Question

Light with an average flux of $20\,\Omega/cm^2$ falls on a non-reflecting surface at normal incidence having surface area $20\,cm^2$. The energy received by the surface during time span of $1\,minute$ is :

• A. $10 \times 10^3J$
• B. $12 \times 10^3J$
• C. $24 \times 10^3J$
• D. $48 \times 10^3J$

Answer 1

The Correct Option is C

To find the energy received by the surface, we can use the formula that relates the energy \(E\) with the average flux \(F\), the area \(A\) of the surface, and the time duration \(t\) for which the light falls on the surface.

The formula is given by:

\(E = F \times A \times t\)

Where:

• \(F = 20\, \Omega/cm^2\) is the average flux.
• \(A = 20\, cm^2\) is the area of the surface.
• Time \(t\) is given in minutes, which we need to convert to seconds since energy is generally calculated in Joules, and usually, the time is considered in seconds in physics calculations.

Given: \(t = 1 \text{ minute} = 60 \text{ seconds}\)

Substituting the given values into the formula, we get:

\(E = 20\, \frac{W}{cm^2} \times 20\, cm^2 \times 60\, s\)

\(E = 20 \times 20 \times 60\)

\(E = 24000 \, J\)

Therefore, the energy received by the surface during one minute is \(24000 \, J\), which matches the option:

\(24 \times 10^3J\)

Hence, the correct answer is \(24 \times 10^3J\).