$A$ stick of $1 \, m$ is moving with a velocity of $2.7 \times 10^8 \, m/s$. What is the apparent length of the stick in meters? (Given: $c = 3 \times 10^8 \, m/s$)

Find the number of photons emitted per second by a $100\, W$ red light source. Assume for simplicity that the average wavelength of each photon is $694\, nm$.

Our eye can detect $5 \times 10^4$ photons per second per square meter of green light $(\lambda = 5000 \ \mathring{A})$. If the ear can detect $10^{-13} \ W/m^2$,how many times more sensitive is the eye compared to the ear?

$A$ blue lamp emits light of mean wavelength $4500 \ \text{Å}$. The lamp is rated at $150 \ \text{W}$ and $8 \%$ efficiency. Calculate the number of photons emitted by the lamp per second.

The energy of a photon whose frequency is $10^{12} \text{ MHz}$ is:

The minimum intensity of light that can be perceived by the human eye is $10^{-10} \ W/m^2$. If the area of the pupil is $10^{-6} \ m^2$ and the wavelength of light is $560 \ nm$,approximately how many photons per second enter the eye? (Use $h = 6.6 \times 10^{-34} \ J \cdot s$ and $c = 3 \times 10^8 \ m/s$).