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GRE Subject Test: Physics : Electromagnetics, Waves, and Optics

Study concepts, example questions & explanations for GRE Subject Test: Physics

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Example Questions

Example Question #44 : Gre Subject Test: Physics

Sirius is a binary star system, consisting of two white dwarfs with an angular separation of 3 arcseconds. What is the approximate minimum diameter lens needed to resolve the two stars in Sirius for an observation at $600\textup{ nm}$ ?

Possible Answers:

$5\textup{ mm}$

$50\textup{ m}$

$5\textup{ cm}$

$5\textup{ m}$

$50\textup{ cm}$

Correct answer:

$5\textup{ cm}$

Explanation:

The Rayleigh Criterion gives the diffraction limit on resolution of a particular lens at a particular wavelength:

$\theta=1.22\frac{\lambda}{D}$

Where theta is the angular resolution in radians, $\lambda$ is the wavelength of light, and is the diameter the lens in question. Solving for and converting 3 arcseconds into radians, one can approximate the diameter to be about $5\textup{ cm}$ :

$D=1.22\frac{\lambda}{\theta}$

$D=1.22\frac{600*10^{-9}}{1.45*10^{-5}}$

$D=0.05\textup{ m}$

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Example Question #1 : Mirrors

A reflective sphere has a diameter of $1\textup{ m}$ . The surface of the sphere makes a convex spherical mirror; what is its focal point?

Possible Answers:

$-25 \textup{ cm}$

$-50 \textup{ cm}$

$25 \textup{ cm}$

$2 \textup{ m}$

$50 \textup{ cm}$

Correct answer:

$-25 \textup{ cm}$

Explanation:

The focal length of a spherical mirror is one half of the radius, which is one quarter of the diameter. In the case of convex mirrors, the focal point is considered behind the surface, which gives the answer its negative sign.

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Example Question #1 : Electromagnetics

What is the total resistance of a circuit containing four resistors ( $5\Omega, 10\Omega, 15\Omega, 20\Omega$ ) hooked up in series.

Possible Answers:

$\frac{1}{50}\Omega$

$20\Omega$

$\frac{1}{20}\Omega$

$50\Omega$

$\frac{12}{5}\Omega$

Correct answer:

$50\Omega$

Explanation:

For a circuit in series, the total resistance is simply given by the sum of each individual resistor:

$R_{Total}=R_1+R_2+R_3+...R_n$

$R_{Total}=5\Omega+10\Omega+15\Omega+20\Omega=50\Omega.$

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Example Question #2 : Electromagnetics

What is the total resistance of a circuit consisting of three resistors in a parallel configuration? The resistors have the following resistance: $R_1=10\Omega, R_2=20\Omega, R_3=30\Omega.$

Possible Answers:

$\frac{11}{60}\Omega$

$60\Omega$

Cannot be determined.

$\frac{1}{60}\Omega$

$\frac{60}{11}\Omega$

Correct answer:

$\frac{60}{11}\Omega$

Explanation:

The total resistance of a circuit in parallel is given by the following equation:

$\frac{1}{R_{Total}}=\frac{1}R_{1}+\frac{1}{R_2}+\frac{1}{R_3}$

Now, we just plug in the values, and solve for the total resistance by inverting!

$\frac{1}{R_{Total}}=\frac{1}{10}+\frac{1}{20}+\frac{1}{30}$

$R_{Total}=\frac{1}{\frac{11}{60}}=\frac{60}{11}\Omega$

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GRE Subject Test: Physics : Electromagnetics, Waves, and Optics

Study concepts, example questions & explanations for GRE Subject Test: Physics

All GRE Subject Test: Physics Resources

Example Questions

Example Question #44 : Gre Subject Test: Physics

Example Question #1 : Mirrors

Example Question #1 : Electromagnetics

Example Question #2 : Electromagnetics

All GRE Subject Test: Physics Resources

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