[Music] I'm welcome back to three science lessons cold okay by the end of this video you should be able to describe how to investigate the effect of light intensity on the rate of photosynthesis and if you're a higher TN student you should also be able to explain how the results of this all affected by the inverse square law lists the required practical so you need to learn the details we start by taking a boiling tube and placing a 10 centimeters away from an LED light source an LED light is used as these don't release very much heat too much heat would change the temperature of the experiment now if we have to use a normal light bulb then we need to place a beaker of water between the light and the boiling tube this absorbs the heat produced by the bulb now we fill the boiling tube with sodium hydrogen carbonate solution sodium hydrogen carbonate solution releases carbon dioxide arm that's needed for photosynthesis next we put a piece of pond weed into the boiling tube with the coat and at the top we then leave this for five minutes to acclimatize to the conditions in the boiling tube we should see bubbles of gas being produced from the cut end of the pond weed this gas is oxygen and that's produced by photosynthesis now we start a stopwatch and we count the number of bubbles produced in one minute we then repeat this two more times and we calculate the mean number bubbles produced in one minute next we do the whole experiment again from the start but this time at 20 centimeters then we do to 30 centimeters and finally at 40 centimeters now there are two main problems with this practical firstly the number of bubbles can be too fast to count accurately secondly the bubbles are not always the same size so for example a large bubble would count the same as a small bubble now we can solve these problems by measuring the volume of oxygen produced instead of counting bubbles we use this equipment we place the pond we don't do a formal and we catch the bubbles in a measuring cylinder filled with water we then use the measuring cylinder to measure the volume of oxygen produced now if your foundation tier student you can stop watching now however higher tier students need to continue watching if we plot the mean number bubbles per minute or the volume of oxygen per minute against the distance from the lump to the pondweed then we get this graph now the key point about this is that if we double the distance then the number of bubbles per minute falls by a factor of four so going from a distance of 10 centimeters to a distance of 20 centimeters causes the number of bubbles per minute to fall by four times and going from 20 centimeters to 40 centimeters the number of as per minute again falls by 4 times scientists call this the inverse square law now the reason for this is that if we double the distance the light intensity falls by 4 times on because we need like a photosynthesis that causes the number of oxygen bubbles to fall by 4 times remember you'll find plenty of questions on this required practical in my vision workbook and you can get up by clicking on the link above ok so hopefully now you should be able to describe how to investigate the effect of light intensity on the rate of photosynthesis and if you're a higher tier student you should also be able to explain how the results of this all affected by the inverse square law [Music]