A2 Daily A Level Physics question
Which statement must be true when a narrow X-ray beam of fixed energy passes normally through two identical aluminium plates, each with thickness equal to the half-value thickness (HVT) for aluminium at that energy?
Answer
The correct answer is D.
Correct: D — After the two plates, the intensity is about one quarter of the incident beam. A says one half, but one HVT halves once; with two HVTs you halve twice, giving a quarter. B suggests a one-third result as if attenuation were linear; HVT implies multiplicative halving, not subtracting in equal chunks. C claims minimal change, but a single HVT leaves 50% transmitted, so two leave far less than 50%. D follows the definition of HVT: halve once (to 1/2), then halve again (to 1/4).