Sat, Dec 06, 2008 - Page 13 News List

WEEKEND SCIENCE: Experimenting with afterimages 後像實驗

This April 25, 2007 photo shows a human eye.


If you have ever stared at a light bulb for more than a few seconds, you may have noticed that you could still see an outline of the image for a few seconds after you stopped looking it. The outline you saw is known as an afterimage, or ghost image. In today’s experiment we will take a closer look at afterimages and learn about the features of the eye that cause them.

What you will need: a flashlight, some cardboard and some adhesive tape.(JOHN PHILLIPS, STAFF WRITER)





Step 1: Use the cardboard to block out most of the light from the flashlight. The remainder of the light should make a recognizable shape, like a square, triangle or star.

Step 2: Stare at the flashlight for about thirty seconds. Make sure you use a flashlight with low luminosity, otherwise you could damage your eyes.

Step 3: Look at a wall. You should notice that the afterimage persists for a few seconds.

Step 4: Hold up your hand and look at it. You will see the afterimage on your hand and it will appear larger than when it was on the wall.

Step 5: Cover your left eye and stare at the flashlight with your right eye. Uncover your left eye and look at the wall and you will see the afterimage. However, when you cover your right eye you won’t see an afterimage.








Your retina works in the same way as photographic paper. When it is exposed to light, the light creates an image on it. This image is then relayed to the brain. When you stare at a bright light for a long time, the photosensitive cells in your retina desensitize to the brightness and stop working as efficiently. When you look at the wall, the desensitized cells create a temporary silhouette. After a few seconds, they readjust and you see the wall normally.


1. afterimage n.

後像 (hou4 xiang4)

2. recognizable adj.

可辨認的 (ke3 bian4 ren4 de5)

3. persist v.i./v.t.

存留(cun2 liu2)

4. photographic adj.

攝影的 (she4 ying3 de5)

5. retina n.

視網膜 (shi4 wang3 mo2)

6. silhouette n.

輪廓 (lun2 kuo4),剪影 (jian2 ying3)

In step 5, the reason you could only see the afterimage in the uncovered eye is because it occurs at the point of input (the retina) and not the point of process (the brain).




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