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List of top Physics Questions on radiation

A radioactive substance has a half-life of 5 years. What is the probability that a single atom of this substance will decay within 5 years?
Two black bodies emit the same amount of radiation per second. The radius of the first is \( R_1 = 2 \, \text{m} \) and its temperature is \( T_1 = 400 \, \text{K} \). If the second body has a radius \( R_2 = 4 \, \text{m} \), what is its temperature \( T_2 \) in Kelvin?
A star 'A' has radiant power equal to 3 times that of the Sun. The temperature of star 'A' is \( 6000 \, \mathrm{K} \) and that of the Sun is \( 2000 \, \mathrm{K} \). What is the ratio of their radii?