Light and Color

Explain how diffraction is used to identify unknown stars or substances.

A diffraction grating can be used to view a star or other light emitting object. Since white light contains all colors of the spectrum, the more white the light, the more colors of the spectrum are seen. The composition of a star can be determined by looking at the spectra produced from a diffraction grating and matching the spectra lines to known spectra.

from New York to Los Angeles?

0.016 seconds

around the equator of the Earth?

0.133 seconds

from the moon to the Earth?

1.29 seconds

from Alpha Centauri (next closest star)?

4 years

How long does it take light to travel one mile?

5.3 ( 10-6 seconds

from the sun to the Earth?

8 minutes

How long (in meters) is a light year?

9.5 10 meters

Why might you choose a window shade that is translucent? Why opaque?

A translucent shade would keep people from being able to see in but still allow light to pass through. An opaque shade would block all light and make the room dark.

What is an opaque object?

An opaque object is something that allows no light through it. Concrete, wood, and metal are some examples of opaque materials. Some materials can be opaque to light, yet translucent to other types of electromagnetic waves. For example, wood does not allow visible light to pass through it, but will allow other types of electromagnetic waves, such as microwaves and radio waves. The physical characteristics of the material determine what type of electromagnetic energy will and will not pass through it.

What is a light year?

The concept of a light year can be rather confusing at first. A year is a unit of time, but a light year represents a unit of distance. Specifically, it is the distance that light travels in one year. Since light travels at 3 ( 108 m/s and a year consists of 31.536 ( 106 seconds, a light year is therefore equivalent to 9.46 ( 1015 meters.

What is the diffraction of light?

The diffraction of light occurs when light bends around the boundaries of an object attempting to create a shadow. When light is sent through an opening that is wider than the wavelength of the light, a clear and distinct shadow is formed. However, if the light needs to pass through a very narrow opening, it bends around the edges of the opening and a fuzzy shadow is created. Diffraction occurs with all types of waves, but can be most often and easily seen with light.

What is black light?

A black light or ultraviolet lamp is almost the same as a fluorescent light. The differences are that ultraviolet lights lack the white phosphor coating and are made with dark blue glass. Fluorescent tubes contain a near vacuum with mercury vapor inside. When excited electrically, the mercury gas gives off visible green, blue, and violet light as well as invisible ultraviolet frequencies. If black light tubes were made from clear glass, the bright blue/green glow would light up the room. Because of this, black lights are made with dyed glass which blocks most of the visible light but passes the ultraviolet. If a perfect dye were available, black light tubes would not glow purple when on; they would look totally black. However, the dye in the glass does let a little visible violet light through as well as passing a lot of invisible ultraviolet rays.

How do rainbows occur?

A rainbow is a spectrum of light formed when sunlight refracts into, reflects, and then refracts once again out of water droplets. Upon entering a water droplet, the white light is spread apart into its individual frequencies, just as in a prism. The light inside the droplet then reflects against the back of the water droplet and spreads apart even more upon exiting the water droplet. The separation of the light frequencies, along with a large number of water droplets exposed to sunlight, creates a circular-shaped rainbow.

What is a supernumerary rainbow?

A supernumerary rainbow is another name for a double rainbow. The secondary rainbow has its color spectrum reversed, is outside of the original rainbow, and is significantly dimmer than the primary rainbow. The reason why a supernumerary rainbow occurs is because an additional reflection of the light takes place inside the water droplets. Instead of reflecting once in the water droplet, the light reflects twice inside the water, creating a dimmer mirror image of the primary rainbow.

Distinguish between transparent, translucent, and opaque objects.

A transparent object allows 100% transmission with no distortion. A translucent object allows some light to pass but it is distorted. An opaque object blocks all light.

When a light wave encounters a different medium, can the refraction of light be determined?

All material has an index of refraction. It can be thought of as a measure of how quickly light moves through the material. The index of refraction for a given medium is the speed of light in a vacuum divided by the speed of light in that medium. Therefore, a vacuum, with no impediment to light propagation, has a refractive index of 1.00, whereas glass has a higher value, typically around 1.50. The higher the index of refraction, the more slowly light travels through the medium. According to the Law of Reflection, light that hits a surface at a given angle is reflected off at that same angle. This principle makes it possible to see around a corner using a mirror held at 45(.

What is an electromagnetic (EM) wave?

An electromagnetic (EM) wave is a vibrating electric charge that produces a wave that is partially electric and partially magnetic and carries energy. All electromagnetic waves are transverse waves that travel at a constant speed (the speed of light).

Why are sunsets often red?

As light travels through the atmosphere it is refracted and scattered. At noon, the sun is often seen as white since no refraction occurs when the angle of light is zero degrees. As the day goes on, the angle of refraction increases. The longer the path light must travel, the more scattering that occurs. As the light is scattered along a long path, it will eventually all get scattered, allowing none of that color to reach you at sunset. Blue gets used up first, then green. The only color left is red, which produces the red sunsets we see. Since orange has a similar wavelength to red, orange is often seen at sunset as well. Pollution produces the most vivid and colorful sunsets. This is due to the varying size of pollution in the air which scatters more blue and green. The left over light produces vibrant sunsets of red and orange.

How fast does light travel?

Because light is found on the electromagnetic spectrum, light travels at the same speed as other electromagnetic waves, which, in a vacuum, is 299,792,458 m/s, or 186,282.4 mi/s. Except when performing experiments that need the exact velocity of light, this number is usually rounded up to 3 ( 108 m/s (186,000 mi/s). Just like any other wave, light does slow down when it enters the Earth's atmosphere.

What is black?

Black is the absence of color. It is not a color. It can be thought of as a shadow

If white is the combination of the colors of the rainbow, what is black?

Black, the exact opposite of white light, is the absence of light or the absorption of all light. A black piece of paper appears black because all the light is being absorbed into the paper - none is reflected back out.

How are objects seen?

In order to see an object, it must be emitting or reflecting light. We can see stars, lightning, and light bulbs because they are emitting or giving off light. And we depend on the light emitted from these objects in order to see objects that don't emit light - we see those objects because they reflect light. A blade of grass, for example, does not emit light; however we see the blade of grass because it reflects existing light, specifically green light.

What are the differences and similarities between incandescent light, fluorescent light, and vapor light?

Incandescent light emits light due to heating (filament light bulb), vapor light emits light due to exciting vapor or gas electrically (street lamp, halogen lamp), and fluorescent light emits light due to exciting mercury and an inert gas electrically (fluorescent light).

What is refraction?

Light can be redirected in three ways: reflection, diffraction, or refraction. Reflection occurs when light bounces off of a surface. The second method of redirection, diffraction, is the deviation from a straight path that occurs when a wave such as light or sound passes around an obstacle or through an opening. The third method, refraction, refers to the bending of light as it goes from one medium to another. Sunlight refracts when it encounters the medium of the Earth's atmosphere, and when it goes through water. An image can look quite different after viewing it through a refracting medium.

What is polarized light?

Light is typically emitted in all directions and with different orientations; that is to say that a light wave may be oriented with the electric component of the electromagnetic wave vibrating up and down or vertically, while in other instances the light might vibrate horizontally or even diagonally. Regardless of the orientation, these light waves are not polarized. To be polarized, all the light waves must be oriented in the same direction. For example, vertically polarized light has its waves all aligned in up and down vibrations. Non-polarized light produces glare, which can be a distraction when driving, skiing, or taking pictures.

What is light?

Light, or visible light, as it is often called, is what enables vision. In fact, light is the only thing people see; every object we think we see is a result of light that is reflected off the objects and into our eyes - in the absence of light, we would see nothing. Light is an electromagnetic wave, located between infrared and ultraviolet on the electromagnetic spectrum. Although light is on a wave spectrum, at times it mimics the behavior of particles.

Is light a wave or a particle? Explain.

No conclusions have been made as to the classification of light as a wave or particle. New theories continue to develop, but until one theory can successfully define light, it is described as a particle carried on a wave.

Why do diamonds sparkle so much?

One of the keys to finding a good diamond is making sure it has a good cut. The sides of the diamond need to be at specific angles so that when light enters the diamond, it is internally reflected instead of being refracted out of the diamond. The critical angle for a diamond is about 25(; since the critical angle of the diamond is so small, this ensures that most of the light that has entered the diamond will emerge not from the sides but from the top of the diamond, resulting in a sparkle of shimmering light.

Why is the sky blue?

Recall the description of red as an easy going, slow mover and violet as a hyper, fast mover; green is somewhere in between. The atmosphere contains mostly oxygen (O2) and nitrogen (N2), which are larger than light rays. As the different colors separate through the atmosphere, red has little chance of hitting any nitrogen or oxygen. Green has a greater chance of hitting the oxygen and nitrogen whereas blue, with its high frequency, has the largest probability of scattering. The scattering of mostly blue makes the sky appear blue.

What is the difference between selective reflection and selective transmission?

Selective reflection is when opaque objects reflect the color seen and absorb all other colors. Selective transmission is when transparent objects transmit the color seen and absorb all other colors.

The speed of light is slower in water than in a vacuum. How does this affect frequency and wavelength?

Since the frequency of light remains constant when the medium change, the wavelength must decrease.

Explain why red is on top of a rainbow and violet is on the bottom

Since violet light has the highest frequency it is refracted the most and comes out quickly (on the bottom). Red has a lower frequency so it is refracted the least, allowing it to go further into the medium (raindrop) and come out last (on the top).

What isn't light?

Some ancient Greeks felt that invisible streamers emitted from our eyes were responsible for sight, while others felt that light consisted of particles flying through the air and striking our eyes. It was not until the time of Isaac Newton and Christian Huygens, two pioneers in the study of light, that scientific theories were formed concerning light.

What direction should sunglasses be polarized to help reduce the amount of glare? Explain.

Sunglasses should be polarized vertically because most glare is reflection from horizontal surfaces. Vertical polarization would block all horizontal light, thus reducing glare.

Could an index of refraction ever be less than 1? What would that imply about the speed of light in that medium?

The index of refraction can never be less than one. If it were less than one, it would imply that the speed of light in the medium is faster than the speed of light in a vacuum.

What changes occur in the characteristics of the interference patterns formed by a diffraction grating containing 10,000 lines/cm and one having 100,000 lines/cm?

The more lines per centimeter, the more spread apart the spectrum appears. Therefore, a 100,000 lines/cm diffraction grating will show a more spread out spectrum.

What is the true color of the ocean?

The ocean behaves much like the sky, except the salt in the ocean is much larger than the oxygen and nitrogen in the sky. Combined with the fact that the ocean naturally absorbs red, orange, and yellow, this allows more green and blue to be reflected, which produces a cyan color. The ocean usually looks blue due to the reflection of the sky.

What is the order of colors in a rainbow?

The order of colors in a rainbow goes from low frequency on the outer arc to higher frequency light on the inside of the arc. The full order from outer to inner is: red, orange, yellow, green, blue, indigo, and violet.

What conditions must be met in order to see a rainbow?

There are only two main conditions for witnessing a rainbow. The first is that the observer must be between the sun and the water droplets. The water droplets can either be rain, mist from a waterfall, or the spray of a garden hose. The second condition is that the angle between the sun, the water droplets, and the observer's eyes must be between 40 and 42(. Therefore, in order to see a rainbow from rain, the rainbow needs to occur in the morning or afternoon when the sun and observer are between 40 and 42( of each other.

What is the difference between transparent and translucent?

Transparent media such as air, water, glass, and clear plastic allow light and images to clearly pass through the material. Translucent materials, on the other hand, allow light to pass through, but clear images cannot be seen through them. For example, frosted glass and thin paper are translucent because they let some light through, but are not transparent because you cannot see clearly through them.

Can the speed of light change?

Waves travel at different speeds when traveling in different materials. In the vacuum of space, light travels at 3 ( 108 m/s. If light could travel in a circular path, it would orbit the Earth 7.5 times in one second. When light encounters a denser medium, however, like that of the Earth's atmosphere, it slows down ever so slightly to 2.91 ( 108 m/s. Upon striking water it slows down rather dramatically, to 2.25 ( 108 m/s, three quarters of its original speed. Finally, when light passes through the dense medium of glass, it slows to only 1.98 ( 108 m/s. Even at this slower speed, light would still travel around the Earth (if it could indeed travel in a circle) 5.6 times in one second.

Why do we see specific colors?

When we see colors, we are actually seeing the effect of light shining on an object. When white light shines on an object it may be reflected, absorbed, or transmitted. Glass transmits most of the light that it comes in contact with and thus appears colorless. Snow reflects all of the light and so appears white. A black cloth absorbs all light and so it appears black. A green blade of grass reflects green light better than it reflects other colors. Most objects appear colored because their chemical structure absorbs certain wavelengths of light and reflects others.

What color is white light?

White light is the combination of all the colors in the visible light spectrum. When separated from each other, the different frequencies create different colors: the lowest frequency light is the color red, and increasing frequencies result in orange, yellow, green, blue, indigo, and finally, the highest frequency visible color, violet.

ROY G BIV

acronym for the colors of the visible spectrum; red, orange, yellow, green, blue, indigo, and violet; red ~ 700 nm, violet ~ 400 nm.

primary color of light

average colors of sunlight or white light; colors of light that when added together produce white light; red, green, and blue.

primary color of pigment

base colors of all pigments (all pigments are made from these colors); colors of pigment that when mixed produce black pigment; yellow, magenta, and cyan.

diffraction grating

material containing many parallel lines very closely spaced used to separate colors of light due to interference; acts as multiple slits that cause overlapping diffraction.

pigment

materials that selectively absorb colored light; some reemit absorbed light, like neon and fluorescent paint; most pigments are dyes, paints, or ink.

black

not a color; absence of color; shadow.

selective reflection

opaque objects reflect the color seen and absorb all other colors.

color by addition

process used to determine the resultant color of combined light; the adding of colored light.

color by subtraction

process used to determine the resultant color of combined pigments; the subtraction of pigments.

secondary color of light

result of adding any two primary colors of light; color of light, that when added to its primary color of light, produces white light; yellow, cyan, and magenta.

secondary color of pigment

result of mixing any two primary colors of pigment; pigment, that when added to its primary pigment, produces black pigment; red, blue, and green.

scattering

the reflection of light in the air.

selective transmission

transparent objects transmit the color seen and absorb all other colors.

prism

triangular piece of glass used to separate white light into its component colors.

color wheel of light

used to show how various colors of light are created.

color wheel of pigment

used to show how various colors of pigment are created.

spectrum

visible light; white light; range on electromagnetic spectrum between infrared and ultraviolet; wavelength range between 400 nm to 700 nm; frequency around 1015 hertz; represents less than one percent of the electromagnetic spectrum; the only thing we can see.