All AP Physics 2 Resources
Example Questions
Example Question #11 : Doppler Effect
How fast towards an observer would a red car have to be moving in order to appear yellow ?
None of these
Using the Doppler effect equation for approaching sources:
Where is the speed of the wave in the medium, which in this case is the speed of light,
Combining equations
Solving for :
Example Question #12 : Doppler Effect
An ambulance is receding at with it's siren on. Normally, the siren has frequency . Determine the perceived frequency if the speed of sound is .
Using the Doppler effect equation for receding sources:
Where is the speed of sound in the current medium
Plugging in values:
Example Question #11 : Doppler Effect
A train is receding at with it's horn on. Normally, the siren has frequency . Determine the perceived frequency if the speed of sound is .
Using the Doppler effect equation for receding sources:
Where is the speed of sound in the current medium
Plugging in values:
Example Question #11 : Doppler Effect
A rocket ship traveling towards an observer flashes a red light . How fast would it have to be traveling for the wavelength to be cut in half?
None of these
Doppler effect:
Where is the change in wavelength
is the original wavelength
is the velocity of the source
is the speed of light
Plugging in values:
Solving for
Example Question #11 : Doppler Effect
A rocket ship traveling towards an observer flashes a light, . How fast would it have to be traveling for the wavelength to be cut to one third it's initial value?
None of these
Doppler effect:
Where is the change in wavelength
is the original wavelength
is the velocity of the source
is the speed of light
Plugging in values:
Solving for
Example Question #15 : Doppler Effect
A certain shade of blue light has a laboratory rest wavelength of . The same shade of blue light is emitted from a newly discovered galaxy at a wavelength of . Using this information, what can we tell about this newly discovered galaxy?
The galaxy is moving towards the Earth at speed
The galaxy is moving away from Earth at speed 
None of these
The galaxy is moving away from Earth at speed
The galaxy is moving towards the Earth at speed
The galaxy is moving away from Earth at speed
Here, we need to use the Doppler effect equation:
Where refers to the wavelength difference between the two sources, is the laboratory wavelength, is the speed of the source, and is the speed of light.
Now, let's plug in all of the values:
.
.
Because the wavelength has been shifted to longer wavelengths (the number is larger than the rest wavelength down on Earth), we say the object is redshifted. Therefore, the source (the galaxy) is moving away from Earth at this speed.
Example Question #1 : Slit Experiments
What did the double-slit experiment (also called Young's Experiment) demonstrate?
Light is composed purely of waves
The double-slit experiment did not tell scientists anything useful
Light is composed purely of particles
Light cannot be diffracted by slits
Light demonstrates properties of both particles and waves
Light demonstrates properties of both particles and waves
The process for the double-slit experiment consisted of a coherent light source aimed at a screen with a plate with two parallel slits in between them. The light traveled through each of the slits and had wave-interference (destructive and constructive interference) to produce bands of alternating light and dark along the screen. This result would not be expected if light consisted solely of particles, as was classically thought. This showed that light is a wave, because waves interfere in that manner. The light also was found to be hitting the screen at discrete points as individual particles (photons), with the alternating bands indicating the density of the particles that hit the screen. In versions of the experiment that featured detectors at the slits, the photon passed through a single slit (as would a particle) and not both (as would a wave). These outcomes demonstrate the wave-particle duality of light.
Example Question #1 : Slit Experiments
Suppose a light with a wavelength of passes a pair of slits with a separation of . Determine the angle which corresponds to the first bright fringe.
To determine the angle, use the formula for bright fringes in a two-slit experiment.
Rewrite the equation so that the angle term is isolated.
represents the first bright fringe.
Substitute all the givens and solve for the angle.
Example Question #3 : Slit Experiments
Suppose light pass through a slit with a width . The diffraction pattern angle is for the first dark fringe. What is the light's wavelength?
To solve for the wavelength of the light, use the formula for dark fringes of a single-slit interference.
Rewrite the equation in order to solve for lambda (wavelength). Since this is the first dark fringe, the value of .
Example Question #1 : Slit Experiments
Suppose that a diffraction experiment is performed in order to determine the wavelength of light. As light of an unknown wavelength is passed through a slit, a diffraction pattern is observed. If the first dark fringe is found to occur at an angle of away from the slit, what is the wavelength of this light?
For this question, we're presented with a scenario in which light is passing through a small slit. In doing so, the light is diffracted, meaning that it is spread out along a viewing screen. In such a case, there will be a central bright fringe, with alternating dark and bright fringes as one moves above and below the central fringe.
In this question, we're told that the size of the slit is , and the first of these dark fringes occurs at an angle of away from the slit. We're then asked to determine the wavelength of the light based on this information.
First and foremost, we'll need to use the light diffraction equation:
In the above equation, represents the wavelength of light, represents the length of the slit, and represents the location of the dark fringes (it can equal ). In this case, is equal to one because we're considering the first dark fringe (on the top/bottom of the central fringe).
Now that we have the equation, we can isolate the wavelength term:
Next all we have to do is plug in the values that we know, and we can solve for the wavelength:
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