a quick look at light Flashcards
visible spectrum
visible spectrum—that is, the wavelengths of light we can see—makes up only a tiny part of the full electromagnetic spectrum. (Simply put, the electromagnetic spectrum is a group of different types of radiation. It includes everything from radio waves to microwaves and gamma rays.)
That entire visible spectrum section you see at the bottom fits into what?
That entire visible spectrum section you see at the bottom fits into that little space between ultraviolet and infrared.
We measure the wavelengths of the visible spectrum in what?
nanometers (nm), or billionths of a meter.
the visible spectrum spans only
the visible spectrum spans only a few hundred nanometers.
we can see wavelengths ranging from
we can see wavelengths ranging from 380 to 760 nm.
ultraviolet wavelengths are shorter than
the visible spectrum
, while infrared wavelengths are longer than
the visible spectrum
Microwaves and radio and TV waves are longer still than.
the visible spectrum
Red is the longest of the
of the wavelengths in the visible spectrum,
violet is the shortest
visible wavelength.
angle of incidence
If our light wave strikes a highly reflective object, it will bounce off at an angle . We call the angle at which the light wave hits the surface the angle of incidence,
angle of reflection.
we call the angle at which it bounces off the angle of reflection.
These angles are always the same.
angle of incidence and angle of reflection.
normal line
normal line, which is the dotted line in our drawing that runs perpendicular to the surface the light is striking.
how we measure the angle of incidence and angle of reflection.
We measure them by calculating their relationship to the normal line,
UVC
is completely absorbed by the atmosphere.
UVB
is the most damaging to our cells, causing cataracts and skin cancer.
UVA
also called black light, causes tanning and thickening of the skin and contributes to cataracts.
The protective lenses you’ll recommend in sunglasses block what
both UVB and UVA rays.
Snell’s Law.
This law allows us to determine the angle at which light will bend as it passes from one substance to another. All we need to know is the refractive indices of both substances. For instance, if we know the refractive index of air (which is 1) and the refractive index of a polycarbonate lens (1.584), we can figure out exactly how far light will bend as it travels through the lens.
Law of Refraction
Light bends toward the normal when it enters a medium more dense than the one it came from.
refraction
the bending of light.
When we talk about refraction, we’re not talking about what?
When we talk about refraction, we’re not talking about light that bounces (reflects) off a surface. Instead, we’re talking about light passing through a substance, bending as it goes.
light slows when …
light slows when it moves from a less resistant or rarer medium (for instance, air) to a denser medium like water.
light travels faster when…
light travels faster when it moves from a denser medium to a rarer medium.
light will keep moving in a straight line unless it strikes the new medium at an angle. When this happens, it will do what
it will bend.
index of refraction.
index of refraction. The higher the index of refraction of a material, the more the light will bend as it goes through it.
why the index of refraction is important
this fact is very important, as it explains why the high-index plastic materials you order will make lenses appear thinner.
So what’s an advantage of making lenses from a material with a high index of refraction?
Simple: We can make the lenses thinner, because they’ll bend the light more powerfully.
High-index plastic lenses are popular with who
High-index plastic lenses are popular with people with strong prescriptions who want to avoid thick lenses.
why high-index lens materials are usually more expensive than regular plastic lenses
it takes more high-tech equipment to produce them.
downside to high-index lens being thinner,
Also, since a high-index lens is thinner, the central optical zone of the lens is more compressed. This means that there’s a smaller area in the center of the lens where the sharpest vision can occur. If you’re wearing high-index lenses and you move your eyes outward toward the sides of the lenses, things will not be as sharp as they are when you’re looking at the periphery of plastic lenses.
light dispersion,
the colors of the rays passing through the lenses separate
when people see colors around images it is called what?
ome people see colors around images, an effect we call chromatic aberration.
how you eliminate chromatic abrassion
We can usually eliminate this effect by putting an antireflection coating on the lenses.
high index lenses can cause what?
High-index lenses can also cause a high degree of light dispersion, meaning that the colors of the rays passing through the lenses separate—much like a light ray passing through a prism separates into the colors of the rainbow. As a result, some people see colors around images, an effect we call chromatic aberration. We can usually eliminate this effect by putting an antireflection coating on the lenses.
downsides to high index lenses:
- usually more expensive
- there’s a smaller area in the center of the lens where the sharpest vision can occur
- can also cause a high degree of light dispersion,
Abbe Value of a lens
the amount of dispersion produced by different lenses.
the Nu Value or Constringent.
The lower the Abbe Value, the higher the amount of light dispersion of that lens material.
It’s typically a good idea to recommend high-index plastic lenses when?
if a prescription is high.
When a prescription is moderate, it may be best to choose what lens material
When a prescription is moderate, it may be best to choose a light lens material with excellent optics, like a Trivex lens. When a prescription is mild, it’s best to choose a material with a lower index of refraction and better optics.
light passing through a prism will always bend toward what
light passing through a prism will always bend toward the base of the prism.
What do prisms have to do with lenses?
A plus lens is made of two prisms placed base to base. A minus lens is made of two prisms placed apex to apex. This is why plus lenses are always thicker in the center, and minus lenses are always thicker at the edges.
A plus lens is also called what
convex lens
The light going through a plus lens will correct the vision of who?
a hyperopic (farsighted) person because the light will converge sooner and focus onto the retina of a short hyperopic eyeball
minus lens is also called what?
a concave lens.
A minus lens bends the light more or less
A minus lens bends the light less, so the light converges farther back in the eye, reaching the retina of a nearsighted person whose myopic eyeball is longer than normal.
vergence.
vergence. Vergence describes how light rays diverge (spread apart) or converge (come together).
Plus (convex) lenses
Plus (convex) lenses: After going through a convex lens, light converges before it reaches the retina of a normal-length eyeball. We call the image created by a plus lens a real image.
Minus (concave) lenses
Minus (concave) lenses: After going through a concave lens, light rays diverge and focus farther out than they normally would. We call the image created by a minus lens a virtual image.
plano.
If the surface of one side of a lens is flat the power at that surface is plano.
biconvex.
If the front and back surfaces of a lens are both convex, the lens is called biconvex.
biconcave.
If the front and back surfaces of a lens are both concave, the lens is called biconcave.
A meniscus lens
A meniscus lens has one convex and one concave side. Meniscus lenses can minimize distortion or blur in images, especially if the lens power is very high. (We’ll talk more about the lens aberrations that cause distortion or blur in a later lesson.)
a minus meniscus lens.
A meniscus lens that’s thinner in the center than in the periphery is called a minus meniscus lens.
a positive meniscus lens.
A meniscus lens that’s thicker in the center than in the periphery is called a positive meniscus lens.
If a client doesn’t know the power of his or her lenses, how can you find this information?
One way is to use a device called a lens clock.
how to use a lens clock
When you use a lens clock, you’ll test both the front surface and the back surface of a lens. Then you’ll add the numbers to determine the power of the lens. For example, if the front of a lens reads -2.00 and the back surface is +4.00, the total power of the lens is -2.00 + 4.00, or +2.00. Simple!
to measure the strength of plus or minus lenses, we use what?
to measure the strength of plus or minus lenses, we use a unit called a diopter, or diopter of sphere. You’ll sometimes see this abbreviated as D, DS, sph, or sphere. This unit of power describes how much divergence or convergence occurs when light goes through a lens. Prescriptions show numbers in units of diopters.
The total power of the eye’s refractive system
about 59 diopters.
The total power of the crystalline lens alone
about 18 diopters.
vergence.
vergence. This is the coming together (convergence) or spreading apart (divergence) of light rays.
Vergence of light entering lens + lens power =
= vergence of light leaving lens
The inverse
The inverse is 1 divided by the power of the lens.
For example, the focal length of a +2.00 lens is .5 meters. This means that light going through a 2-diopter lens will focus .5 meters behind the lens. Here’s another way of looking at it:
Inverse of 2 =
1/2 =
.5 meters
But what happens when you’re dealing with a minus lens? In this case, your focal length will be negative as well—because the image (that virtual image we talked about earlier) will fall in front of the lens rather than behind it. Here are the numbers for a -2.00 lens:
Inverse of -2 =
- 1/2 =
- .5 meters
