the terminology: An incident beam is a stream of light waves hitting the interface between two optical media. Radiation that returned to the initial matter is called reflected radiation. If we construct an imaginary perpendicular to the reflecting surface (normal) at the point of light incidence, the incidence angle will be calculated as the angle between the perpendicular and the incident light flux. The angle of light return, respectively, is the angle between the normal and the reflected light.

The fundamentals of physical knowledge are the most accessible to understand, since we observe their principles with our own eyes every day around us. The same is true of the law of light reflection. This law describes the moment when light waves hit a surface, change their direction and come back only at a different angle. This doesn't just apply to mirrored surfaces. We see any object because it reflects natural sunlight or artificial light. As the rays change their direction, they pass in one medium and collide with another, some of them returning back to the primary medium. If part of the spectrum penetrates into another substance we observe the phenomenon of refraction.

Q: Who nominated 2H4MBP and EHMC; why? A: 2H4MBP and EHMC were nominated for study by the National Cancer Institute due to high prevalence of the chemical in commercial products such as sunscreens and cosmetics, and lack of carcinogenicity data.

Refraction oflight

From these definitions, the reflection coefficient can be derived. This coefficient shows how much of the light flux will return back to the original medium. The return coefficient is primarily influenced by the nature of the rays and the angle of incidence on the surface.

Specularreflection

The phenomena we observe on a daily basis have their own principles and descriptions. We do not always think about why we see our reflection in a body of water, or a distorted portrait in a laughing room. However, these patterns are actively applied in the manufacture of optics. Where else can we observe the effect of the law of light reflection in everyday life, share in the comments and social networks.

Total internalreflection

Q: What is the endocrine disruptor screening panel? A: The Endocrine Disruptor Screening Panel (EDSP) is a series of guideline tests that aim to determine if chemicals and environmental contaminants interact the estrogen, androgen, and thyroid hormone systems.

The law of light reflection describes the pattern of the phenomenon in which a ray passing through one substance, on the surface of contact with another substance, returns back. If the medium is transparent, the spectrum passes through it and we will not see a return.

Q: What are toxicokinetic studies? A: A toxicokinetic study measures the concentration of the chemical in blood as a function of time. Toxicokinetic studies characterize how much of the chemical gets in and how long it stays in the body.

See table below for the most up-to-date information on the variety of projects taking place at NTP. For general descriptions of the various studies, please see the FAQs.

NTP is working to assess the safety of UV filters through a variety of studies in rodent and in vitro models. NTP conducted modified one-generation studies in rats to determine if 2-hydroxy-4-methoxybenzophenone (2H4MBP) and 2-ethylhexyl p-methoxycinnamate (EHMC), common UV filters, would adversely affect the animal’s ability to develop and reproduce. Previously NTP conducted a long-term study on 2-hydroxy-4-methoxybenzophenone (2H4MBP) to test the toxicity and carcinogenicity of chronic exposure to this chemical in rats and mice. NTP conducted endocrine disruptor panel screening studies to determine if EHMC and 2H4MBP (TR-597) activated the endocrine system. Six other chemicals are also being evaluated for endocrine activity in vitro.

Millions of people use sunscreen lotions, creams, and sprays on their skin to prevent sunburn and resulting skin damage. Specific chemicals, known as ultraviolet (UV) filters, are added to sunscreens to absorb or block UV radiation from the sun. UV filters are regularly used in cosmetics for sun protection purposes, and in other products like plastics, toys, or furniture finishes to limit UV degradation. People may be exposed to these chemicals when food comes into contact with plastics that contain UV filters. The UV filter and the body’s breakdown products of the UV filter have been found in urine, indicating that they make their way into the body.

Diffusereflection

Q: In your Modified One-Generation and two-year chronic toxicity and carcinogenicity studies, why did you give 2H4MBP to animals through food instead of applying it to skin? A: Our study aimed to mimic continuous exposure of 2H4MBP in the animals. Rodents eat throughout the day and thus had continual exposure, similar to applying and reapplying sunscreen to skin. This method models the amount of UV filters people could be exposed to via repeated application of sunscreens to the skin or by eating food that has touched UV filter-containing plastics.

Our vision perceives light from its emitter or from objects reflecting light waves. If an object reflects back some of the energy, it itself becomes an object of radiation to our eyes. To describe the laws of geometrical optics, there are two laws: The first law: incident radiation, reflected radiation and normal radiation (conditional perpendicular to the surface) are in the same plane relative to each other. This means that the light beam is flat. The second law: the angle of reflection of the incident beam is equal to the angle of incidence with respect to the normal. That is, first the light beam hits the mirror surface, and at the point of incidence it becomes the source of secondary radiation. This will happen after milliseconds. Based on the Huygens principle, if we consider the incidence and return of the flux in terms of isosceles triangles (∠ABC = ∠DAC). The second law can be represented as an equality: ƒот = ƒп

Reflection, refraction diffraction

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The foundation of theoretical knowledge about the laws of light propagation was laid by the ancient Greek mathematician Euclid and Aristotle. They were the first to try to describe the processes of transformation of solar activity in terms of physics in the 3rd century BC. Further theoretical materials were studied and confirmed experimentally by Newton, Huygens. It was he who first explained the geometrical regularities of optical phenomena in terms of the wave nature of radiation. His proof is based on geometric axioms about isosceles triangles. We will discuss these principles in a little more detail.

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If the surface is perfectly flat and mirror-like, we can observe the process of back reflection. This is the phenomenon where waves completely return after hitting the mirror base to the source of their radiation in a parallel straight line. That is, if you take a mirror and point the light directly perpendicular to it, it will come back exactly back. This phenomenon can be clearly observed by placing two mirrors perpendicular to each other. Whichever way the light is directed, the spectrum will return back parallel to the original emission.

There are two types of rays returning to the substance from whence they fell: specular and diffuse. It depends on the structure of the surface. Diffuse reflection occurs from non-smooth substrates (wood, paper, asphalt). Such materials have many micro-bends, depressions, broken recesses, which have different angles. Therefore, parallel waves of energy, hitting such an object, are reflected at different angles. That is, for each wave, the second law is fulfilled, and in general, the scattering of the flux occurs in different directions. We observe specular reflection from glossy flat substrates (mirror, mercury, tinted glass, polished metal, stone). It is a phenomenon where each wave comes back at the same angle for all rays. Radiation falls on an object in parallel lines and is reflected, also in parallel streams. We recommend watching a video on "Mirror and Diffuse Reflection".

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Q: There are some who say that UV filters are an endocrine disrupting chemical. Did your studies find this to be true? A: In our cell-based studies, 2H4MBP at very high doses added to the culture media interacted with the estrogen and androgen receptors but did not result in the estrogen receptor or androgen mediated biological effects. EHMC did not interact with estrogen or androgen receptors or biological effects. Our studies on these two UV filters did not show endocrine disruption in animal models. In the larger MOG studies, 2H4MBP and EHMC did not induce effects consistent with estrogenic, androgenic, or anti-androgenic activity. Together these data suggest that in cell models at high concentrations, these UV filters can interact with receptors of the endocrine system, however effects were not observed in vivo.

In practice, we can observe these physical laws everywhere. To make it clearer, take a laser flashlight with a thin beam of light. Turn off the light and point it at the mirror at different angles. If you change the direction of illumination, the plane of its return will also change. This effect is used in the optical equipment of modern experimental technique. Concave mirror planes are used to focus the rays at one point. Convex, on the contrary, scatter the spectrum falling on them. This increases the angle of view. The principle of full internal energy spectrum return, is used in the manufacture of fiber-optic cables for high-speed digital data transmission.

Q: How do NTP's studies on UV filters relate to humans? A: Results from the NTP MOG study reports on the toxicity of UV filters, 2H4MBP and EHMC, suggest that they do not show endocrine disruption in animal models. In the 2H4MBP MOG, NTP found uncertain evidence of reproductive toxicity in rats based on a decrease in F2 (offspring) litter size and evidence of developmental toxicity in rats based on postnatal growth retardation. In the EHMC MOG, NTP found no evidence of reproductive toxicity in rats. NTP found uncertain evidence of developmental toxicity in rats based on lower offspring weights, though they partially recovered by study end. When 2H4MBP was given to animals through repeated doses for two years, NTP found uncertain evidence of carcinogenic activity in one of the two rodent species tested. These findings do not suggest that UV filters will cause harm if used by humans in a typical application. It is also important to note that sun exposure without application of a sunscreen that blocks the UV rays can damage skin, and sustained exposure is associated with skin cancer in both animals and humans. The Centers for Disease Control and Prevention (CDC) offers tips to protect yourself from UV radiation.

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Q: What is a modified one-generation study? A: The modified one-generation study, or MOG study, measures developmental and reproductive toxicity parameters, and enables the setting of appropriate dose levels for a cancer bioassay through evaluation of target organ toxicity based on chemical exposure that starts during gestation. This gives scientists the power to detect adverse effects where there is a prenatal exposure, but evaluations occur postnatally.

Q: What’s next for NTP on UV filter research? A: We are working on a report summarizing cell-based and short-term animal studies conducted on six additional UV filters: avobenzone, ensulizole, homosalate, octocrylene, octylsalicylate, and padimate-O.

That is, all the energy will be directed to the reflection of light, and there will be no refracted light at all. This phenomenon is called the phenomenon of total reflection of light.

Q: What is an Absorption, Distribution, Metabolism, & Excretion (ADME) study? A: This type of study describes in more detail what happens to a chemical in the body in four different stages: