Answer
d. There are more photons per second
Work Step by Step
Let's write out equations that may be relevant to better see what's going on:
$Intensity=I=\frac{Power}{Area}=\frac{P}{A}$
$Power=P=\frac{Energy}{Time}=\frac{E}{t}$
$Frequency=f=\frac{c}{\lambda}$
$Energy=E=h*f$
$\textbf{a. The photons travel faster. --> INCORRECT}$
Since frequency is unchanged, let's look at our equations involving frequency and speed $c$ = speed of light = constant.
$f=\frac{c}{\lambda}$
where $\lambda$ = wavelength.
Since $f$ is unchanged and $c$ is a constant, $\lambda$ must also be unchanged.
This option can't be correct.
$\textbf{b. Each photon has more energy. --> INCORRECT}$
Recall $f=\frac{c}{\lambda}$
$Energy=h*f=\frac{hc}{\lambda}$
Since $f$ and $\lambda$ are unchanged and $hc$ are both constants, $E$ is also unchanged, so this can't be the answer either.
$\textbf{c. The photons are larger. --> INCORRECT}$
Size is not mentioned in any equation we have, so omit this.
$\textbf{d. There are more photons per second --> CORRECT}$
Therefore we are left with this answer, but let's check if it makes sense anyways.
$Energy$ is unchanged, so $power$ can only be changed if we change the amount that's coming in per unit $time$.
$Power=\frac{Energy}{Time}=\frac{E}{t}$
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The correct answer is C because Intensity is directly proportional to photons emitted.
