 Have a look at these two images. Can you tell which bulb which light source is brighter or are they of the same brightness? Take around five seconds and and think of which which bulb you think is glowing brighter. Alright, well if you're like me when I first saw these images, I thought the left one, the left one is glowing brighter, right? But turns out both of them are of the same brightness. Both of them are glowing with the same brightness. Now, how could that be? Why do we perceive the first light source to be glowing brighter than the second light source? Turns out the key is in the amount of light present in the environment. The same light source appears brighter to us in the dimly lit environment. That is the first image. The room is dimly lit. There is not enough daylight coming in. But in a room with abundant of daylight coming in, that is the second image, the light source does not seem that bright, right? It seems a little dim. How could that be really? Does that mean that we cannot trust our eyes? Well, turns out, yes, you cannot really trust your senses. Our eyes are adapted to the lighting conditions in each room. And so our perception of the brightness of the light source is relative to the surrounding environment. So the surrounding environment in the first case is dimly lit. So it seems to us that the light source is glowing brighter and the lighting conditions in the second the second image The room is very well lit. There is a lot of daylight coming in. So it seems to us that the light source is not glowing as bright as in the first image. The light source in the second image is a little dim. But in reality, both of them are glowing with the same brightness. So we can't entirely trust our eyes. Let's let's take one more example. Here I have three glasses of water. The glass on the left. This one has cold water. You can see some ice cubes. The glass on the right. This one has warm water and the glass in the middle. This one has room temperature or normal water. Now what I'll do is I will put the index finger of my left hand in the cold water that is on the left and the index finger of the right hand in the warm water on the right. And I will place my fingers in those glasses of water for around two minutes. Then I will place both of these fingers, left hand and right hand in the glass of water in the middle like this. Now what I sense is what I perceive is that my left hand, the index finger of my left hand is telling me that the water in the middle is warm. And the index finger of the right hand is telling me that the water in the middle is cold. So this experience of heat that is hot tense or cold tense of an object through the sense in this case a sense of touch is not completely reliable, right? Let's compare this with the images that we saw in the beginning. The blue dot at the top shows cold hand and the red dot at the top shows hot hand. Okay, so if you look at this image, the room was very dimly lit. So the light source appeared brighter to us just like the left hand is very cold. So the water in the middle up, it seems warmer to us. And if you look at this image, the room was very well lit. There's a lot of daylight coming in. So the light source appeared dimmer to us just like when our hand was hot, the water in the middle seems colder to us. So turns out we cannot even completely rely on a sense of touch. Then how can we ever measure anything? How can we ever know if an object is hot or cold? Is there a way to do that? Think about what do you do when you get a fever? I'm sure all of us have used a thermometer, right? A thermometer that might look like this. And we use it to measure if our body temperature has gotten a little high. So the exact measure of coldness or hotness of an object is expressed by a physical quantity which is called temperature. And to measure this temperature, a thermometer is used. So this is a clinical thermometer that we can use in our homes. Now we can see the markings on this thermometer. It goes from 35 to around 42. We can see those markings. And the unit of these markings is degree Celsius. So the numbers that you see that will be 35 degree Celsius, 36 degree Celsius, 37 degree Celsius. Our body temperature is 37 degree Celsius. But this clinical thermometer does not cover a wide range, right? It only starts from 35 goes to 42. And for laboratory purposes, you need something like a laboratory thermometer, which has a very wide scale, very wide range. So a laboratory thermometer, it starts from zero degree Celsius. I hope you can see the marking. It starts from zero and goes still 100 or 110 degrees Celsius. Yeah, now it's visible. So it starts from zero and goes still 100. Okay, now we will measure the temperature of the water in these three glasses. And for that, we will use something like this. This is a digital thermometer where you can get the reading digitally. You will actually just see the exact reading digitally. You will be able to tell how hot or cold an object is. So to start with, let's measure the temperature of the water in the first glass, which is a cold water. So it will take some time to arrive at a reading. And when it stops changing, that should be the temperature of the water in that glass. So it's around 15.5 degrees Celsius. Okay, now let's put the thermometer in the warm water. Okay, so it's around 44.3 degrees Celsius. Now let's measure the temperature of the normal water that was kept in between. So this comes to be around 31 degrees Celsius. So in this activity, we saw that in order to exactly measure the coldness or hotness of an object, which is called a temperature, you need an instrument like a thermometer. You cannot rely completely on your senses.