Monday, November 10, 2008

Properties of light - 2

I almost can't show to you how my fan is rotating. Here is a photo of a rotating fan:
It looks stationary!
So I changed the setting on my camera from auto to night mode:
Night mode has lot of light gathering due to large aperture and longer exposure. So you see the blurred image, and because of flash reflection, you see the 3 pans too. Two bright lights are tubelights.
And when I put it on fireworks mode:
Its a very long exposure mode. Its more or less what my eye also sees. So what our eyes see and what a camera sees are different worlds! Here is a video of the same fan. Which way is it rotating?


In the video, it apears to rotate in the opposite way than in reality! This can be seen by switching off the fan:


!!! ??? So how come this illusion!!! ???
You may have taken photos like this one:
Now as a traveller, your eyes won't see such an image, wherever you look, you would see things pretty much clear only( Unless you are travelling in some super fast train!).

Whether eye or camera, each observing instrument has a characteristic exposure time, the time for which the instrument gathers light information, stores it, and then takes next information to process.
This also shows how versatile our eyes are! They are very powerful cameras!
The opposite motion of fan is due to the frequecy of camera capturing the frames of video. If it was a single pan rotating, the video would be different. It would maintain the same sense of rotation. Here, three pans replace each others' positions during the frame re-capture time, and it is done with a lag, so gives a sense of opposite motion than real.

The time taken by eye to take next frame is called Persistance of Vision. Long exposure photos often show nice slow motion, as people observe in motion of stars:

Of course, the stars appear to rotate because we are rotating!

Anytime the exposure is longer enough than the motion, we get a streak photo, as seen in the fireworks here:

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