Stare at a bright source of light behind a star-shaped cutout on a piece of cardboard for a minute in a dimly lit room. Now turn on the room lights and look at your hand … and then look at the wall. What happens to that dark star?

We have been studying the function of the eye in the context of lenses and geometrical optics and the biology of the visual processing system to unravel questions like those above. The students have been immersing themselves in activities that make analogies between the eye and the camera as well as probing their visual interpretation of images through optical illusions. In the process, they are learning about photography from its earliest days and the relationship between film and digital imaging sensors, and exploring how these extend to the eye.

While the eye lens and retina have a direct analogy with the camera, our class discovered that the dilation of the pupils in dim light couldn’t account for the enormous range of intensity our eye can detect, a job that is done by the rods and cones that act as our photoreceptors. They have recounted stories about stumbling into seats at movie theaters, and have found out why. They have recalled that they have sometimes been woken up by someone switching on the light, and have shut one eye. To their delightful surprise, they could see perfectly well in the dark with the eye that remained shut when the lights were shut off again, but not the other eye! One "aha" moment followed another when one of the students asked if that was why pirates wear eye patches.

Our exploration of the camera also led the students to ask what the role of the camera shutter plays in the eye, which remains open continuously, expect for involuntary blinking. This led to the next stage of discussion related to the network of cells that transmit information to the brain, and how it is pre-processed at the retinal level. We found out what it was like to experiment with different video frame rates and what effect tweaking the shutter speed on a video camera had on our perception. They also explored the phenomena of lateral inhibition, where contrasting edges are sharpened, and how this not only compresses transmissions to the brain, but also sends information about what changes, and the biological importance of this. The students were given different gray-scale images, and asked to graph how they perceived the brightness to vary, with surprising results they could explain using the idea of lateral inhibition.

We recently explored retention of images in our eye through positive afterimages and had an invited speaker--one of our high school students--who gave a talk on the 3D zoetrope she is building for Project Studio. The last weeks of the block will be spent working on interesting projects the students have identified and will be fabricating. A few will involve building exhibits in the spirit of the Exploratorium, which publishes their designs. Yet others will extend ideas on persistence of vision by programming LED arrays. We are all looking forward to a noisy lab with whirring machinery soon bringing the students' ideas to fruition.

-- Kaushik Basu