Worlds of David Darling > Children's
Encyclopedia of Science > Making Light Work > Chapter 4
MAKING LIGHT WORK:
The Science of Optics
a book in the eXperiment! series by David Darling
4. When Light Bends
Spear fishermen use a strange trick to catch their prey. They aim at a point
slightly below where the fish seems to be.
The reason is that light changes direction when it moves from one transparent
substance, such as water, to another of different density, such as air.
This change of direction is called REFRACTION.
You can find many examples of refraction in everyday life. A bar of soap
at the bottom of a bath appears to be higher than it really is. A pencil
seen through the side of a water-filled jar seems to be broken. And a riverbed,
seen from the bank, looks shallower than its true depth.
Refraction in Action
You will need:
- The beam tank
- A 12" ruler
What to do:
Lay down the ruler so that it extends from the light source to the
base of the holed card. Line up the ruler with the center of the incoming
First, arrange for the light to come in at right angles to the card.
Look down on the beam tank. Has anything happened to the direction
of the light as the beam goes from air to water?
Now move the light source around to one side slightly so that the
incoming beam is still pointing at the holes in the card but is no
longer square on the card. Again, place the ruler so that it lies
exactly along the center of the incoming beam. Look closely at the
direction of the light after it has entered the water. What do you
A Change of Course
Light travels faster through air than it does through a denser substance,
such as water or glass. So, when light rays travel through air and meet
the surface of the denser substance, they are suddenly slowed down. When
this happens, the light rays bend.
|A straw in water appears bent because of
You can see a similar effect if you throw a stone at a slant into water.
As it leaves the thinner air and enters the thicker water, it slows and
is dragged on a more downward path. The only exception to this is when the
incoming rays (or stone) meet the surface at right angles. In this case,
there is no change of direction.
On a hot day, you can sometimes see what looks like a pool of water on the
ground – even though the ground is completely dry. In fact, the "pool"
is caused by refracted light from the sky. The air close to the ground becomes
hotter and, therefore, less dense than the air higher up. This change in
density causes light from the sky to be bent upward so that it reaches your
eyes from the direction of the ground. The effect is called a MIRAGE and
is common on hot roads and in deserts. As well as the sky, mirages of distant
objects on the horizon such as mountains may be seen.
Refraction can be put to good use by shaping pieces of glass so that one
or both sides are smoothly curved. These curved pieces of glass are called
LENSES. As light passes through them it is bent in a special way, following
a certain order.
|How convex and concave lenses bend light
as it passes through them
Just as there are convex and concave mirrors, so there are convex and concave
lenses. In a convex lens, the glass bulges out from the edge to the center,
while in a concave lens it curves the other way.
You will need:
- A convex lens, such as that in a magnifying glass
- A concave lens, such as those on spectacles of people who cannot
see distant objects clearly
- A 12" ruler
- A piece of white card
- The beam tank
What to do:
Hold the convex lens a few inches above the words on this page. Do
the words look bigger, smaller, or the same size? Are they upright
or not? Measure how far the lens can be placed from the page before
the letters start to become blurred. Call this measurement A.
Now put the convex lens upright about six feet away from a source
of light such as an electric light bulb. Let the light that passes
through the lens fall onto the white card. Move the card (or the lens)
until you obtain a small, sharp image of the light source on the card.
Measure the distance from the lens to the card. This distance is known
as the focal length of the lens. Call it measurement B. Compare measurements
A and B. Which is greater?
Hold the convex lens at armís length and look at a distant object.
What do you observe? Does the object look large, smaller, or the same
size? Which way up is it?
Now take the concave lens. Hold it a few inches from this page. What
effect does it have on the words? What happens as you move it farther
and farther from the page? Do the words eventually become blurred
as they did with the convex lens? Hold the concave lens at armís length
and look at a distant object. What do you observe?
Taking it further:
Hold the convex lens upright in the beam tank in the path of the light
rays. Look down from above and from the side. What do you observe?
Measure the distance from the lens to the point where the light rays
cross. Does this agree with your earlier measurement of the focal
Replace the convex lens with the concave lens. What happens to the
Larger and Smaller
Placed near an object, a convex lens acts as a magnifying glass. The amount
by which it makes things seem bigger depends on the curvature of its sides.
A convex lens will also focus, or bring to a point, the light rays from
an object that is far away. The distance from the lens to the point of focus
is called the FOCAL LENGTH. An object will only be magnified by a convex
lens if it lies within the focal length. Exactly the same is true of a concave
mirror. Held at a distance from the eye that is greater than the focal length,
a convex lens will show distant objects upside down and reduced in size.
A concave lens works in a similar way to a convex mirror. It spreads light
rays apart rather than bringing them together, and it makes objects at any
distance seem smaller than normal.