Pin Image and Shadow
Here is a ray tracing diagram showing how a pin held close to the eye can produce a fuzzy image of the pin which appears right-side-up and a sharper shadow of the pin which appears inverted.
The pin is shown in black at the left. The pin shadow on the retina is in red, the pin image on the right is in blue.
Light rays scatter from the top of the pin, they spread out and are bent by the eye to form an image of the pin shown in blue behind the retina. The image of the pin on the retina is fuzzy since the black rays do not come together at one point on the retina. In this case the pin is so close that the eye cannot create an image of the pin on the retina.
A single light ray from the bottom of the pin is shown in blue. This shows that the image of the pin is inverted. The brain also inverts this image creating a right side up impression of the fuzzy silvery pin.
A point of light, such as a minimaglight is shown in red.
Light from the point spreads out and is shown in red. This creates a shadow of the pin on the retina, which is shown as a red pin. This shadow is right-side-up.
The brain invertys the shadow and so the shadow of the pin is seen upside down.
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25 June 2002