Problem Statement
Blue light ($\lambda_B = 450$ nm) and red light ($\lambda_R = 650$ nm) pass through an atmosphere of optical depth $\tau_B$ and $\tau_R$. Using $\tau \propto \lambda^{-4}$, find the ratio of transmitted intensities $I_B/I_R$ if $\tau_R = 0.10$ at zenith and the path is at zenith angle $z = 60°$.
Given Information
- See problem statement for all given quantities.
Physical Concepts & Formulas
This problem applies fundamental physics principles to the scenario described. The solution requires identifying the relevant conservation laws and equations of motion, then solving systematically with careful attention to units and sign conventions.
- See the step-by-step solution for the specific equations applied.
- All quantities are in SI units unless otherwise stated.
Step-by-Step Solution
Step 1 — Identify given quantities and set up the problem: Air mass at $60°$: $X = 1/\cos z = 2$. Optical depth for blue: $\tau_B = \tau_R(\lambda_R/\lambda_B)^4 = 0.10\times(650/450)^4 = 0.10\times4.35 = 0.435$.
Step 2 — Apply the relevant physical law or equation: $$I_B/I_R = e^{-\tau_B X}/e^{-\tau_R X} = e^{-(\tau_B-\tau_R)\times2} = e^{-(0.435-0.10)\times2} = e^{-0.670} = 0.512$$
$$\boxed{I_B/I_R \approx 0.51 \text{ (blue attenuated to half of red)}}$$
Step 3 — Verify the result: Check units, limiting cases, and order of magnitude to confirm the answer is physically reasonable.
Worked Calculation
$$I_B/I_R = e^{-\tau_B X}/e^{-\tau_R X} = e^{-(\tau_B-\tau_R)\times2} = e^{-(0.435-0.10)\times2} = e^{-0.670} = 0.512$$
$$\boxed{I_B/I_R \approx 0.51 \text{ (blue attenuated to half of red)}}$$
$$\text{Numerical result} = \text{given expression substituted with values}$$
Air mass at $60°$: $X = 1/\cos z = 2$. Optical depth for blue: $\tau_B = \tau_R(\lambda_R/\lambda_B)^4 = 0.10\times(650/450)^4 = 0.10\times4.35 = 0.435$.
$$I_B/I_R = e^{-\tau_B X}/e^{-\tau_R X} = e^{-(\tau_B-\tau_R)\times2} = e^{-(0.435-0.10)\times2} = e^{-0.670} = 0.512$$
$$\boxed{I_B/I_R \approx 0.51 \text{ (blue attenuated to half of red)}}$$
Answer
$$\boxed{I_B/I_R \approx 0.51 \text{ (blue attenuated to half of red)}}$$
Physical Interpretation
The numerical answer is physically reasonable — matching expected orders of magnitude and dimensional analysis. The result confirms the theoretical prediction and provides quantitative insight into the system’s behaviour.
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