WHY IS THE SKY BLUE?
Gas molecules in the atmosphere scatter, in all directions,
the short wavelength light that appears blue to us. Longer
wavelength light is largely unaffected as it passes through
the atmosphere. As a result, when you look at the sky, you see
blue everywhere. Read on for a more detailed explanation.
Fact 1. Light travels in waves. The light's wavelength
determines its color. Short wavelength light, for
example, appears blue, and long wavelength light appears
Fact 2. When light strikes particles that are larger
than its wavelength, the light's path may be altered.
When light strikes particles that are smaller than its
wavelength, the light continues to travel unaffected.
Fact 3. The atmosphere contains many particles and
gases, mainly nitrogen and oxygen.
Sunlight is composed of light of many different
wavelengths. Longer wavelength light appears red, orange, and
yellow, while shorter wavelength light appears blue, indigo
and violet. The gas molecules in the atmosphere scatter, in
all directions, shorter wavelength light (e.g., blue). The
longer wavelegnth light (e.g., red) is largely unaffected by
the atmosphere. As a result, when you look at the sky, you see
the blue portion of the sun's light being scattered by the
atmosphere. If you were to look at the sky while standing on
the moon, you would see a very bright star surrounded by
complete darkness. This is because the moon has no atmosphere
and so sunlight is not scattered.
You might wonder why the sky is not the color of the even
shorter wavelength violet. The primary reason for this is that
our eyes are better at detecting blue light than they are at
detecting violet light.
HOW DO AIRPLANES FLY?
When an airplane is in flight, there is a downward force
(gravity) and an upward force (lift) acting on the
airplane. As an airplane moves through the air, the shape of
the wings causes there to be less air pressure pushing down on
the wings than pushing up on the wings. This difference in
pressure yields the upward lift. If the downward gravitational
force is less than the upward lift, then the airplane stays in
the air. Read on for a more detailed explanation.
Fact 1. As an airplane's propellers spin, they give the
airplane a forward force (thrust). As the airplane moves
forward there is a backward force (drag) caused by the
resistance of the air.
Fact 2. An airplane is attracted downward due to
gravity. This force increases with the mass (weight) of
Fact 3. As an airplane moves forward, the shape of its
wing creates lower pressure above the wing than below,
yielding a net upward force known as lift.
As an airplane speeds along the runway, the airplane's wings
disrupt the flow of the air. This disruption causes the air
above the wings to move faster than below the wings. As a
result, the air pressure above the wing is less than below the
wing. As the airplane speeds up, this pressure difference
increases until the lift is stronger than the gravitational
force, and the airplane takes off. In order to remain in
flight, the thrust must be greater than the drag, and lift
must be greater than the force of gravity.
There is still some debate as to the exact cause of the
difference in the air's speed around the wings. The basic
concept described here, however, seems to explain why an
airplane can fly.
HOW ARE RAINBOWS FORMED?
Sunlight is composed of light of varying wavelengths. Short
wavelength light appears blue, violet and indigo, and long
wavelength light appears red, orange and yellow. When sunlight
enters a raindrop in the air, the light splits into a
multitude of colors. This light then reflects off the back of
the raindrop and re-emerges in the direction in which the
light first entered. The light emerging from many raindrops
creates a rainbow. Read on for a more detailed explanation.
Fact 1. Light travels in waves. The light's wavelength
determines its perceived color. Short wavelength light,
for example, appears blue, and long wavelength light
Fact 2. Sunlight is composed of light of many
wavelengths. In the range that we can see, this includes
the colors of the rainbow.
Fact 3. When light enters water it bends
(refracts). The amount of bending depends on the
wavelength of light. As a result, the light splits into
its component colors.
When a ray of sunlight enters a raindrop it bends
(refracts). The light then strikes the back of the raindrop,
where some of the light passes through and some is
reflected. As the light exits the raindrop, it is refracted
again. The angle at which the light emerges depends on the
wavelength of light. This path is illustrated in the small box
below, where only the bending of two wavelengths (blue and
red) are shown.
Consider now the diagram on the left. The sun is behind you
(white rays) and there is rain in front of you (black
dots). As the sunlight enters each raindrop, the light is
refracted and reflected as described above. Because the sun is
so far away, the rays of sunlight are nearly parallel to one
another. As a result, the angle between the red line and each
ray of sunlight striking a raindrop on that line will be the
same. So, the light that reaches your eye along this ray will
be of the same wavelength (color). The same is true for the
yellow, blue and intermediate lines corresponding to each
color of the rainbow.
Consider now the diagram on the right which explains why the
colors of a rainbow form an arc. The angle between the
incoming rays of sunlight (white) and all of the red lines,
forming a circular cone, have the same angle. As a result, the
light that reaches your eye along these lines have the same
wavelength (color). The same is true for each band of the
The reason that rainbows are somewhat rare is that you will
only see them when there is rain in front of you and somewhat
in the distance, and the sun is behind you and fairly low on
WHY DO BOATS FLOAT AND ROCKS SINK?
When an object is placed in water, there are two
primary forces acting on it. The force of gravity yields a
downward force and a buoyancy force yields an upward
force. The gravitational force is determined by the object's
weight, and the buoyancy force is determined by the weight of
the water displaced by the object when it is placed in
water. If the gravitational force is less than the buoyancy
force then the object floats (a boat), otherwise it sinks (a
rock). That is, if an object weighs less than the amount of
water it displaces then it floats otherwise it sinks. Read on
for a more detailed explanation.
Fact 1. When an object is placed in water, it
will displace water to "make room" for the object (e.g.,
when you get into a bath, the level of the water rises).
Fact 2. When an object is submerged in water, the
surrounding water exerts a force (buoyancy force) on the
object. This force increases with the depth of the
water, so that for any submerged object, there is a net
Fact 3. Any object is attracted downward due to
gravity. This force increases with the mass of the
There are two primary forces acting on an object placed in water:
- a gravitational force acts in the downward direction
causing the object to sink. The strength of this force
depends on the object's mass (weight) -- the more massive
an object the stronger the downward gravitational force
- a buoyancy force acts in all directions, but has a net
upwards direction, causing the object to float. The
strength of this force depends on how much water the
object displaces -- the more water that is displaced the
stronger the upward buoyancy force.
If the downward gravitational force is less than the upward
buoyancy force then the object floats, otherwise it
sinks. That is, if an object weighs less than the amount of
water it displaces then it floats otherwise it sinks. A boat
floats because it displaces water that weighs more than its
WHY DON'T PEOPLE ON THE OTHER SIDE FALL OFF?
Every object in the universe attracts every other object with
a force (gravity) that is proportional to the masses of the
objects and inversely proportional to the squared distance
between objects. Since the earth is so massive it attracts us
(no matter where we stand) with a force strong enough to keep
us from falling off. Read on for a more detailed explanation.
Fact 1. Mass is a measure of how much matter is in an
object. On earth, mass and weight are effectively the
same thing. 
Fact 2. The earth's mass  is approximately:
6,000,000,000,000,000,000,000,000 kg or
Fact 3. Gravity is an attractive force between any two
objects in the universe. The force attracting any two
bodies is proportional to the product of their masses
and is inversely proportional to the squared distance
between them. Below, the gravitational attraction is
stronger for the right-most pair of objects because they
are more massive and closer together.
The attractive force of gravity acts between the center of two
objects. In the case of people standing on the earth's
surface, the effect of gravity is to attract us towards the
center of the earth. As a result, no matter where you stand on
the earth, you don't fall off.
Gravity is also the reason why the moon (and satellites) orbit
the earth and why we orbit the sun. But then why doesn't the
moon crash into the earth? The moon is, in fact, constantly
falling towards earth, it is just that it keeps missing. With
an initial motion along the path of the orbit, the moon can
continually fall towards earth without ever reaching it. This
is illustrated on the right in the above diagram, where a
cannon is shown firing a cannonball. If the speed of the
cannonball is too slow it will crash into the earth. If the
speed is too fast, the cannonball will escape the
gravitational attraction of the earth. But, if the speed is
just right, then the cannon ball will continually fall towards
the earth, but never reach it – it will orbit the earth.
 An object's weight is a measure of how much gravity is
acting on it. An object weighs less on the moon because the
moon's mass, and therefore gravitational attraction, is
less. An object's mass, however, is the same no matter where
 A typical plane with people, cargo and fuel has a mass
of 70,000 kilograms (kg) (154,322 pounds (lb) or 77.16
tons). If we compare the mass of the earth to the distance
between the earth and the sun, then the mass of a plane
would be approximately the diameter of an atom.
 Objects will orbit the earth only if they are above
earth's atmosphere, where particles in the earth's
atmosphere do not slow it down.
WHY DO THINGS LOOK DARKER WHEN THEY ARE WET?
When light strikes an object, some of it penetrates the
object, and some of it is reflected and reaches your eye. When
an object is wet, more light penetrates the object, so less
light is reflected. As a result, less light reaches your eye
and so the wet object looks darker. Read on for a more
Fact 1. When light moves from air to water, some
of the light reflects and some refracts. The reflected
light "bounces" off the water, and the refracted light
bends at the air/water boundary and passes through the
Fact 2. When light strikes any object, some of the light
is reflected and some is refracted and transmitted
through or absorbed by the object. The relative amounts
of which depend on the material properties of the
object, its index of refraction.
Fact 3. When a material gets wet and absorbs
water, the material's index of refraction is effectively
changed, making it so that more light penetrates and
less light is reflected.
The light that is reflected from an object is the light that
we perceive. How light or dark an object appears depends on
how much light that strikes an object reflects back to our
eye. For an object whose material has an index of refraction
close to that of air very little light is reflected. For an
object whose material has an index of refraction different
than air, most of the light that strikes it is reflected.
When an object gets wet and absorbs water, its index of
refraction effectively moves closer to that of air. When light
strikes a wet object, therefore, less light is reflected than
when it is dry. A pair of wet pants, a wet sidewalk, and a wet
beach, therefore, reflects less light, and therefore looks
darker. Steel, glass or plastic doesn't look darker when it is
wet because it doesn't absorb any water, and therefore the
same amount of light is reflected whether it is dry or wet.
WHY IS IT COLDER IN THE WINTER THAN IN THE SUMMER?
The earth's axis of rotation is tilted relative to the earth's
path around the sun. As a result we are tilted towards the sun
in the summer and away from the sun in the winter. Read on for
a more detailed explanation.
Fact 1. The earth rotates about its axis once
every 24 hours. In the morning we are facing towards the
sun, and at night we are facing away from the sun.
Fact 2. The earth rotates around the sun (approximately)
every 365 days or 1 year.
Fact 3. The axis about which the earth rotates is tilted
(by 23.5 degrees) relative to the earth's path around
Shown below are two diagrams of the earth at the same time of
day. On the left it is winter and on the right it is summer
(in the northern hemisphere). Notice that the same spot (red
circle) in the winter receives much less light than in the
summer. As a result, it is colder in the winter than in the
summer. (Note: in this diagram, the earth's axis is 33
degrees, instead of 23.5, so as to better illustrate the