The difference is the consequence of interpreting the participle φανερουμενον "being made manifest" as either passive or middle (middle: themselves make manifest). The word form is passive, but sometimes passive form also covers the middle. Majority of versions are taking it passive: made manifest, illuminated. WEB and NHEB: treats it as active, as in whatever reveals others is light. KJV seemingly middle. I think the word light is simply used as a simplified adjective bright G5460 φωτεινός photeinos meaning full of light. Cf Luke 11:34 when your eye is good, your whole body is also full of light; but when it is bad, your body also is full of darkness. The word should be treated as passive.
So, the meaning in most translations should be: which has beeen made manifested, is light, as it brightens others as a lamp acting as light. BSB has put it nicely for everything that is illuminated becomes a light itself.
Word Studies Vincent comments:
Whatsoever doth make manifest is light [παν το φανερουμενον φως εστιν] Wrong. The A. V. renders doth make manifest, as in the middle voice, but the verb is in the passive voice. It occurs nearly fifty times in the New Testament, and never as middle. Hence, RV., correctly, everything that is made manifest.
13.] but (opposition to τὰ κρυφῆ γιν.) all things (not only, all the κρυφῆ γινόμενα, as Ellic. after Jer. al.: the Apostle is treating of the general detecting power of light, as is evident by the resumption of the πᾶν in the next clause) being reproved, are made manifest by the light: for every thing which is made manifest is light (the meaning being, ‘the light of your Christian life, which will be by your reproof shed upon these deeds of darkness, will bring them out of the category of darkness into light’ (ἐπειδὰν φανερωθῇ, γίνεται φῶς, Chr.). They themselves were thus once darkness,’ but having been ‘reproved’ by God’s Spirit, had become ‘light in the Lord.’ There is in reality no difficulty, nor any occasion for a long note here. The only matters to be insisted on are, 1) ὑπὸ τοῦ φωτός belongs to φανεροῦται, not to ἐλεγχόμενα: for it is not the fact of φανεροῦται that he is insisting on, but the fact that if they reproved the works of darkness, these would become no longer works of darkness, but would be ὑπὸ τοῦ φωτὸς φανερούμενα. And 2) φανερούμενον is passive, not middle, in which sense it is never used in N. T.; ‘every thing which is made manifest, is no longer darkness, but light: and thus you will be, not compromised to these works of darkness, but making an inroad upon the territory of darkness with the ὅπλα τοῦ φωτός.’ And thus the context leads on easily and naturally to the next verse. The objection to this (Eadie) that ‘light does not always exercise this transforming influence, for the devil and all the wicked are themselves condemned by the light, without becoming themselves light,’ is null, being founded on misapprehension of the φῶς ἐστιν. Objectively taken, it is universally true: every thing shone upon IS LIGHT. Whether this tend to condemnation or otherwise, depends just on whether the transforming influence takes place. The key-text to this is Joh 3:20, πᾶς γὰρ ὁ φαῦλα πράσσων μισεῖ τὸ φῶς, κ. οὐκ ἔρχεται πρὸς τὸ φῶς, ἵνα μὴ ἐλεγχθῇ τὰ ἔργα αὐτοῦ,—His works being thus brought into the light,—made light, and he being thus put to shame. Notice also φανερωθῇ in the next verse, which is the desire of him who ποιεῖ τὴν ἀλήθειαν. The E. V. is doubly wrong—1) in ‘all things that are reproved’ (π. τὰ ἐλεγχόμενα): 2) in ‘whatsoever doth make manifest is light’ (πᾶν τὸ φανεροῦν): besides that such a proposition has absolutely no meaning in the context. The meaning is discussed at length in Harl., Eadie, who however fall into the error of rendering φανερούμενον active (not middle),—Stier, Ellicott,—and best of all, Meyer)
πᾶν γὰρ τὸ φανερούμενον φῶς ἐστιν. Πᾶν τό. Winer, § 18, 4. The meaning depends greatly on this-whether φανερούμενον be taken in a middle or passive sense. Many prefer the passive sense, which is certainly the prevailing one in the New Testament, and occurs in the previous clause. The exposition of Olshausen, Stier, Ellicott, and Alford is—“whatever is made manifest is light”—“all things illuminated by the light are themselves light.” Well may Olshausen add—“this idea has somewhat strange in it,” for he is compelled to admit “that light does not always exercise this transforming influence, for the devil and all the wicked are reproved by the light, without becoming themselves light.” Alford calls this objection “null,” as being a misapprehension of φῶς ἐστι, but φῶς in his exegesis changes its meaning from the previous verse. This opinion of Olshausen is virtually that of the Greek patristic expositors, who are followed by Peter Lombard. Theophylact says-ἐπειδὰν δὲ φανερωθῇ, γίνεται φῶς. Harless renders, “what has been revealed is no longer a hidden work of darkness: it is light.” The view of Röell, Robinson, and Wilke is not dissimilar. Thus also Ellicott—“becomes light, as of the nature of light.” A dark object suddenly illumined may indeed be said to be all light, because it is surrounded with light, and this is the notion of Bretschneider. But if this be the view, it seems to make the apostle use a tautology, “whatever is revealed, is enlightened;” unless you understand the apostle to say, that by such a process they themselves who were once darkness become light. De Wette's explanation of the same rendering is-without φῶς there is no φανερούμενον, and where there is φανερούμενον there is light. But the apostle doe s not utter such a truism-where everything is manifested there is light. Piscator's hypothesis is equally baseless—“whatever is manifested is light, that is, is manifested by the light.” The passive meaning may be adopted, with the proviso that the apostle does not say whether the light be for conversion or condemnation. But while this view may thus be grammatically defended, still we feel as if the context led us to take the last clause as a reason of the statement contained in the first. Thus, some prefer, with Beza, Calvin, Vatablus, Grotius, Rollock, Zanchius, Morus, Wahl, Turner, and the Peschito, to give the participle a reflexive or medial signification. Meyer affirms that φανεροῦμαι is always passive, but the passive may have a medial signification, as it seems to have sometimes in the New Testament. Mar 16:12; Joh 1:31; Joh 9:3; 2Co 4:10-11; Jelf, § 367, 2. Olshausen takes up the exegesis of Grotius, which is also that of Bodius and Dickson—“for the light is the element that makes all clear,” and then argues grammatically against such a rendering. But according to the accurate position of subject and predicate, the meaning is—“whatever makes manifest or renders apparent, is light.” Such manifestation is the nature and function of light. These clandestine sins, when reproved, are disclosed by the light so cast upon them, for it belongs to light to make such disclosures. The apostle urges his readers to reprove such sins, which, though done in secret, will and must be exposed; yea, all of them being reproved, are shone upon by the light-that light which radiates from Christianity. And this power of unveiling in Christianity is properly called “light,” for whatever causes such things to disclose themselves is of the essence of light. Such is a natural and simple view of the verse. See Lücke-Commentar, Joh 3:21, vol. i. p. 550, 3rd ed.