a quick look at light

Question 1

visible spectrum

Answer 1

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.)

Question 2

That entire visible spectrum section you see at the bottom fits into what?

Answer 2

That entire visible spectrum section you see at the bottom fits into that little space between ultraviolet and infrared.

Question 3

We measure the wavelengths of the visible spectrum in what?

Answer 3

nanometers (nm), or billionths of a meter.

Question 4

the visible spectrum spans only

Answer 4

the visible spectrum spans only a few hundred nanometers.

Question 5

we can see wavelengths ranging from

Answer 5

we can see wavelengths ranging from 380 to 760 nm.

Question 6

ultraviolet wavelengths are shorter than

Answer 6

the visible spectrum

Question 7

, while infrared wavelengths are longer than

Answer 7

the visible spectrum

Question 8

Microwaves and radio and TV waves are longer still than.

Answer 8

the visible spectrum

Question 9

Red is the longest of the

Answer 9

of the wavelengths in the visible spectrum,

Question 10

violet is the shortest

Answer 10

visible wavelength.

Question 11

angle of incidence

Answer 11

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,

Question 12

angle of reflection.

Answer 12

we call the angle at which it bounces off the angle of reflection.

Question 13

These angles are always the same.

Answer 13

angle of incidence and angle of reflection.

Question 14

normal line

Answer 14

normal line, which is the dotted line in our drawing that runs perpendicular to the surface the light is striking.

Question 15

how we measure the angle of incidence and angle of reflection.

Answer 15

We measure them by calculating their relationship to the normal line,

Question 16

UVC

Answer 16

is completely absorbed by the atmosphere.

Question 17

UVB

Answer 17

is the most damaging to our cells, causing cataracts and skin cancer.

Question 18

UVA

Answer 18

also called black light, causes tanning and thickening of the skin and contributes to cataracts.

Question 19

The protective lenses you’ll recommend in sunglasses block what

Answer 19

both UVB and UVA rays.

Question 20

Snell’s Law.

Answer 20

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.

Question 21

Law of Refraction

Answer 21

Light bends toward the normal when it enters a medium more dense than the one it came from.

Question 22

refraction

Answer 22

the bending of light.

Question 23

When we talk about refraction, we’re not talking about what?

Answer 23

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.

Question 24

light slows when …

Answer 24

light slows when it moves from a less resistant or rarer medium (for instance, air) to a denser medium like water.

Question 25

light travels faster when…

Answer 25

light travels faster when it moves from a denser medium to a rarer medium.

Question 26

light will keep moving in a straight line unless it strikes the new medium at an angle. When this happens, it will do what

Answer 26

it will bend.

Question 27

index of refraction.

Answer 27

index of refraction. The higher the index of refraction of a material, the more the light will bend as it goes through it.

Question 28

why the index of refraction is important

Answer 28

this fact is very important, as it explains why the high-index plastic materials you order will make lenses appear thinner.

Question 29

So what’s an advantage of making lenses from a material with a high index of refraction?

Answer 29

Simple: We can make the lenses thinner, because they’ll bend the light more powerfully.

Question 30

High-index plastic lenses are popular with who

Answer 30

High-index plastic lenses are popular with people with strong prescriptions who want to avoid thick lenses.

Question 31

why high-index lens materials are usually more expensive than regular plastic lenses

Answer 31

it takes more high-tech equipment to produce them.

Question 32

downside to high-index lens being thinner,

Answer 32

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.

Question 33

light dispersion,

Answer 33

the colors of the rays passing through the lenses separate

Question 34

when people see colors around images it is called what?

Answer 34

ome people see colors around images, an effect we call chromatic aberration.

Question 35

how you eliminate chromatic abrassion

Answer 35

We can usually eliminate this effect by putting an antireflection coating on the lenses.

Question 36

high index lenses can cause what?

Answer 36

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.

Question 37

downsides to high index lenses:

Answer 37

1. usually more expensive
2. there’s a smaller area in the center of the lens where the sharpest vision can occur
3. can also cause a high degree of light dispersion,

Question 38

Abbe Value of a lens

Answer 38

the amount of dispersion produced by different lenses.

Question 39

the Nu Value or Constringent.

Answer 39

The lower the Abbe Value, the higher the amount of light dispersion of that lens material.

Question 40

It’s typically a good idea to recommend high-index plastic lenses when?

Answer 40

if a prescription is high.

Question 41

When a prescription is moderate, it may be best to choose what lens material

Answer 41

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.

Question 42

light passing through a prism will always bend toward what

Answer 42

light passing through a prism will always bend toward the base of the prism.

Question 43

What do prisms have to do with lenses?

Answer 43

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.

Question 44

A plus lens is also called what

Answer 44

convex lens

Question 45

The light going through a plus lens will correct the vision of who?

Answer 45

a hyperopic (farsighted) person because the light will converge sooner and focus onto the retina of a short hyperopic eyeball

Question 46

minus lens is also called what?

Answer 46

a concave lens.

Question 47

A minus lens bends the light more or less

Answer 47

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.

Question 48

vergence.

Answer 48

vergence. Vergence describes how light rays diverge (spread apart) or converge (come together).

Question 49

Plus (convex) lenses

Answer 49

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.

Question 50

Minus (concave) lenses

Answer 50

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.

Question 51

plano.

Answer 51

If the surface of one side of a lens is flat the power at that surface is plano.

Question 52

biconvex.

Answer 52

If the front and back surfaces of a lens are both convex, the lens is called biconvex.

Question 53

biconcave.

Answer 53

If the front and back surfaces of a lens are both concave, the lens is called biconcave.

Question 54

A meniscus lens

Answer 54

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.)

Question 55

a minus meniscus lens.

Answer 55

A meniscus lens that’s thinner in the center than in the periphery is called a minus meniscus lens.

Question 56

a positive meniscus lens.

Answer 56

A meniscus lens that’s thicker in the center than in the periphery is called a positive meniscus lens.

Question 57

If a client doesn’t know the power of his or her lenses, how can you find this information?

Answer 57

One way is to use a device called a lens clock.

Question 58

how to use a lens clock

Answer 58

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!

Question 59

to measure the strength of plus or minus lenses, we use what?

Answer 59

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.

Question 60

The total power of the eye’s refractive system

Answer 60

about 59 diopters.

Question 61

The total power of the crystalline lens alone

Answer 61

about 18 diopters.

Question 62

vergence.

Answer 62

vergence. This is the coming together (convergence) or spreading apart (divergence) of light rays.

Question 63

Vergence of light entering lens + lens power =

Answer 63

= vergence of light leaving lens

Question 64

The inverse

Answer 64

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