Final answer:
The statement (A) that the equal intensity of light from the two slits in Young's experiment is necessary for the best contrast is true, and the reason (R) that intensity is proportional to the square of amplitude is also true. However, R does not directly explain A because equal amplitude is specifically required for the clearest contrast in the interference pattern.
Step-by-step explanation:
Understanding Young's Double Slit Experiment
In Young's double slit experiment, the assertion (A) is that for the best contrast between maxima and minima in the interference pattern, the intensity of light emerging out of the two slits should be equal. This is true because when the amplitude of the waves from the two slits is the same, the interference pattern produced on the screen results in maxima and minima with the greatest contrast. Reason (R) states that the intensity of an interference pattern is proportional to the square of the amplitude, which is also true. However, R is not the direct explanation for A, because while it's correct that intensity is related to amplitude, the assertion about contrast depends specifically on both light sources having equal amplitude to produce the clearest interference pattern.
The intensity of bright fringes (maxima) in the interference pattern is a result of constructive interference when the waves from the two slits arrive in phase, and the intensity of dark fringes (minima) results from destructive interference when the waves arrive out of phase. When the amplitudes from both slits are the same, the constructive interference is maximized and the destructive interference leads to complete cancellation, thus producing the best contrast.
The answer to the question is B. A and R are true but R is not the correct explanation of A.