# Ask-a-Physicist by Topic

## Light & Optics

If you had a light in a room which was entirely sealed with mirrors, then switched the light off, would the room stay lit?

Would the light just keep reflecting off the mirrors?

Would it slow down? Perhaps to the point where it is no longer visible?

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With the latest telescopes, we can look at some galaxy's some 13 billion light years away, correct?

And that is much older than the earth is thought to be. I also heard that we have seen what the universe looked like only a short time from its birth. So how can all this be true? If nothing can travel faster than light and the light from that moment in time would have passed the spot the earths current location long ago, how can we see that light? What am I missing? I can not fathom how we beat the light from the early universe to this spot. Something doesn't add up for me, so can someone break it down for me?

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How does one measure the kinetic energy of the ejected electrons resulting from the photoelectric effect?

My understanding is that the KE of the ejected electrons was dependent on the frequency of the incident light, not the intensity. A higher intensity light, however, would [have] ejected MORE electrons; provided that the light's frequency is high enough to overcome the work function of the metal.

Thus, it seems to me that an ammeter would show a higher current either way. A higher frequency light would produce faster electrons and a higher intensity light would produce more electrons. Given that current is a rate of charge (I = dq/dt), how did Einstein know that the frequency was the primary factor for the KE and not the intensity?

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Assume two photons are moving in opposite directions from each other from a common light source. How fast would they be traveling relative to each other? Twice the speed of light? If the speed of light is the ultimate speed limit in the universe, how can something travel twice that speed?

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If photons have no mass, why is it impossible for them to reach the escape velocity of a black hole? It seems that a massless particle/wave would have no difficulty escaping the gravity of a black hole, no matter how massive the singularity.
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When carrying out the double slit experiment using electrons or buckyball molecules, do the particles have to be traveling at near light speed velocities to produce an interference pattern?

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If you shake the sun, how long would it take before it had an effect on the position of the earth?

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Since the earthquake in Japan the country has been having trouble with their nuclear energy plants and possible meltdown. The question is why aren’t they using lead to absorb the radiation?

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Are airport whole body x-ray scanners safe for frequent travelers?

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Explain how beautiful sunrises and sunsets are the result of dust in the atmosphere?
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How is the photoelectric effect used to produce a current in a photovoltaic cell?
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I understand now why the sky is blue, but why are sunsets red and orange? - AB, Oak Ridge, TN
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What happens when you use the tab on an inside rearview mirror to diminish the brightness of the headlights behind you? RC, Vail Colorado
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How does MP3 recording compress digital music by factors near 10? I roughly understand zip encoding, but that usually only reduces file sizes by 2 or less. — J, Greenbelt, MD
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I've heard that digital encoding on audio CDs represents a pressure wave. But audio has so many components like volume, pitch, and timbre; how can the binary encoding of a pressure wave encompass all these attributes? RY, Madison Wisconsin.
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Why is the sky blue? CL, Hong Kong
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Do bubbles last longer in cold weather or hot weather? Why? — A, Alpharetta, Georgia
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What does physics have to do with global warming? — ML, New York
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An electron beam, such as the one found in a TV picture tube, is composed of negatively charged electrons. Why is it that this beam does not rapidly spread out owing to the electric repulsion? I realize that the tube has various focusing magnets and such, but I would think the electronic repulsion would be a serious problem. - JT, Buffalo Grove, IL
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How can we see anything? - D
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We're confused about the difference between fluorescent and halogen bulbs. Fluorescent bulbs are classified as hazardous waste because they contain mercury. Do halogen bulbs also contain mercury? - LR and APM, Washington, DC
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Which color is hotter, red or violet? — LL, Falls Church, Virginia
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