Light is a form of electromagnetic radiation, and when white light passes through a prism it separates into a range of colors—violet, indigo, blue, green, yellow, orange, and red. These are the colors of a rainbow and they are made in the same way, because raindrops act as prisms. When light crosses from one medium, such as air, to another, such as water, its speed and direction change. The light bends. This is called refraction. Light striking a raindrop is refracted as it enters the drop, crossing from air to
The spectrum and the rainbow
water. It then strikes the rear of the drop and is reflected. As it leaves the drop it is refracted for a second time. The degree by which radiation is refracted varies according to the wavelength. White light is composed of radiation with a number of wavelengths, and each of these is refracted by a different amount. That is how the white light comes to be separated into its constituent wavelengths. We see each wavelength as a different color. Red light emerges from a raindrop at an angle of approximately 42° to the incident sunlight; violet light emerges at 40°; and the other colors emerge at angles between 40° and 42°. When raindrops are refracting and reflecting light in this way, if you stand with your back to the Sun you will see a band of red light at 42° to the sunlight in every direction and bands of the other colors at other angles down to 40°. Consequently, the red light will form an arch with the other colors below it and violet on the inside. The diagram illustrates what happens.
Although every raindrop breaks light into all of its colors, we see only one color from a particular raindrop. If light of one color reaches our eyes, the other colors will either pass above us or disappear onto the ground before they reach us. There are so many raindrops, however, that enough light of each color reaches us for the complete rainbow to be visible.
Sometimes a secondary rainbow is visible above the primary bow and with the colors reversed. This happens when light is reflected twice inside the raindrops. The double reflection results in the red light being at the bottom of the bow and the violet light at the top.
The bands of colors produced by a prism are called a spectrum. They demonstrate that white sunlight comprises radiation at a number of wavelengths. We see the light as white when all the wavelengths reach our eyes together. When the colors of light are mixed together they form white. Rainbow
Was this article helpful?