Light is electromagnetic energy that is emitted in the form of waves. The visual system begins with the eyes. At the back of the eye is the retina which contains photoreceptors specialized to covert light energy into neural activity. The output of the retina is not a reproduction of the intensity of the light falling on it but is specialized to detect differences in the intensity of light falling on different parts of it. The first synaptic relay in the pathway that serves visual perception occurs in a cell group of the dorsal thalamus called the lateral geniculate nucleus (lgn). Lgn, visual information ascends to the cerebral cortex where it is interpreted and remembered. Wavelength: the distance between successive peaks or troughs. Frequency: the number of waves per second. Amplitude: the difference between wave trough and peak. The energy content of electromagnetic radiation is proportional to its frequency [i. e. high frequency (short wavelengths) has the highest energy content].