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University Physics Volume 3

Conceptual Questions

University Physics Volume 3Conceptual Questions

Conceptual Questions

1.1 The Propagation of Light


Under what conditions can light be modeled like a ray? Like a wave?


Why is the index of refraction always greater than or equal to 1?


Does the fact that the light flash from lightning reaches you before its sound prove that the speed of light is extremely large or simply that it is greater than the speed of sound? Discuss how you could use this effect to get an estimate of the speed of light.


Speculate as to what physical process might be responsible for light traveling more slowly in a medium than in a vacuum.

1.2 The Law of Reflection


Using the law of reflection, explain how powder takes the shine off of a person’s nose. What is the name of the optical effect?

1.3 Refraction


Diffusion by reflection from a rough surface is described in this chapter. Light can also be diffused by refraction. Describe how this occurs in a specific situation, such as light interacting with crushed ice.


Will light change direction toward or away from the perpendicular when it goes from air to water? Water to glass? Glass to air?


Explain why an object in water always appears to be at a depth shallower than it actually is?


Explain why a person’s legs appear very short when wading in a pool. Justify your explanation with a ray diagram showing the path of rays from the feet to the eye of an observer who is out of the water.


Explain why an oar that is partially submerged in water appears bent.

1.4 Total Internal Reflection


A ring with a colorless gemstone is dropped into water. The gemstone becomes invisible when submerged. Can it be a diamond? Explain.


The most common type of mirage is an illusion that light from faraway objects is reflected by a pool of water that is not really there. Mirages are generally observed in deserts, when there is a hot layer of air near the ground. Given that the refractive index of air is lower for air at higher temperatures, explain how mirages can be formed.


How can you use total internal reflection to estimate the index of refraction of a medium?

1.5 Dispersion


Is it possible that total internal reflection plays a role in rainbows? Explain in terms of indices of refraction and angles, perhaps referring to that shown below. Some of us have seen the formation of a double rainbow; is it physically possible to observe a triple rainbow?

A photograph of a double rainbow.
(credit: "Chad"/Flickr)

A high-quality diamond may be quite clear and colorless, transmitting all visible wavelengths with little absorption. Explain how it can sparkle with flashes of brilliant color when illuminated by white light.

1.6 Huygens’s Principle


How do wave effects depend on the size of the object with which the wave interacts? For example, why does sound bend around the corner of a building while light does not?


Does Huygens’s principle apply to all types of waves?


If diffraction is observed for some phenomenon, it is evidence that the phenomenon is a wave. Does the reverse hold true? That is, if diffraction is not observed, does that mean the phenomenon is not a wave?

1.7 Polarization


Can a sound wave in air be polarized? Explain.


No light passes through two perfect polarizing filters with perpendicular axes. However, if a third polarizing filter is placed between the original two, some light can pass. Why is this? Under what circumstances does most of the light pass?


Explain what happens to the energy carried by light that it is dimmed by passing it through two crossed polarizing filters.


When particles scattering light are much smaller than its wavelength, the amount of scattering is proportional to 1λ1λ. Does this mean there is more scattering for small λλ than large λλ? How does this relate to the fact that the sky is blue?


Using the information given in the preceding question, explain why sunsets are red.


When light is reflected at Brewster’s angle from a smooth surface, it is 100%100% polarized parallel to the surface. Part of the light will be refracted into the surface. Describe how you would do an experiment to determine the polarization of the refracted light. What direction would you expect the polarization to have and would you expect it to be 100%100%?


If you lie on a beach looking at the water with your head tipped slightly sideways, your polarized sunglasses do not work very well. Why not?

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