Inverse square law
Used on the ASP, CSP and CHST. The worked example below is computed by code and checked against the source.
I2 = I1 × (d1 ÷ d2)²
I = intensity or dose rate at distance d from a point source.
Worked example
100 mR/h at 2 ft; what at 6 ft?
100 × (2 ÷ 6)² = 11.1 mR/h
Where it appears on the exam
ASP: ASP11 Domain 1, Mathematical Calculations
Blueprint: ASP11 Domain 1, task 15: radiation exposure (dosage)
Practice: Radiation Calculations: Decay, Distance, Shielding
CSP: CSP11 Domain 6, Occupational Health and Applied Science
Blueprint: CSP11 Domain 6, task 1: occupational exposures (radiation)
Practice: Applied Science: Radiation, Ventilation, Heat and Force
CHST: CHST5 Domain 1, Hazard and Risk Identification and Control
Blueprint: CHST5 Domain 1, knowledge 19: radiation exposure
Source
The formula comes from
OSHA, Ionizing Radiation: Introduction to Health Physics handout(opens osha.gov in a new tab)“Estimating Attenuation in Air Using Inverse Square Law I 2 = I 1 [ d 1 / d 2 ] 2”
Questions
- What is the formula for inverse square law?
- I2 = I1 × (d1 ÷ d2)². I = intensity or dose rate at distance d from a point source.
- Will this formula be given on the BCSP exam?
- BCSP says the formula is included in the question whenever one is needed, unless the question is testing whether you know the formula. You can't bring a formula sheet into the exam, so learn what each formula does and when to use it.
BCSP on formulas at the exam
BCSP, What to Expect When Sitting for a BCSP Exam (item 7)(opens bcsp.org in a new tab)“For any question requiring a mathematical formula, the formula will be included within the exam question (unless the question is specifically testing your knowledge of a formula). You will not be provided or permitted to bring a formula sheet.”
Related
- Radioactive decay
- Shielding with half-value layers
- ASP formula sheet
- CSP formula sheet
- CHST formula sheet
Checked 2026-10-16. Spot a problem? Email hello@passcited.com.