Physics for Kids
Science of Light
What is light made of?
This is not an easy question. Light has no mass and is not really considered matter. So
does it even exist? Of course it does! We couldn't live without light. Today scientists say
light is a form of energy made of photons. Light is unique in that it behaves like both a
particle and a wave.
Why does light go through some things and not others?
Depending on the type of matter it comes into contact with, light will behave differently.
Sometimes light will pass directly through the matter, like with air or water. This type of
matter is called transparent. Other objects completely reflect light, like an animal or a
book. These objects are called opaque. A third type of object does some of both and
tends to scatter the light. These objects are called translucent objects.
Light helps us to survive
Without sunlight our world would be a dead dark place. Sunlight does more than just
help us see (which is pretty great, too). Sunlight keeps the Earth warm, so it's not just a
frozen ball in outer space. It also is a major component in photosynthesis which is how
most of the plant life on Earth grows and gets nutrients. Sunlight is a source of energy
as well as a source of vitamin D for humans.
The speed of light
Light moves at the fastest known speed in the universe. Nothing moves faster than (or
even close to) the speed of light. In a vacuum, where there is nothing to slow it down,
light travels 186,282 miles per second! Wow, that's fast! When light travels through
matter, like air or water, it slows down some, but it's still pretty fast.
To give you and idea as to how fast light is, we'll give you some examples. The sun is
almost 93 million miles from the Earth. It takes around 8 minutes for light to get from the
sun to the Earth. It takes around 1.3 seconds for light to go from the moon to the Earth.
Refraction
Normally, light travels in a straight path called a ray, however, when passing through
transparent materials, like water or glass, light bends or turns. This is because different
materials or mediums have different qualities. In each type of medium, whether it is air
or water or glass, the wavelength of the light will change, but not the frequency. As a
result, the direction and speed of the traveling light wave will change and the light will
appear to bend or change directions.
One example of refraction is a prism. Prisms are unique in that each color of light is
refracted to a different angle. So it can take white light from the sun and send out light of
various colors.
Lenses use refraction to help us see things. Telescopes help us to see things far away
and microscopes enable us to see very small things. Even glasses use refraction so
that we can see everyday things more clearly.
Physics for Kids
The Science of
Light Spectrum
Frequency and Wavelength
One of the characteristics of light is that it behaves like a wave. As a result, light can be
defined by its wavelength and frequency. The frequency is how fast the wave vibrates
or goes up and down. The wavelength is the distance between two peaks of the wave.
Frequency and wavelength are inversely related, meaning that a low frequency wave
has a long wavelength and vice versa.
We can only see light within a certain range of wavelengths and frequency. This range
is called the visible spectrum. The frequency range of the visible spectrum is from 405
Terahertz to 790 Terahertz.
The Electromagnetic Spectrum
The electromagnetic spectrum includes a wide range of light waves, some that we can't
see. Some of the non-visible types of waves are radio waves, microwaves, infrared
rays, and X-rays. These types of waves have all sorts of uses in science and
technology.
In the visible spectrum of light, the color of the light depends on the frequency. The
visible spectrum is always the same for a rainbow or the separated light from a prism.
The order of colors is red, orange, yellow, green, blue, indigo, and violet. A fun way to
remember this is by using the first letter in each color in order to spell out the name
ROY G. BIV.
Colors of light
What we are seeing when we see an object is reflected light. When light hits an object
some wavelengths are absorbed by that object and some are reflected. Light of different
wavelengths looks like different colors to us. When we see an object of a certain color
that means that light of that color's wavelength is being reflected off the object. For
example, when you see a red shirt, the shirt is absorbing all the colors of light except for
the red color. The frequency of light that we see as red is being reflected and we see
that shirt as red.
Black and white are a bit different from other colors. White is a combination of all colors,
so when we see white, the object is reflecting all the colors of light the same. Black is
the opposite. When we see a black object that means almost all the colors of light are
being absorbed.
Additive colors
Light from the three additive primary colors may be combined to make any other color.
These three colors are red, blue, and green. This fact is used all the time in technology
such as computer screens and televisions. By combining just the three main types of
light in various ways, any color can be made.
Subtractive colors
If you have white light and want to subtract colors to get any other color, you would use
the primary subtractive colors to filter or remove light of certain colors. The primary
subtractive colors are cyan, magenta, and yellow.
Physics for Kids
Behavior of Light as a Wave
In physics, light is type of electromagnetic radiation that is visible to the eye. Light has
the unique property that it can be described in physics as both a wave and as a stream
of particles calledphotons.
On this page we will describe some of the behaviors of light as a wave including
reflection, refraction, and diffraction.
Reflection
One of the most important wave-like behaviors of light is reflection. It is reflected light
that we see with our eyes. How light reflects off objects affects the colors we see as
well.
When a wave strikes a new medium, some of the wave will bounce off the surface. How
reflective the surface is will determine how much light (and what wavelengths of light)
will be reflected and how much will be absorbed or transmitted.
When light is reflected it obeys the law of reflection that is followed by waves. This
means that the angle of the reflected wave of light will equal the angle of incidence of
the incoming light wave. See the picture below for an example:
Types of Reflection
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Specular reflection - A specular reflection is when rays of light are reflected off a
surface in a single outgoing direction. An example of this type of reflection is a
mirror. Specular reflections occur on surfaces that are flat at the microscopic
level such as polished silver or a smooth body of water.
Diffuse reflection - A diffuse reflection is when a surface reflects rays of light in a
broad range of directions. Diffuse reflections occur when a surface is rough at the
microscopic level. The surface may appear or feel smooth, like a piece of paper,
but it is actually rough at the microscopic level. This causes the beams of light to
reflect at different angles.
Refraction
When light moves from one medium (like air) to another medium (like water) it will
change directions. This is a "wave-like" behavior and is called refraction. In this way
light behaves like other waves such as sound waves. The speed of the light wave also
changes when it moves from medium to medium.
You can see an example of refraction of light in water if you put a straw in a glass of
water. You will see how the straw seems to move to the side. This is the light wave
bending as it enters the water.
Index of Refraction
In order to measure how light will behave in different substances, scientists use the
index of refraction. This gives a ratio of the speed of light in a vacuum over the speed of
light in the substance. The equation for the index of refraction is:
n = c/v
where n is the index of refraction, c is the speed of light in a vacuum, and v is the speed
of light in the substance.
As an example, take the index of refraction for water which is 1.33. This means that the
speed of light in a vacuum is 1.33 times faster than the speed of light in water.
Diffraction
Another wave-like property of light is
diffraction. When light waves encounter an
obstacle or pass through an opening they
will bend. The diffraction of light can be
seen in the silver lining around clouds as
well as the patterns of light from the
surface of a compact disc (see picture).
Physics for Kids
Lenses and Light
A lens is a curved piece of glass or plastic designed to refract light in a specific way.
Lenses are used in glasses and contacts to help correct vision. They are used in
telescopes to help view items that are far away and are used in microscopes to help
view very small items.
Refraction
When a light wave moves from one medium (like air) to another medium (like glass) the
light rays are bent. This is called refraction. By using refraction, lenses can bend
multiple light rays. Most of the lenses we use in everyday life are designed to bend light
rays to a specific focal point where items will be in focus (clear).
Types of Lenses
There are different ways to classify lenses. One way to classify lenses is by how they
bend light.
Converging
A converging lens will cause the light rays to bend to a specific focal point. Another
name for this type of lens is a positive lens.
Diverging
A diverging lens will cause light rays from a specific focal point to be spread out.
Another name for this type of lens is a negative lens.
Other Types of Lenses
Another way to classify lenses is by the curve of the glass on each side of the lens.
There are terms used to describe each side. Then the two sides are combined to come
up with the name of the lens.
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Convex - A convex lens is one where the center of the lens is thicker than the
edges.
Concave - A concave lens is one where the center of the lens is thinner than the
edges. One way to remember the difference between the two lenses is to think of
"caving in" with the concave lens.
Plano - A plano lens is a flat lens. This is used when one side is flat and the other
side is concave or convex. You can think of flat as a "plain."
Meniscus - A meniscus lens is one where one side is concave and one side is
convex.
Putting the Names Together
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Biconvex - A lens in which both sides are convex is biconvex. Biconvex lenses
are converging lenses.
Plano-convex - A lens in which one side is convex and the other is plano. Planoconvex lenses are converging lenses.
Biconcave - A lens in which both sides are concave is biconcave. Biconcave
lenses are diverging lenses.
Plano-concave - A lens in which one side is concave and the other is plano.
Plano-concave lenses are diverging lenses.
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Positive meniscus - A converging lens where one side is concave and the other
convex.
Negative meniscus - A diverging lens where one side is concave and the other
convex.
Focal Point
The focal point of a lens is generally noted by the capital letter "F." This is the point in
space where the light rays will converge to after passing through a converging lens. A
diverging lens will have a negative focal point where the rays originate from before
diverging through the lens.
Focal Length
The focal length is the distance from the center of the lens to the focal point.
Principal Axis
The principal axis is a horizontal imaginary line drawn through the center of the lens. In
a perfect lens the focal point will reside on the principal axis at a distance of the focal
length from the center of the lens.