Physics Ch. 13

Why does a red apple appear to be the color red? Would the red apple appear to be the color red if blue light illuminated the apple?

A red apple appears red because when white light illuminates the red apple all of the colors in the white light are absorbed except the red light which is reflected. Since blue light one of the primary colors (and does not contain red light), when it illuminates a red apple the apple would appear black since there is no red light to be reflected back.

How is the wavelength of light related to its frequency?

As with all waves, the wavelength and frequency are inversely proportional to one another. As one increases the other decreases.

What has the higher frequency, red light or blue light?

Blue light has a higher frequency.

What do radio waves and light have in common? What is different about them?

Both travel at the same speed in air. (3.00 X 108 m/s). They have different frequencies and wavelengths.

What part of the electromagnetic spectrum is unable to penetrate the Earth's atmosphere? Why is this important?

The atmosphere is opaque (meaning that the electromagnetic waves can not be transmitted through) for high frequency ultraviolet light and other high frequency electromagnetic waves such as X-rays and Gamma rays. This is important because high frequency electromagnetic waves carry a great deal of energy. In fact, lower frequency ultraviolet light that makes it through the atmosphere is what causes sunburns. If other high frequency electromagnetic waves were able to penetrate the atmosphere, life on this planet would not be possible as we exist now.

How does the average speed of light in glass compare with its speed in a vacuum?

The average speed of light is glass is less than in a vacuum. Light always travels slower in a medium as compared to air (or a vacuum).

What causes the variety of colors seen in gasoline splotches on a wet street? In the same regard, what accounts for the variety of color in a soap bubble?

The colors we see in gasoline and soap bubbles are a result of interference of light when the light strikes a thin film. The light that passes through the thin film and reflects and the light that simply reflects off the surface of the thin film interfere with one another. Some frequencies of light cancel and we see the different colors.

How does the frequency of an electromagnetic wave compare with the frequency of the vibrating electrons that produce it?

The frequencies are the same. All electromagnetic waves originate from a vibrating electron

Short wavelengths of visible light interact more frequently with the atoms in glass than do longer wavelengths. Does this interaction time tend to speed up or slow down the average speed of light in glass?

The greater the number of interactions, the slower the speed of light in the material

What are the primary colors of light? What color do you get if you mix blue and red? Green and red? Green and blue? What happens if you mix blue, red, and green light?

The primary colors are Red, Green, and Blue. Blue + Red = Magenta Red + Green = Yellow Green + Blue = Cyan If you mix the primary colors (red, blue, and green) you get white light.

Which has the shorter wavelengths, ultraviolet or infrared? Which has the higher frequencies?

Ultraviolet has shorter wavelengths and higher frequencies. (This is the type of electromagnetic waves that cause sunburns.)

What is the fate of the energy in ultraviolet light incident on glass? Visible light?

Ultraviolet light is absorbed by glass while visible light passes through.

What is the principal difference between a radio wave and light? Between light and an X ray?

All electromagnetic waves travel at the same speed in air (or a vacuum), 3.00 X 108 m/s. What makes the waves different is the fact that they have different frequencies and wavelengths. Radio waves have long wavelengths and low frequencies compared to visible light while X-rays have short wavelengths and high frequencies compared to visible light.

The sound coming from one tuning fork can force another to vibrate. What is the analogous effect for light? (Think about light traveling through a transparent material and what the light does to the atoms of the material.)

In a transparent medium, light can cause the electrons in the atom that make up the transparent material to vibrate at the same frequency of the light. The electrons can then give off photons and that energy is transmitted to the next electron. Basically, the frequency of the light can cause the electrons in the material and sometimes the atoms themselves to vibrate at the same frequency.

You can get sunburn on a cloudy day, but you can't get sunburn even on a sunny day if you are behind glass. Explain.

Ultraviolet light is the cause of sunburns and glass is an opaque material for ultraviolet light. In other words, ultraviolet light can not pass through the glass it is absorbed.

Is interference restricted only to some types of waves or does it occur for all types of waves?

Interference can occur with any type of wave.

Which have the longest wavelengths: light waves, X rays, or radio waves?

Radio waves have the longest wavelengths and the smallest frequencies.

Do radio waves travel at the same speed of sound, at the same speed of light, or at some speed in between?

Radio waves travel at the speed of light in air (3.00 X 108 m/s) which is much larger than the speed of sound waves in room temperature air (340 m/s)

What is the color of light that is transmitted through a piece of red glass?

Red light is transmitted through a piece of red glass.

Why does the Sun look reddish at sunset and sunrise but not at noon?

Referring to the previous question, light scatters in the atmosphere. Blue light scatters the most and yellow, orange, and red the least. Light that isn't scattered is transmitted through the atmosphere. When the sun is low in the sky, the light must travel through more of the atmosphere. The thicker the atmosphere, the more scattering that can occur. The higher frequencies are scattered away and the red light is transmitted through the most since hardly any red light was scattered. At noon, when the sunlight travels through the least amount of atmosphere, and thus the yellow and orange light have not been scattered with the blue. The sun will appear to be a yellow or orange color as that color is transmitted through.

How could you use the spotlight at a play to make the yellow clothes of the performers suddenly change to black? (same type of question: Under which light will a ripe banana appear black?) a. Red light b. Yellow light c. Green light d. Blue light

Since yellow light is a combination of both red and green light you would want to use a color of light that does not contain yellow. Blue is a primary color and does not contain yellow light.

Does visible light make up a relatively large part or a relatively small part of the electromagnetic spectrum?

Small part

Which requires a physical medium in which to travel, light, or sound? Or do both require a physical medium? Explain.

Sound requires a physical medium to travel through.

What does it mean to say that light is scattered? How does this explain a very blue sky?

When light is incident on the atmosphere which is made up of atoms, the light actually interacts with the atoms. The atoms behave as tiny optical tuning forks. When the light strikes the atoms, the atoms reemit light. The upper atmosphere is made up primarily of Nitrogen and Oxygen molecules. These particular atoms are "tuned" to higher frequency light (Blue). When white light in incident on the atoms, they reemit the higher frequency light more often and in all directions. This is termed scattering. The other colors are scatters in order of frequency with red being scattered the least. Since blue light is scattered the most, we see the sky as being very blue especially on a clear day. If there is a lot of water vapor in the air or dust particles, lower frequencies of light also undergo scattering. This causes the sky to appear less blue or even whitish.

Distinguish between the white of this page and the black of this ink, in terms of what happens to the white light that falls on both? In other words, why does the page appear to be white and ink black?

When white light falls on this page, the white parts of the page reflect all the white light while the black ink absorbs all the white light. In fact, this particular page contains red ink. The red appears red because all but the red light is absorbed by the red lettering and only the red light is reflected back.