6.30 am to 9.30 am

11.00 am to 1.30 am

2.30 am to 5.00 am

Theory Behind the Project :

What is photosynthesis … Photosynthesis is the synthesis of glucose from carbon dioxide and water in living plant cells containing chlorophyll in the presence of sunlight. The by product of this reaction is oxygen. In this reaction the radiant energy of light is absobred and is converted into the ‘potential’ chemical energy of glucose. The equation which sums up photosynthesis is

6CO₂ + 6H₂O + Radiant energy C₆H₁₂O₆ + 6O₂

It can be noticed that the amount of oxygen evoloved can donate how much photosynthesis has taken in other words if in a certain case (of photosynthesis) more oxygen is evoloved than in another case, it can be said that more photosynthesis has taken place in the first case than in the second one. So by comparing the voloume of oxygen evolved by a plant, during photosynthesis in the amount of photosynthesis occuring in the different cases can be compaired and that is how the amount of photosynthesis taking place under different coloured lights is compared

The Expected Results :

Sunlight falling on the leaves of a living plant is absorbed partially i.e. source of it is absorbed and some of it is replaced. Naturally the colours that are absorbed are the ones which are used in photosynthesis and the ones that are reflected causes the least. It is seen that a leaf appears green under sunlight. This means that the green part of the spectrum of sunlight which comprises of violet, blue, green, yellow orange and red is given off while the others are absorbed so it can be expected that the red- orange and the blue- violet part of the spectrum are the ones which causes the most photosynthesis and the green-yellow region causes the least if at all any.

If equal amounts of red-orange and blue-violet would cause more photosynthesis than the other.This is because the energy possessed by a certain wave length of light varies directly with the frequency of wave.The above statement can be derived from the equation : Energy = hv, which is in accordance with the quantum theory of light. In this equation ‘h’ is a constant called the plank’s constant and ‘v’ is the frequancy of the the light wave.

Therefore since blue and voilet light have higher frequency than orange and red. the blue- voilet part of the solar spectrum has more energy and so it is expected to cause more photosynthesis than the red-orange part of the solar spectrum.

But during the early and late hours of the day the sunlight has to travel a longer distance through the earth’s atmosphere than at mid-day. Due to the longer distance of the atmosphere which the sunlight has to travel in the early and late hours of the day, the blue and violet wave lengths are absorbed the most on account of their shorter wave so at these hours lesser amount of the earth. Thus the photosynthesis caused by the blue-violet part of the solar spectrum at these hours will be lesser than what it will cause at mid-day.

To sum up the expected results of the project the following may be said:

During 6.30 to 9.30 am and 2.30 to 5.00 pm the blue-violet part of the solar spectrum will cause the most photosynthesis and that cause by the red-orange part will follow closely.

During 11.00 to 1.30 pm the blue-violet part of the solar spectrum will cause the most photosynthesis followed closely by the red-orange part

During the three time interval the green part of the solar spectrum will cause the least photosynthesis the amount being very little compared with the red-orange and blue-violet parts.

In the experiment coloured cellophane sheets are used for the filteration of sunlight in order to get the particular wave lengths required. So before doing the actual experiment it is necessary to perform an auxilliary experiment to see whether the celophone sheets can filter the sunlight properly (to the required degree).

Apparatus : Mirror, Prism, Lens, Screen (Whitewall) and a dark room with a small square hole in the curtain of a window on which the sun shines.

Procedure :

1) The mirror is kept outside the room (on the window sil.) and it is adjusted in such a way that it reflects a horizontal beam of sunlight through the hole in the curtain in the darkroom.

2) The prism is placed on a stool with its parellel sides horizontal such that the beam of sunlight makes an angle of about 45’ with one of the refracting surface of the prism.

3) The lens is placed on a stool such that it is at a distance equal to its own focal length from the white wall and such that it allows all of the beam of light emerging from the prism to fall almost normally on it (making 90’ with the lens’s plane)

Observation :

A spectrum comprising of the following colours appeared on the wall: Violet, blue, green, yellow,orange and red. The width of the individual colours are given below.

4) A sheet of red cellophane is held between the prism and the mirror so that the spectrum formed on the wall is of the filtered light.

Observation : The spectrum comprises of the red and the orange colours only. It looked like the following

Inference : One sheet of red cellophane is sufficient to let only the red and orange light to reach the plant being used in the main experiment.

5) Similarly (as in procedure number 4), a green cellophane sheet is used instead of red one.

Observation : The spectrum comprises of orange, yellow, green and blue colours.

6) So two green cellophane sheets are now held in the same position.

Observation : The spectrum now obtained comprises of green and yellow lights who are respectively fringed with a very narrow streak of blue and orange colours.

Orange fringe Yellow Green     Blue fringe

Inference : Two green cellophane sheets are needed in order to let only the green and yellow lights to reach the plant being used in the main experiment.

7) A blue cellophane paper is now used instead of a red one as in procedure number 4.

Observation : The spectrum thus obtained consists of violet and blue colours the latter being fringed by a very narrow band of green light.

Orange fringe Blue       Violet

Inference : One blue cellophane sheet is enough to let only the blue and violet wavelengths of light to reach the plant being used in the main experiment.

Procedure :

1 ) Some hydrilla is in the cone of the funnel and it is inverted into a beaker filled with water in which some sodium bicarbonate is added. The narrow end of the funnel should be completely within the the water level in the beaker.Then a test tube is filled completely with water and a funnel is placed at its mouth to prevent the water from falling when it is inverted. The water filled test tue is inverted into the beaker of water so that its open mouth is well within the water surface. Then the funnel is removed from the mouth of the test tube and the latter is placed over the stem of the funnel, so that the stem of the funnel lies within the test tube.

2) The beaker is then wrapped with a red cellophane sheet so that just one layer of latter encloses the former.(There was no need to cover the top with red cellophane paper because it was winter season when the experiment was performed and the sun was always slightly towards the south. So the light that struk the hydrilla was the filtered light.)

3) At 6.30 am the whole apparatus is kept on the roof top where it is exposed to direct sunlight.

4) The test tube Is tapped gently a few times every half an hour so that it shakes the funnel also.This causes the oxygen bubbles sticking to the plant and to the inner wall of the funnel to rise inside the test tube and collect over the water in it (This is one of the pecautionary measures taken).

5) At 9.00 am the apparatus is brought inside a room and the test tube is tapped gently many times till no more bubbles appear to rise inside the test tube .Then the height of the oxygen column inside the test tube is measured .This measurement is done by placing the scale in such a way that its ‘0’ mark coincide with the lower minimum of the water level in the test tube and the marking which coincide with the closed flat end of the test tube is recorded.This marking represents the height of the oxygen coloumn.

(The closed end of the test tube is covered and in order to match it easy to measure the column of oxygen collected in the test tube the cover end has to be made flat.To do this that end is heated till the glass is soft.Then it is held vertically on a tube so that the closed end touches the table top.Then the test tube is pressed downwards till all the covered plants become flat.

6) At 11.00 am the apparatus is again set up as before using the same hydrilla but the water is changed and sodium bicarbonate same amount as before is also added.

7) Procedure : 4 is repeated and at 1.30 pm the apparatus is taken into the room and the test tube is tapped as usual.Then the height of the oxygen coloumn in the test tube is measured.

8) Similarly the apparatus is again set up at 2.30 pm and the oxygen is collected in the test tube by the usual way (tapping the test tube every half an hour)

9) Then at 5.00 pm the height of the oxygen column is again measured by the usual way.

10) Procedures 2 to 9 are repeated for two more days.Thus a reading of the height of the oxygen column , 3 for each times interval is obtained.

11) Procedures 2-10 is repeated but this time a green cellophane sheet is used instead of a red one and it is wrapped around the beaker in such a way that 2 layers of cellophane paper enclose the beaker.

12) Procedure 2-10 is repeated using a blue cellophane sheet instead of red one.

During the interval 6.30 am to 9 am the amount of oxygen liberated during photosynthesis under the blue- violet and red- orange parts of the solar spectrum are equal and are greater than that liberated during photosynthesis under the yellow-green part of the spectrum.This means that during this time interval almost equal photosynthesis is caused by the solar spectrum while it is caused the least by the green-yellow part of the spectrum.

During the time interval 11.00 am to 1.30 pm the most of oxygen is liberated during photosynthesis under the blue-violet part of the solar spectrum followed by the red-orange part and the yellow –green part liberated the least. This means that during this time interval the maximum photosynthesis is caused by the blue-violet part of the solar spectrum and the red-orange part comes next.

During the time interval 2.30 pm to 5.00 pm the maximum amount of oxygen is liberated during photosynthesis under the blue-violet part the solar spectrum followed closely by the red-orange part.This means that at this time interval the blue-violet part of the solar spectrum causes the maximum photosynthesis and the red-orange part comes next.

The experimental results are more or less in fact that in the experimental it was found that during the early hours of the day the red-orange part of the spectrum caused almost equal photosynthesis as the blue-violet part did where as the expected value is that the blue-violet part would cause slightly more photosynthesis than the red-orange part. This is because the eastern sky is full of smoke due to the presesnce of an iron and steel factory towards the east of the place where the experiment was performed and in the early morning a slight mist prevails in the sky, which takes some time to evaporate. This causes more hindrance to the short wave length blue-violet light to reach the earth’s surface.

So even though the blue-violet part has the more energy per quantum, the photosynthesis caused by that part of the spectrum dirung the early hours of the day is lesser than the expected value because the amount of it which reaches the plant is much less than thea amount of red-orange light which reches them.

The variation of photosynthesis from day to day is due to the increase or decrease in the brightness of the sun from day to day.

The principal source of error is that some bubbles of oxygen stick to the leaves even if the apparatus is shaken well.So the volume of the oxygen liberated can not be measured correctly.