Why the image in a mirror is behind the mirror?

Have you ever observed birds or animals encountering a mirror? They misidentify their image as someone looking at them from the backside of the mirror. If possible, they run around the mirror to verify whether someone is there! Of course, they find nothing there and get puzzled by this behaviour. Such an image is called virtual image as opposed to the real image as in the case of a camera lens. When the lens of the camera forms an image of an object, the image is real. That’s why we can put a photographic plate or sensor at the image location and actually record the image.

Suppose the object is in the front of the mirror at a distance of, say, five meters. The image of the object will be behind the mirror at a distance of five meters. This is because the light travels in a straight line and reflects back when hits a mirror. The light rays from the object reflect back from the mirror and our eyes detect them. Yet, our mind doesn’t interpret the whole event this way. The mind creates a perception in which the light rays travelled from the other side of the mirror toward our eyes. Hence, the rays appear to form an image behind the mirror.

But, sometimes we have to ignore the perception created by the mind. Instead, we need to reconstruct the location of the original objects by looking in the mirror. This is a difficult process requiring conscious thinking. One simple example is cutting our own hair by looking into the mirror. You will notice how difficult is it to judge the location of the hair and the scissors. The problem doesn’t arise in combing the hair. We have combed our hair for such a long time that our minds have adjusted to this reconstruction in this case. Another example is driving your car backward by looking in the mirrors.

My dear scientists, if you have understood this, answer this question. Can you find why the image formed in the mirror is at the same distance from the mirror as the object?

Can you explain why summers are hot and winters are cold?

A documentary filmmaker went to Harvard’s 1987 graduation ceremony and asked the students the science behind the seasons. Only two out of 23 were able to give a correct explanation. Yet, they all have studied about the seasons in their schools. The lesson of the story is that an educated person is not necessarily scientific.

Sun shines with equal power throughout the year. Yet, it doesn’t mean that the intensity on the surface of the earth is also same throughout the year. To understand this, consider the following analogy.

Imagine that a spherical ball is spinning along its axis in the vertical direction. Imagine a light bulb few meters away from the ball. The light of the bulb falls normally in the equatorial region of the ball. But, it falls tangentially on the regions near the poles. Hence, the equator will be brighter (hotter) than the poles.

What happens if the axis of rotation of the ball is not vertical but inclined towards the bulb? The northern hemisphere of the ball tilts towards the bulb. So, in the northern hemisphere of the ball, most of the light rays hit almost normally. In the southern hemisphere of the ball, most of the light rays will hit only tangentially. This makes the northern hemisphere hotter than the southern hemisphere.

Now, suppose the ball starts revolving around the bulb on a circular path.  The inclination of the axis of rotation remains unchanged during the revolution. After the half rotation, the southern hemisphere of the ball tillts toward the bulb. Hence, it is hotter than the northern hemisphere.

That’s exactly what happens during the summer. The earth is like the tilted ball and the bulb is like the Sun. It takes one year for the earth to complete one revolution around the Sun. For half of the revolution, the northern hemisphere tilts towards the Sun. This is the time when there is summer in the northern hemisphere and winter in the southern hemisphere. For the other half of revolution, it is the other way around.

My dear scientists, if you have understood this, can you answer why the inclination of the earth’s axis remains unchanged? What will happen if there is a slow  change in the inclination of the axis of the earth like that in a revolving top?

How far away is the thunderstorm?

Have you ever observed carefully a thunderstorm and accompanied lightning in the sky?

If yes, you must have noticed that first you see the lightning and then the thunderstorm. Of course, the sound of the thunderstorm and the lightening are produced simultaneously amongst the clouds. Yet, the light travels almost instantaneously to us while the sound travels with a speed of about 330 meters per second. So, if the sound takes 2 seconds to reach us, it means that the thunderstorm is 660 meters from us. If the sound takes six seconds, the thunderstorm is 1980 meters away from us.

My dear scientists, if you have understood this, can you tell why the time interval between the sight of lightning and the sound of a thunderstorm can never be greater than about 10 seconds?

Chai and Whi?

I was trying to find out whether other people have played with the idea of discussing science with a cup of tea. I was amazed to discover that Tata Institute of Fundamental Research (TIFR), Mumbai has been doing this the last six years. The public outreach program has been named “Chai and Why?” (tea is called chai in many indian language). They are sharing the science done in TIFR with the general public under this probram in form of the public talks that can be viewed at their youtube channel.

My aim of writing this blog is much narrower in scope. I will focus on writing about the topics of general science that can be understood by a high school student.

What makes the sky blue and the setting sun red?

Light travels in space just like the waves travel on the sea, rising and falling periodically.  The distance between two successive crests or troughs is called the wavelength.

Imagine a boat floating on the sea. When the waves hit it, they lose track of their original path and scatter all around the boat. Smaller the wavelength, larger will be the scattering.

Unlike waves on the surface of the water, we can’t see the crest and troughs of the light waves. so, we can’t find their wavelength the way we do it for the water waves. Our eyes perceive the wavelength of the light waves as colour. Blue light has a smaller wavelength than the red light.

The white light of the Sun contains the waves of all colours. When the sunlight comes toward Earth and enters its atmosphere, it encounters numerous dust particles. The light waves get scattered all around just like water waves were get scattered by the boat. Smaller wavelengths scatter more as compared to the larger wavelengths. So, the blue light gets scattered all around making the sky blue. What happens to red light? It manages to come directly towards us making the Sun appears red.

My dear scientists, if you have understood this, can you tell why the Sun is red only at sunrise and sunset and not always.

Things do not have color: it’s all our perception

Have you ever wondered what color is? Is it in the object you see? Or, is it in the light that makes it possible to see? Or, is it in the eyes of someone who is looking at the object?

These questions are not philosophical. They are scientific questions found in Science textbooks!

The color is a perception of the mind. The objects do not have any color, neither do the light. We do not perceive color even in our retina. Retina detects color but not perceive it. It is not generated even by the sensitive nerve cells that send the visual signals of color to our brain. We perceive color when our brain processes these signals.

If you are not convinced, google about some optical illusions about color. You will see for yourself how your mind can see where there is no color at all. You may get surprised when your mind mistakes one color for another.

My dear scientists, can you explain some of these popular color illusions?