Genèse 1:4

וירא אלוקים את האור, He looked at its appearance and it was good. The statement is similar to Yocheved saying ותרא אותו כי טוב, (Exodus 2,2) although he had been born three months prematurely, just as had Samuel. In practice, the fact that all his vital features including the nails of his toes and fingers and the hairs on his head had grown, gave her the feeling that he was “good,” i.e. a viable human being, so that she proceeded to hide him for three months because she knew that he was not premature in the sense of not being viable.

ויבדל אלוקים בין האור ובין החושך, He arranged for the day to be divided into 12 hours of daylight and 12 hours of night. The day commences with a period of light and concludes with a period of darkness. The reason is that the beginning of directives issued during the creative process commenced with the command: “let there be light.”

וירא אלוקים את האור כי טוב, This describes the reason why G’d created the light, not that He “saw”” something new which He had not been aware of previously. It was because it was “good,” that G’d had created the light.

ויבדל אלוקים בין האור ובין החשך, the days (prior to the fourth day when the sun was placed in orbit) during which use was made of the “original light,” periods of light and darkness alternated due to causes other than the revolving of the planet around its own axis, i.e. due to the direct expression of G’d’s desire. He ensured that periods of light alternated with periods of darkness.

וירא אלוקים, He “saw” in His knowledge and wisdom, that the light would be good for the creatures which inhabit the “lower” universe. Even though darkness is also good for them, i.e. useful for them, as we already explained, darkness is not something to which the adjective, attribute טוב, “good,” can be applied. The reason is that darkness entails the removal of light. How can this be described as “good?” Darkness had existed already, was not a new phenomenon, it was therefore appropriate to state that the new phenomenon light was good (in the sense that it added a positive element to the universe which it had lacked.)
Moreover “light” is something evident, as opposed to darkness. That which emanates from darkness is generally evil. Kohelet 11,7 already speaks of “how sweet is the light, what a delight for the eyes to behold the sun!” Whenever he mentions darkness he does so in the context of something evil, whereas he relates to light as something good, if only because it is something in the open, something that is not “ashamed” to be seen. The evil of darkness is “visible,” as opposed to its positive aspects, which are concealed. Similarly, every time the Torah uses the phrase כי טוב in the report of creation it means that the phenomenon which has been so described is good for the creatures in the lower regions, seeing that all the acts of the creative process were good, did not contain a negative element.
The reason for the word ויבדל, “He separated,” after the word וירא, “He saw,” is that once G’d had “seen” that the light was good, He did not want it to be called by the same name as darkness, although both periods are part of the same “day.” This is why the Torah writes afterwards (verse 5) ויקרא, “He called, He named,” to tell us that the separation between light and darkness was confined to their being called by different names.
It is impossible to interpret the word ויבדל as referring to anything else but the change of the name each part of the day is known by. Normally, the expression הבדל is applied to different substances, whereas light and darkness are neither of them a substance, so that the whole expression appears extraordinary at this point. If darkness is nothing more than the absence of light, how could the term “separated, distinguished,” be properly applied to the absence of something? This is why the Torah wrote ויבדל אלוקים וגו' instead of writing ויבדל האור, or ויבדל without adding a subjective or objective which would have meant that the light itself formed the division, the separation, as the Torah had done when describing the separation of the רקיע and the מים in verse 7. G’d effected the separation between darkness and light by calling them by different names.
As to the expression ולהבדיל בין האור ובין החשך, “and to separate between the light and between the darkness,” (verse 18) this refers to the same day, i.e. a separation between parts of the same entity, the daylight hours enjoying the light of the sun, whereas the night receives only the relatively insignificant light of the moon. Relative to the brilliant light of the day such light is minor, whereas relative to the absolute darkness prevailing during the plague of darkness in Egypt it is quite significant.