Light comes in different colors, and there are kinds of light that our eyes cannot see. The physical origin of this difference is that the waves of light can have differing lengths. Even though the wavelength is very small, it does give rise to observable phenomena.

For example, the sky at noon is blue - why not green or red or yellow?  The sky overhead is blue because air contains very tiny dust particles that are even smaller than light waves.  Of the many wavelengths of visible light coming from the sun, blue light has the shorter wavelength.  The small dust particles have a much bigger effect on the short wavelength light, just as bicycles are more affected by a bump in the road than are buses. The result is that more blue light is scattered than is the longer green and red wavelengths.  When the blue light bumps into the dust, it scatters in all directions, some of it reaching our eyes.  The red and green light on the other hand do not so easily scattered by the dust, so they continue straight on through, not scattering to reach our eyes. The sky is red at dawn and sunset because the blue light has been scattered out to make a blue sky for someone else!

The finite size of the wavelength of light becomes important when you study things as small as bacteria -- they are hardly bigger than the light waves, and this means that our view of them must always be fuzzy. In everyday life we deal mainly with objects that are much larger than the wavelength of light, and then light acts like a stream of tiny bullets that travel in straight lines. This justifies the drawings we were making to study shadows, and we will continue to draw lines to represent the path of light beams when we discuss mirrors and lenses. However, a diffraction grating works because light is a wave.
Diffraction gratings