3 Turn over 3 A light source radiates a power P onto a surface, covering a circular area of radius r. Which of the following is the correct expression for the intensity I of the radiation at the surface? A I = 2 P πr B I = Pπr 2 C I =  2 P πr D I = 2Pπr (Total for Question 3 = 1 mark) 4 A string is held under tension. When it is plucked it vibrates with a frequency f. Which of the following would result in a lower value for f ? A decreasing the cross-sectional area of the string B decreasing the density of the material of the string C increasing the length of the string D increasing the tension (Total for Question 4 = 1 mark) 5 The image shows a diffraction pattern observed when a beam of electrons is fired at thin gold foil. (Source: © The Reading Room/Alamy Stock Photo) What property of electrons does this observation demonstrate? A they exist in discrete energy levels B they have a negative charge C their small mass D their wave nature (Total for Question 5 = 1 mark)