# A-Level Physics Help Required

1. Nov 9, 2009

### OJ-Hick

Last year I sat my physics A-Level paper and despite achieving A's in the first 3 modules I took a nosedive and underachieved. I have finished school now and am retaking modules in January time and upon my preparation I have come across a few problems which I can't seem to answer! Any help would be greatly appreciated. Here goes:

1) Explain the problem and state how the measurement can be made more accurate when measuring the fringe width in Young's Double Slit experiment (laser/monochromatic light source)

2) Explain how the following formula follows from the definition of intensity:
I= P/(4 x pi x r2)

3) Photoelectical Emission:
(i)When p.d is set to zero the microammeter indicates a small current. Why?

(ii) The cathode is made from sodium - work function 3.7 x 10^-19 J. Show the stopping potential for radiation of frequency 7.5 x 10^14 Hz is approx 0.8V

(iii) Sketch a graph to show how current changes as the reverse potential difference V is increased from 0 to 1.0 V

THANKS GUYS HOPE YOU CAN HELP