Total internal reflection. Next lesson.
Current timeTotal duration Video transcript We know from the last few videos we have light exiting a slow medium. Let's say I have light ray exiting a slow medium there Let me draw. This is its incident angle right over there Though it's not the true mechanics of light, you can imagine a car was coming from a slow medium to a fast medium; it was going from the mud to the road If the car was moving in the direction of this ray, the left tires would get out of the mud before the right tires and they are going to be able to travel faster So this will move the direction of the car to the right So the car will travel in this direction, like that where this angle right over here is the angle of refraction This is a slower medium than that.
This is the fast minimum over here We get theta 2 is going to be greater than theta 1 What I want to figure out in this video is is there some angle depending on the two substances that the light travels in where if this angle is big enough--because we know that this angle is always is always larger than this angle that the refraction angle is always bigger than the incident angle moving from a slow to a fast medium Is there some angle--if I approach it right over here Let's call this angle theta 3 Is there some angle theta 3 where that is large enough that the reflected angle is going to be 90 degrees if that light is actually never going to escape into the fast medium?
And if I had a incident angle larger than theta 3, like that So whatever that is, the light won't actually even travel along the surface it definitely won't escape. It won't even travel on surface. It will actually reflect back So you actually have something called total internal reflection To figure that out, we need to figure out at what angle theta three do we have of a fraction angle of 90 degrees? That incident angle is going to be called our critical angle Anything larger than that will actually have no refraction It's actually not going to escape the slow medium It's just going to reflect at the boundary back into the slow medium Let's try to figure that out and I'll do it with an actual example So let's say I have water.
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This is water It has an index of refraction of 1. I'll call it theta critical and so if I have any incident angle less than this critical angle, I'll escape At that critical angle, I just kind of travel at the surface Anything larger than that critical angle, I'll actually have total internal reflection Let's think about what this theta, this critical angle could be So I'll break out Snell's Law again We have the index of refraction of the water 1. To figure that out, you need to think about the unit circle You can't just do the soh-cah-toa This is why the unit circle definition is useful Think of the unit circle You go 90 degrees.
Moreover, metaphysical thought prioritises presence and purity at the expense of the contingent and the complicated, which are considered to be merely aberrations that are not important for philosophical analysis. Basically then, metaphysical thought always privileges one side of an opposition, and ignores or marginalises the alternative term of that opposition.
Deconstruction cannot limit itself or proceed immediately to neutralisation: it must, by means of a double gesture, a double science, a double writing, practise an overturning of the classical opposition, and a general displacement of the system. It is on that condition alone that deconstruction will provide the means of intervening in the field of oppositions it criticises" M Derrida's terms change in every text that he writes. This is part of his deconstructive strategy. He focuses on particular themes or words in a text, which on account of their ambiguity undermine the more explicit intention of that text.
It is not possible for all of these to be addressed Derrida has published in the vicinity of 60 texts in English , so this article focused on some of the most pivotal terms and neologisms from his early thought. The most prominent opposition with which Derrida's earlier work is concerned is that between speech and writing.
According to Derrida, thinkers as different as Plato, Rousseau, Saussure, and Levi-Strauss, have all denigrated the written word and valorised speech, by contrast, as some type of pure conduit of meaning. Their argument is that while spoken words are the symbols of mental experience, written words are the symbols of that already existing symbol.
As representations of speech, they are doubly derivative and doubly far from a unity with one's own thought.
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Without going into detail regarding the ways in which these thinkers have set about justifying this type of hierarchical opposition, it is important to remember that the first strategy of deconstruction is to reverse existing oppositions. In Of Grammatology perhaps his most famous work , Derrida hence attempts to illustrate that the structure of writing and grammatology are more important and even 'older' than the supposedly pure structure of presence-to-self that is characterised as typical of speech. For example, in an entire chapter of his Course in General Linguistics , Ferdinand de Saussure tries to restrict the science of linguistics to the phonetic and audible word only In the course of his inquiry, Saussure goes as far as to argue that "language and writing are two distinct systems of signs: the second exists for the sole purpose of representing the first".
Language, Saussure insists, has an oral tradition that is independent of writing, and it is this independence that makes a pure science of speech possible. Derrida vehemently disagrees with this hierarchy and instead argues that all that can be claimed of writing - eg.
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But as well as criticising such a position for certain unjustifiable presuppositions, including the idea that we are self-identical with ourselves in 'hearing' ourselves think, Derrida also makes explicit the manner in which such a hierarchy is rendered untenable from within Saussure's own text. Most famously, Saussure is the proponent of the thesis that is commonly referred to as "the arbitrariness of the sign", and this asserts, to simplify matters considerably, that the signifier bears no necessary relationship to that which is signified.
Saussure derives numerous consequences from this position, but as Derrida points out, this notion of arbitrariness and of "unmotivated institutions" of signs, would seem to deny the possibility of any natural attachment OG After all, if the sign is arbitrary and eschews any foundational reference to reality, it would seem that a certain type of sign ie.
However, it is precisely this idea of a natural attachment that Saussure relies upon to argue for our "natural bond" with sound 25 , and his suggestion that sounds are more intimately related to our thoughts than the written word hence runs counter to his fundamental principle regarding the arbitrariness of the sign. In Of Grammatology and elsewhere, Derrida argues that signification, broadly conceived, always refers to other signs, and that one can never reach a sign that refers only to itself.
He suggests that "writing is not a sign of a sign, except if one says it of all signs, which would be more profoundly true" OG 43 , and this process of infinite referral, of never arriving at meaning itself, is the notion of 'writing' that he wants to emphasise. This is not writing narrowly conceived, as in a literal inscription upon a page, but what he terms 'arche-writing'. Arche-writing refers to a more generalised notion of writing that insists that the breach that the written introduces between what is intended to be conveyed and what is actually conveyed, is typical of an originary breach that afflicts everything one might wish to keep sacrosanct, including the notion of self-presence.
This originary breach that arche-writing refers to can be separated out to reveal two claims regarding spatial differing and temporal deferring. To explicate the first of these claims, Derrida's emphasis upon how writing differs from itself is simply to suggest that writing, and by extension all repetition, is split differed by the absence that makes it necessary.
One example of this might be that we write something down because we may soon forget it, or to communicate something to someone who is not with us. According to Derrida, all writing, in order to be what it is, must be able to function in the absence of every empirically determined addressee M Derrida also considers deferral to be typical of the written and this is to reinforce that the meaning of a certain text is never present, never entirely captured by a critic's attempt to pin it down. The meaning of a text is constantly subject to the whims of the future, but when that so-called future is itself 'present' if we try and circumscribe the future by reference to a specific date or event its meaning is equally not realised, but subject to yet another future that can also never be present.
The key to a text is never even present to the author themselves, for the written always defers its meaning. As a consequence we cannot simply ask Derrida to explain exactly what he meant by propounding that enigmatic sentiment that has been translated as "there is nothing outside of the text" OG Any explanatory words that Derrida may offer would themselves require further explanation. So, Derrida's more generalised notion of writing, arche-writing, refers to the way in which the written is possible only on account of this 'originary' deferral of meaning that ensures that meaning can never be definitively present.
This problematises efforts like Saussure's, which as well as attempting to keep speech and writing apart, also suggest that writing is an almost unnecessary addition to speech. If the spoken word requires the written to function properly, then the spoken is itself always at a distance from any supposed clarity of consciousness.
The widespread conviction that the sign literally represents something, which even if not actually present, could be potentially present, is rendered impossible by arche-writing, which insists that signs always refer to yet more signs ad infinitum , and that there is no ultimate referent or foundation.
This reversal of the subordinated term of an opposition accomplishes the first of deconstruction's dual strategic intents. Rather than being criticised for being derivative or secondary, for Derrida, writing, or at least the processes that characterise writing ie. Just as a piece of writing has no self-present subject to explain what every particular word means and this ensures that what is written must partly elude any individual's attempt to control it , this is equally typical of the spoken.
Utilising the same structure of repetition, nothing guarantees that another person will endow the words I use with the particular meaning that I attribute to them. Even the conception of an internal monologue and the idea that we can intimately 'hear' our own thoughts in a non-contingent way is misguided, as it ignores the way that arche-writing privileges difference and a non-coincidence with oneself SP In this respect, it needs to be pointed out that all of deconstruction's reversals arche-writing included are partly captured by the edifice that they seek to overthrow.
For Derrida, "one always inhabits, and all the more when one does not suspect it" OG 24 , and it is important to recognise that the mere reversal of an existing metaphysical opposition might not also challenge the governing framework and presuppositions that are attempting to be reversed WD Deconstruction hence cannot rest content with merely prioritising writing over speech, but must also accomplish the second major aspect of deconstruction's dual strategies, that being to corrupt and contaminate the opposition itself.
Derrida must highlight that the categories that sustain and safeguard any dualism are always already disrupted and displaced.
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To effect this second aspect of deconstruction's strategic intents, Derrida usually coins a new term, or reworks an old one, to permanently disrupt the structure into which he has intervened - examples of this include his discussion of the pharmakon in Plato drug or tincture, salutary or maleficent , and the supplement in Rousseau, which will be considered towards the end of this section.
To phrase the problem in slightly different terms, Derrida's argument is that in examining a binary opposition, deconstruction manages to expose a trace.
This is not a trace of the oppositions that have since been deconstructed - on the contrary, the trace is a rupture within metaphysics, a pattern of incongruities where the metaphysical rubs up against the non-metaphysical, that it is deconstruction's job to juxtapose as best as it can. The trace does not appear as such OG 65 , but the logic of its path in a text can be mimed by a deconstructive intervention and hence brought to the fore.
The logic of the supplement is also an important aspect of Of Grammatology.
Writing is itself an example of this structure, for as Derrida points out, "if supplementarity is a necessarily indefinite process, writing is the supplement par excellence since it proposes itself as the supplement of the supplement, sign of a sign, taking the place of a speech already significant" OG Another example of the supplement might be masturbation, as Derrida suggests OG , or even the use of birth control precautions. What is notable about both of these examples is an ambiguity that ensures that what is supplementary can always be interpreted in two ways.
For example, our society's use of birth control precautions might be interpreted as suggesting that our natural way is lacking and that the contraceptive pill, or condom, etc. On the other hand, it might also be argued that such precautions merely add on to, and enrich our natural way. It is always ambiguous, or more accurately 'undecidable', whether the supplement adds itself and "is a plenitude enriching another plenitude, the fullest measure of presence", or whether "the supplement supplements… adds only to replace… represents and makes an image… its place is assigned in the structure by the mark of an emptiness" OG Ultimately, Derrida suggests that the supplement is both of these things, accretion and substitution OG , which means that the supplement is "not a signified more than a signifier, a representer than a presence, a writing than a speech" OG It comes before all such modalities.
This is not just some rhetorical suggestion that has no concrete significance in deconstruction. Indeed, while Rousseau consistently laments the frequency of his masturbation in his book, The Confessions , Derrida argues that "it has never been possible to desire the presence 'in person', before this play of substitution and the symbolic experience of auto-affection" OG By this, Derrida means that this supplementary masturbation that 'plays' between presence and absence eg.
In a sense, masturbation is 'originary', and according to Derrida, this situation applies to all sexual relations. All erotic relations have their own supplementary aspect in which we are never present to some ephemeral 'meaning' of sexual relations, but always involved in some form of representation. Even if this does not literally take the form of imagining another in the place of, or supplementing the 'presence' that is currently with us, and even if we are not always acting out a certain role, or faking certain pleasures, for Derrida, such representations and images are the very conditions of desire and of enjoyment OG Despite this complexity, two main aspects of Derrida's thinking regarding phenomenology remain clear.
Firstly, he thinks that the phenomenological emphasis upon the immediacy of experience is the new transcendental illusion, and secondly, he argues that despite its best intents, phenomenology cannot be anything other than a metaphysics SP 75, In this context, Derrida defines metaphysics as the science of presence, as for him as for Heidegger , all metaphysics privileges presence, or that which is. While they are presented schematically here, these inter-related claims constitute Derrida's major arguments against phenomenology.
According to Derrida, phenomenology is a metaphysics of presence because it unwittingly relies upon the notion of an indivisible self-presence, or in the case of Husserl, the possibility of an exact internal adequation with oneself SP In various texts, Derrida contests this valorisation of an undivided subjectivity, as well as the primacy that such a position accords to the 'now', or to some other kind of temporal immediacy. For instance, in Speech and Phenomena , Derrida argues that if a 'now' moment is conceived of as exhausting itself in that experience, it could not actually be experienced, for there would be nothing to juxtapose itself against in order to illuminate that very 'now'.
Phenomenology is hence envisaged as nostalgically seeking the impossible: that is, coinciding with oneself in an immediate and pre-reflective spontaneity.
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Following this refutation of Husserlian temporality, Derrida remarks that "in the last analysis, what is at stake is Instead of emphasising the presence of a subject to themselves ie. Every time you try to stabilise the meaning of a thing, try to fix it in its missionary position, the thing itself, if there is anything at all to it, slips away" cf. SP , Caputo DN To put Derrida's point simplistically, it might be suggested that the meaning of a particular object, or a particular word, is never stable, but always in the process of change eg. Moreover, the significance of that past change can only be appreciated from the future and, of course, that 'future' is itself implicated in a similar process of transformation were it ever to be capable of becoming 'present'.
The future that Derrida is referring to is hence not just a future that will become present, but the future that makes all 'presence' possible and also impossible. For Derrida, there can be no presence-to-self, or self-contained identity, because the 'nature' of our temporal existence is for this type of experience to elude us. Our predominant mode of being is what he will eventually term the messianic see Section 6 , in that experience is about the wait, or more aptly, experience is only when it is deferred.
Derrida's work offers many important temporal contributions of this quasi-transcendental variety. In its first and most famous instantiation, undecidability is one of Derrida's most important attempts to trouble dualisms, or more accurately, to reveal how they are always already troubled. An undecidable, and there are many of them in deconstruction eg. For example, the figure of a ghost seems to neither present or absent, or alternatively it is both present and absent at the same time SM. However, Derrida has a recurring tendency to resuscitate terms in different contexts, and the term undecidability also returns in later deconstruction.
Indeed, to complicate matters, undecidability returns in two discernible forms. In his recent work, Derrida often insists that the condition of the possibility of mourning, giving, forgiving, and hospitality, to cite some of his most famous examples, is at once also the condition of their impossibility see section 7. In his explorations of these "possible-impossible" aporias, it becomes undecidable whether genuine giving, for example, is either a possible or an impossible ideal.
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