Physics 26 - Homework Questions
What is the wavelength of a wave that has a frequency of 1 Hz and travels at 300,000 km/s?
300,000 km
What does a changing *magnetic* field induce?
A changing *electric* field.
What does a changing *electric* field induce?
A changing *magnetic* field.
What produces an electromagnetic wave?
An oscillating or accelerating electric charge.
What do electric and magnetic fields contain and transport?
Energy.
Why are metals shiny?
Free electrons in metals vibrate when light strikes them, thereby reemitting the light as reflected light.
How is the fact that an electromagnetic wave in space never slows down consistent with the law of conservation of energy?
If light slowed down, its energy would decrease, thereby violating the law of conservation of energy.
How is the fact that an electromagnetic wave in space never speeds up consistent with the law of conservation of energy?
If light speeded up, its energy would increase, thereby violating the law of conservation of energy.
Why are infrared waves often called heat waves?
Infrared light vibrates entire atoms and molecules, and this vibration increases the temperature of the absorbing substance.
What is the fate of the energy in infrared light incident on glass?
Infrared light will make atoms vibrate, thus becoming internal energy and a temperature increase.
About how much of the measured electromagnetic spectrum does light occupy?
Less than 1 millionth of 1%
Why do wet objects normally look darker than the same objects when dry?
Light bounces around inside the transparent water region covering a wet object, getting absorbed more on each bounce.
Distinguish between an umbra and a penumbra.
No light from the source enters the shadow in an umbra, while some, but not all, light from the source enters the penumbra.
What are the principal differences between a radio wave and light? Between light and an X-ray?
Radio waves have a lower frequency and longer wavelength than visible light waves. Light waves have a lower frequency and longer wavelength than X-rays.
What is the color of visible light of the lowest frequencies? Of the highest frequencies?
Red, violet
How does the average speed of light in glass compare with its speed in a vacuum?
The average speed of light in glass is about 67% of the speed of light in a vacuum.
The sound coming from one tuning fork can force another to vibrate. What is the analogous effect for light?
The light emitted by resonant vibrations of an electron around one atom can be absorbed by an electron with the same resonant frequency of vibration in another atom.
How do the rods in the eye differ from the cones?
The rods are rod-shaped and are more sensitive to dim light; cluster in periphery.
What do we mean when we say that outer space is not really empty?
The vacuum of space is full of electromagnetic wave energy.
How is the wavelength of light related to its frequency?
The wavelength is inversely proportional to the frequency.
How does the frequency of a radio wave compare to the frequency of the vibrating electrons that produce it?
They are the same.
How does the frequency of reemitted light in a transparent material compare with the frequency of the light that stimulates its reemission?
They are the same.
How does the speed of light that emerges from a pane of glass compare with the speed of light incident on the glass?
They are the same.
In what region of the electromagnetic spectrum is the resonant frequency of electrons in glass?
Ultraviolet
What is the fate of the energy in ultraviolet light that is incident upon glass?
Ultraviolet light will be absorbed by the resonant vibration of electrons and turned into internal energy and a temperature increase.
Why do opaque materials become warmer when light shines on them?
Vibrations given by the light to their electrons, atoms, and molecules that absorb the light become internal energy, thus leading to a temperature increase.
What is the fate of the energy in visible light that is incident upon clear glass?
Visible light will be transmitted by the glass, losing little energy in the process, so the energy remains visible light energy.
When are objects on the periphery of your vision most noticeable?
When they are moving.
Do Earth and the Moon always cast shadows? What do we call the occurrence where one passes within the shadow of the other?
Yes, they both cast shadows. When one passes into the shadow of the other, there is an eclipse.
What besides the amount of light affects the size of the pupil of the eye?
our emotions!
