Question
An observer can hear sound from around a comer but cannot see light from around the same comer. Which of the following helps to explain this phenomenon?
(A) Sound is a longitudinal wave, and light is an electromagnetic wave.
(B) Sound is a mechanical wave, and light is a transverse wave.
(C) Light travels at a speed much faster than that of sound.
(D) Light has a much smaller wavelength than sound.
▶️Answer/Explanation
Ans:D
The point source model shows us that the larger the wavelength, the greater the bending of the wave around the corner. We can also see this in the equation for diffraction: \(\sin\theta=\frac{m\lambda}{d}\). As the wavelength gets smaller, the diffraction bending angle also gets smaller. Visible light has a very small wavelength and only bends a tiny amount around corners.
Question
What is the equation for the electric field given by the above plot?
The electric field crosses the axis at times o ps, 1.43 ps, 2.85 ps, and 4.28 ps.
(A) \(E = \left ( \frac{0 N}{C} \right )sin ((2.85 THz)t)\)
(B) \(E = \left ( \frac{10 N}{C} \right )sin ((2.85 THz)t)\)
(C) \(E = \left ( \frac{10 N}{C} \right )sin ((2.20 THz)t)\)
(D) \(E = \left ( \frac{10 N}{C} \right )cos ((0.45 THz)t)\)
▶️Answer/Explanation
Ans:C
The field is a sine curve with an amplitude of 10 N/C. From the graph, the period of the wave is 2.85ps. The argument of sine needs to be 2πt/T.
Question
Which of the following best represents the electric field (E) measured in mV/m as a function of time measured in nanoseconds (ns)?
(A) E = 6cos(25.1t)
(B) E = 6cos(12.6t)
(C) E = 3cos(12.6t)
(D) E=3cos(25.1t)
▶️Answer/Explanation
Ans:C
The amplitude is 3 mV/m, and the time period is 0.5 ns. Using the wave equation, we get
\(E=A\cos(\frac{2\pi t}{T})=3\cos(\frac{2\pi t}{0.5})=3\cos(4\pi t)\)
Question
An electromagnetic wave travels through space. Which of the following best describe the orientation of the magnetic field? Select two answers.
(A) The magnetic field is parallel to the velocity of the wave.
(B) The magnetic field is perpendicular to the velocity of the wave.
(C) The magnetic field is parallel to the electric field.
(D) The magnetic field is perpendicular to the electric field.
▶️Answer/Explanation
Ans: (B) & (D)
The magnetic field is oriented perpendicular to both the velocity of the wave and the electric field.
Question
On a day that is warm and sunny, a car is parked in a location where there is no shade. The car’s windows are closed. The air inside the car becomes noticeably warmer than the air outside. Which of the following factors contribute to the higher temperature? Select two answers.
(A) Hotter air rises to the roof of the car and cooler air falls to the floor.
(B) The body of the car insulates the air inside the car.
(C) Electromagnetic radiation from the Sun enters the car and is absorbed by the materials inside.
(D) The body of the car reflects electromagnetic radiation.
▶️Answer/Explanation
Ans:B and C