In a perfectly simple world -- that is, a vacuum -- light travels in a straight line. (A vacuum here refers to space that has no material or pressure.) Light, however, zooms around in our complicated, imperfect Universe and that’s when things tend to get interesting.
When light encounters a transparent substance – like air or water or glass – it changes direction. (If light hits something solid, it can be blocked or bounce.) This change from one medium to another is akin to driving along a smooth road at a reasonable and proper speed of say, 60 mph. Gradually your car might drift to the right onto a smooth gravel shoulder. If your hands are not on the wheel to keep you steady, the car will veer sharply to the right because it slows down once it hits the gravel.
In the same way, light changes direction when it moves from one medium (such as air) to another (glass or water) because its speed changes. Wait a minute, you might say, we’ve already heard that light’s speed is fixed and never changes. Einstein told us so!
German-born physicist Albert Einstein (1879–1955) correctly predicted many things, including ideas from his theory of special relativity. Einstein’s fingerprints can be found on an incredible amount of today’s scientific achievements, including many in X-ray astronomy. Photo: Oren Jack Turner, Princeton, N.J.
Driving, Dancing & Visiting the Eye Doctor
In any given medium, light does travel at a fixed speed, which is the absolute maximum speed limit anything can travel. The catch is in the words, “in any given medium.” (Where the word “medium” refers to “substance” or “stuff.”) In general, light moves more slowly through a medium containing particles, than through a vacuum. This is because light can collide with the particles, and these collisions act like a friction that slows the light down – just like how the car slowed down when it went from smooth pavement to gravel due to the increased friction on the tires. [The speed of light in a vacuum is about 671 million miles per hour. The speed of light in water, for example, is about ¾ that in a vacuum.]
The slowing down of light is like the movement of a very popular dancer trying to make her way across a crowded dance floor. Even though she is moving at the same speed between partners, the encounters with the many partners who want to twirl around with her slow her down. In contrast, when the dancer reaches a part of the dance floor that is less crowded, she will move faster.
The change of light’s speed in going from one medium to another has the consequence that the path that light takes can bend, just like the car we were driving in our analogy. The bending of light is critical to our existence. If our eyes didn’t bend light to make images, the world would be an incomprehensible blur. Though, for many of us, our eyes don’t do the job perfectly, so we wear eyeglasses or contacts to precisely control how light’s path is bent.
When the path of a light ray is bent, the image of the light source becomes distorted. For example, light paths can be bent through the lenses of eyeglasses. Photo: Hackfish, CC BY-SA 3.0
From Sunsets to Rainbows
Have you ever noticed that the Sun looks like it’s a little squashed as it gets near the horizon during the sunset? The particles in the Earth’s atmosphere bend light as it travels through it. Since sunlight must pass through more of the atmosphere during sunset – think of cutting through an orange rind – then it gets bent more.
The Sun is a sphere, not the flattened, oval shape that appears in this picture. The distortion happens because the Earth's atmosphere is acting as a lens. Light from the bottom of the Sun is being bent more than from the top because the light must pass through more of the atmosphere the closer we look to the horizon. The effect is that the apparent location of the bottom of the Sun is raised more than the top, making the whole Sun look oval-shaped. U.S. Navy photo by Cmdr. Ed Thompson
Or rainbows? Light is bent by raindrops, with each color bent a slightly different amount.
A beautiful example of refraction. The rays of Sunlight are bent by passing through droplets of water in the air, separating out into different colors. The light is also reflected and spreads out. When we see a rainbow, the Sun is behind us, so we are seeing the light from the direction of the water droplets.
Photo: Lisa & Jeffrey Smith
The story of how light bends doesn’t end with eyeglasses, sunsets, and rainbows. We’re talking about what happens when a straight line isn’t a straight line anymore. One of Einstein’s greatest accomplishments was proving that space itself can be altered, or curved, by an object with gravity. The effects of Earth’s gravity on “space-time” is not entirely negligible for our lives – the GPS (Global Positioning Systems) that we rely on to navigate to a favorite restaurant or other locale -- must take it into account or we would get lost (though it’s possible to get lost even with these devices!).
Next, in part 2: Beyond Earth