in ultraviolet light but are not noticeable as any sort of color difference
in the visible region of the spectrum. These patterns are sometimes
called nectar guides and are believed to act as visual features that at-
tract bees and other pollinating insects and birds to them.
Ultraviolet radiation is slightly higher in energy than visible light.
The other direction on the electromagnetic spectrum, with radiation
slightly lower in energy than visible light, is the infrared region.
Infrared radiation also is not visible to us—its energy is too low to
activate the photoreceptors in the eye. However, infrared radiation is
absorbed by many molecules in such a way as to set them vibrating.
This molecular vibratory motion, if it is strong enough, may be expe-
rienced as heat.
Figure 11.5. African daisy (Dimorphotheca aurantiaca) photographed using an
ordinary glass lens (left) and a lens made of quartz, which allows ultraviolet
light to pass through (right). I took these photographs when I was a graduate
student at Caltech using a Hasselblad camera, a duplicate of the one used by
NASA astronauts on lunar landing missions. The camera was kindly loaned to
me by folks at NASA’s Jet Propulsion Laboratory.There is a group of snakes, the pit vipers (rattlesnakes are in this