Question

If the wavelength of light is $4000\dot{A}$ then the number of waves in $1mm$ length will be:

• A. $2.5$
• B. $2500$
• C. $250$
• D. $25000$

Answer 1

The correct option is B

$2500$

Step 1: Given

Wavelength of light: $\lambda =4000\dot{A}=4000\times {10}^{-10}m$

Length: $L=1mm=1\times {10}^{-3}m$

Step 2: Formula Used

$Wavelength\left(\lambda \right)=\frac{Length\left(L\right)}{Numberofwavesinlength\left(n\right)}$

Step 3: Calculate the number of waves using the formula

$$\begin{gathered} 4000\times {10}^{-10}=\frac{1\times {10}^{-3}}{n} \\ \Rightarrow n=\frac{1\times {10}^{-3}}{4000\times {10}^{-10}} \\ \Rightarrow n=\frac{1}{4}\times {10}^4 \\ \Rightarrow n=2500 \end{gathered}$$

Hence, the number of waves in $1mm$ length are $2500$.