 abiotic factors, the light, first the contents, photo period, effect of photo period, circadian rhythms in human, circadian rhythms in other animals, and the photo-tactic responses. So first the photo period, so friends actually here we are discussing the abiotic factor of ecosystem, the ecosystem called abiotic factors, one of them is the light. So light is also effecting the living organism or the life present in the ecosystem, the living organism which is present in the ecosystem, the other abiotic factors will effect the light. So normally the duration of light in 24 hours, day is called the photo period, in 24 hours day the time of light available is called the photo period, this means that in winter the duration, day light comes down, so we say photo period is short, similarly during summer day light is more, so we say photo period is long, so photo period is actually the time period or the time span during which sunlight is available. So photo period is actually the index for the different times or different seasons or for different climatic changes, so photo period actually represents different seasons or they are closely related to the change in seasons, different seasonal changes, so what is the effect of photo periods? They are accurate index, index means that this is an indication, this is an indication that climate or the environment is going to change. So what is the effect of photo periods, the effect of photo periods is simply if we look at the summer or the spring season, summer is spring, so during spring season automatically the photo periods start increasing, so photo period or day length will start increasing this is actually the indication for animals that the favourable conditions are going to come, its result is that as photo period is increasing the animal will start breeding, animals will start breeding, similarly the other effect of photo period is that during the autumn, because autumn is winter, so during autumn the photo period starts squeezing, so photo period starts decreasing, as photo period starts decreasing the animals, they are aware that the next conditions are very severe, so animal will start piling up with their food, animals will start piling up with their food and they will migrate to some other places or they will migrate from there and go to another place, so it means photo period has direct link to the daily life or the activities of the animals. So the other term used with reference to light is the circadian rhythms, circadian rhythms normally they are actually the daily activities in the 24 hour cycle, during 24 hour cycle or 24 hour day we perform activities on daily basis in the rhythm, in the sequence they are called circadian rhythms, in circadian rhythms there are more important things, like the important thing, in circadian rhythms we have a sleep wake cycle, as soon as the sun sets, what will happen automatically because we start sleeping at a specific time, the sun rises and wakes up, so this sleep wake cycle is in our circadian rhythm, it is a daily activity in the 24 hour cycle, normally if the photo period changes, it will affect our circadian rhythms, so it will affect our circadian rhythms, our daily life activities will affect them, how? We are staying here, the sun light starts, if we see the summer, during summer the sunrise is about 5 pm, the sunrise is at 5 pm, if we migrate from here to the world, we go to America, Australia or Europe, the sunrise time there is different, now the reason is that our routine is a sleep wake cycle, our environmental cycle is set, when we go to a new photo period, the environmental condition there is different, what will happen that when we don't get proper sleep conditions, sleep or sleep, the reason is that the photo period is different, so this difference is also called the jet lag, so the change in the photo period affects our daily activities, which is called the jet lag in the case of a human, similarly the photo period is also responsible for setting our biological clock, the photo period also sets our biological clock, how? As the photo period changes, our routine of daily activities changes, normally if a person wakes up in the morning with sunrise, or half an hour before sunrise, like if the sun is coming out at 5 pm, it is 4.30 pm, now in winter the same condition is that sunrise goes from 7 am to 7 pm, so from 5 am to 7 am, now if sunrise is at 7 pm, then the sleep wake cycle, the time of waking up, so this also means that the change in the photo period changes along with the activities that are coordinated, so the internal biological clock is set for the photo period, for animals, in the case of animals, the circadian rhythms of the animals are related to the photo period, that is the daily activities of the animals they are also related to the light, for example the animals, if we talk about water bodies of animals, there are a lot of animals that when there is sunlight for feeding, they come on the upper column and they move to the downward, so if the water is above and below, they move in different layers with reference to the light, so this means that their daily activities, suppose an animal has to feed in the morning, so what will happen as soon as the photo period starts, it will feed on the top, then when the peak day is possible, it will go back to the bottom, similarly the feeding animals of the animals, the dawn and dusk time of feeding, that is the daily activities are related to the photo periods, similarly the terrestrial animals, their circadian rhythms, daily activities mean circadian rhythms, so circadian rhythms are linked to the photo period, how is it linked to the photo period, there are a lot of nocturnal animals, so when the time of night in the photo period is not available, when the photo period is not going on, it will be dark, they will feed, there are a lot of animals that are diurnal, they feed only when there is sunlight, so what happens when they feed only when the sunlight is available, so the animals have different daily activities, they are set by the photo period, daily activities mean circadian rhythms, which means human and other animals daily activities are circadian rhythms, they are actually, they are set by the photo period. Usually the activities that we do on a special time, we are not told, but despite that we are told, like if you have breakfast at 7 o'clock in the morning, so as soon as it is 7 o'clock, you start feeling hungry, similarly when you have to sleep, you are already asleep, then you are feeling sleepy, this means that our internal system also has a clock that switches us on at that time, knocks us on, the internal body clock is called the biological clock, it is called the biological clock, photo, photographic photographic response, students who are animals with response to the intensity of light, the intensity of light, we do the response, usually when the intensity of light is more, in that case the animals go away from their position quickly, so in this case we have, like the protozoa, protozoa are usually in the dim light, and the intensity of light changes their position, so such animals which avoid the intense light, they are called the negatively photo tactic, it means for light, it affects the habitat of different animals where the light intensity is more, their habitat will not be there.