The Nature of Light

Light is a transverse electromagnetic (EM) wave — which basically means that light consists of ripples in electric and magnetic fields. Here is an animation to help you visualize this.We'll discuss this in more detail after we have studied electricity and magnetism.

Unlike sound, light can travel through a vacuum — it doesn't need a medium.It can travel through transparent materials such as glass, water, and air. (A material light can't travel through is said to be opaque.)

In a vacuum, light travels with speed 3.00 x 10⁸ m/sec.This speed is given the symbol c. In a transparent medium, light slows down to a velocity

where n is the index of refraction of that particular material. Shown here is a table of the indices of refraction for some transparent substances.

substance	n
diamond	2.42
flint glass	1.66
crown glass	1.52
water	1.33
ice	1.31
air (@STP)	1.00029

Example As an example, take water. In water, light moves at a speed of

Activities & Practice
to do as you read

What is transparent and what is opaque? That depends. See in this video.

Here's an animation showing wavefronts of light in air entering some transparent material where its speed is cut in half.

The Spectrum The equation is valid for light, just as it is for all waves. Different colors of light have different wavelengths. The range of wavelengths humans can see goes from 700 nm (the deepest red) down to 400 nm (violet). The colors we can see, in order from long to short wavelengths, are red, orange, yellow, green, blue, indigo and violet. You can remember this order by the following mnemonic: think of the imaginary man's name Roy G. Biv. Together, all these colors make up the visible spectrum. Here's a song about the visible spectrum, with a different way to remember the colors.

There are also "colors" with wavelengths longer than red and shorter than violet. The electromagnetic waves longer than red are infrared (IR) and radio. And at the short end, we have ultraviolet (UV), x-rays and gamma rays. Our (human) eyes are not sensitive to these wavelengths, but just because we can't directly see them doesn't make them any less real. Some species of snakes can detect infrared radiation.

Together, all these kinds of electromagnetic radiation make up the electromagnetic spectrum. The various kinds of EM radiation differ not just in their wavelengths, but also frequency and energy. The shortest wavelengths (x-rays and gamma rays) have the highest frequencies and also the highest energies. That's why x-rays are used to take pictures of bones: they can penetrate flesh and show bones as shadows on photographic film.

Here's another song, this one about the entire EM spectrum.

The above diagram shows a "side-view" of the light waves: they are sine curves. We don't usually draw light like that, though, because it's too hard. Instead we draw a "top view". The lines representing the crests are called wavefronts. Think about ripples on a pond. Rays are arrows representing the direction of propagation of the wavefronts. Rays are always perpendicular to the wavefronts.

Filters & Pigments Many molecules absorb (subtract) particular wavelengths, or ranges of wavelengths, but don't affect others. We call these molecules pigments. This process is what gives objects around us what we usually think of as their "color". For example, the leaves of most plants contain chlorophyll, a molecule that absorbs most of the blue and violet that hits it, and also the red end of the visible spectrum. The middle part of the visible spectrum (i.e. green) is NOT absorbed. It is reflected instead, Since the green light is what gets to our eyes, we think of leaves as green. I qualified that last sentence oddly, because the color we perceive an object to be depends not only on what pigments are present, but also what colors are hitting it and therefore available to be reflected. For instance, if we shine only blue light onto a plant, it will appear black: the blue light is absorbed by the chlorophyll, and there isn't any green light to be reflected to our eyes.

Paints are made by mixing pigments into a base, like oil or latex. If you mix pigments into glass or plastic, so the light shines through instead of only reflecting off, you've made a filter.

If you combine pigments (like mixing red paint and yellow paint) the combination will absorb all the wavelengths either one of the individual pigments absorbed. Take the red and yellow paint example. Red paint reflects red, but also a large percentage of the orange. The other colors are mostly absorbed. Yellow paint reflects yellow, but also some of the orange and green. What's the only color that is relected by both red and yellow paint: orange. Mixing pigments like this is subtractive color mixing.

Additional Activities & Practice

1. The microwaves used in microwave ovens are a particular wavelength of radio wave, about 1 centimeter. What is the speed, frequency, and period of those waves.

2. (a) Calculate how far light travels in one year. (This distance is called a lightyear.)
(b) The diameter of Earth is 12700 km. How many times bigger is a lightyear compared to the Earth's diameter?

3. What do you think is the index of refraction for a vacuum?

4. From the table of index of refraction (above), in what material is the speed of light the slowest?

5. How fast does light travel in crown glass?

6. Most airplanes have a Emergency Locator Transmitter (ELT) that serves as a radio beacon in case the plane crashes in a remote region. Rescue teams can home in on the beacon to find the crash site and any survivors. ELTs broadcast on the frequency 121.5 MHz. (a) What is the period of one oscillation of the radio wave? (b) What is the wavelength?

7. If you mix blue paint with yellow paint, what will be the resulting color? Explain why.

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