| Author: |
Benjamin Crowell
|
| Published in: | Light and Matter |
| Release Year: | 2006 |
| ISBN: | 0-9704670-5-2 |
| Pages: | 114 |
| Edition: | First Edition |
| File Size: | 12 MB |
| File Type: | |
| Language: | English |
Description of Optics
Ads for one Macintosh computer bragged that it could do an arithmetic calculation in less time than it took for the light to get from the screen to your eye. We find this impressive because of the contrast between the speed of light and the speeds at which we interact with physical objects in our environment. Perhaps it shouldn’t surprise us, then, that Newton succeeded so well in explaining the motion of objects, but was far less successful with the study of light.
These Optics books are billed as the Light and Matter series, but only now, in the fifth of the six volumes, are we ready to focus on light. If you are reading the series in order, then you know that the climax of our study of electricity and magnetism was the discovery that light is an electromagnetic wave. Knowing this, however, is not the same as knowing everything about eyes and telescopes. In fact, the full description of light as a wave can be rather cumbersome. We will instead spend most of this bookmaking use of a simpler model of light, the ray model, which does a fine job in most practical situations. Not only that, but we will even backtrack a little and start with a discussion of basic ideas about light and vision that predated the discovery of electromagnetic waves.
Despite its title, this chapter is far from your first look at the light. That familiarity might seem like an advantage, but most people have never thought carefully about light and vision. Even smart people who have thought hard about vision have come up with incorrect ideas. The ancient Greeks, Arabs, and Chinese had theories of light and vision, all of which were mostly wrong, and all of which were accepted for thousands of years.
One thing the ancients did get right is that there is a distinction between objects that emit light and objects that don’t. When you see a leaf in the forest, it’s because three different objects are doing their jobs: the leaf, the eye, and the sun. But luminous objects like the sun, a flame, or the filament of a light bulb can be seen by the eye without the presence of a third object. Emission of light is often, but not always, associated with heat. In modern times, we are familiar with a variety of objects that glow without being heated, including fluorescent lights and glow-in-the-dark toys.
How do we see luminous objects? The Greek philosophers Pythagoras (b. ca. 560 BC) and Empedocles of Acragas (b. ca. 492 BC), who unfortunately was very influential, claimed that when you looked at a candle flame, the flame and your eye were both sending out some kind of mysterious stuff, and when your eye’s stuff collided with the candle’s stuff, the candle would become evident to your sense of sight.
Bizarre as the Greek “collision of stuff theory” might seem, it had a couple of good features. It explained why both the candle and your eye had to be present for your sense of sight to function. The theory could also easily be expanded to explain how we see nonluminous objects. If a leaf, for instance, happened to be present at the site of the collision between your eye’s stuff and the candle’s stuff, then the leaf would be stimulated to express its green nature, allowing you to perceive it as green.
Modern people might feel uneasy about this theory, since it suggests that greenness exists only for our seeing convenience, implying human precedence over natural phenomena. Nowadays, people would expect the cause and effect relationship in vision to be the other way around, with the leaf doing something to our eye rather than our eye doing something to the leaf. But how can you tell?
The most common way of distinguishing cause from effect is to determine which happened first, but the process of seeing seems to occur too quickly to determine the order in which things happened.
These Optics books are billed as the Light and Matter series, but only now, in the fifth of the six volumes, are we ready to focus on light. If you are reading the series in order, then you know that the climax of our study of electricity and magnetism was the discovery that light is an electromagnetic wave. Knowing this, however, is not the same as knowing everything about eyes and telescopes. In fact, the full description of light as a wave can be rather cumbersome. We will instead spend most of this bookmaking use of a simpler model of light, the ray model, which does a fine job in most practical situations. Not only that, but we will even backtrack a little and start with a discussion of basic ideas about light and vision that predated the discovery of electromagnetic waves.
Despite its title, this chapter is far from your first look at the light. That familiarity might seem like an advantage, but most people have never thought carefully about light and vision. Even smart people who have thought hard about vision have come up with incorrect ideas. The ancient Greeks, Arabs, and Chinese had theories of light and vision, all of which were mostly wrong, and all of which were accepted for thousands of years.
One thing the ancients did get right is that there is a distinction between objects that emit light and objects that don’t. When you see a leaf in the forest, it’s because three different objects are doing their jobs: the leaf, the eye, and the sun. But luminous objects like the sun, a flame, or the filament of a light bulb can be seen by the eye without the presence of a third object. Emission of light is often, but not always, associated with heat. In modern times, we are familiar with a variety of objects that glow without being heated, including fluorescent lights and glow-in-the-dark toys.
How do we see luminous objects? The Greek philosophers Pythagoras (b. ca. 560 BC) and Empedocles of Acragas (b. ca. 492 BC), who unfortunately was very influential, claimed that when you looked at a candle flame, the flame and your eye were both sending out some kind of mysterious stuff, and when your eye’s stuff collided with the candle’s stuff, the candle would become evident to your sense of sight.
Bizarre as the Greek “collision of stuff theory” might seem, it had a couple of good features. It explained why both the candle and your eye had to be present for your sense of sight to function. The theory could also easily be expanded to explain how we see nonluminous objects. If a leaf, for instance, happened to be present at the site of the collision between your eye’s stuff and the candle’s stuff, then the leaf would be stimulated to express its green nature, allowing you to perceive it as green.
Modern people might feel uneasy about this theory, since it suggests that greenness exists only for our seeing convenience, implying human precedence over natural phenomena. Nowadays, people would expect the cause and effect relationship in vision to be the other way around, with the leaf doing something to our eye rather than our eye doing something to the leaf. But how can you tell?
The most common way of distinguishing cause from effect is to determine which happened first, but the process of seeing seems to occur too quickly to determine the order in which things happened.
Content of Optics
1 The Ray Model of Light 11
2 Images by Reflection 31
3 Images, Quantitatively 43
4 Refraction 59
5 Wave Optics 77
2 Images by Reflection 31
3 Images, Quantitatively 43
4 Refraction 59
5 Wave Optics 77
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