Light What is light? A few weeks ago, we learned about waves. The way to make any wave is to move something,
usually up or down or back and forth. You can move your hand up and down against some water,
for example to make water waves. You can snap the end of a rope up and down to make a wave
that travels along the rope. When you talk, your vocal cords move back and forth really fast. They
vibrate. Those vibrations make sound waves that travel through the air. What about light waves?
What moves up and down or back and forth to make them? Teeny tiny charged particles, that’s
what! Charged particles have positive or negative charges that make them push away from or pull
toward each other.
Charged particles are also called electric charges. When electric charges vibrate, they give off
energy. Some of that energy is in the form of light waves. ​That’s what light is! ​ When we think of
light, many of us think of sunlight or a source of light like a light bulb, which can brighten up a room.
Light is a ​form of energy​ that comes to us from the sun in the form of a wave called
electromagnetic radiation​. Some forms of light are visible, while other forms are not. The
electromagnetic spectrum, provides us with information of the many different forms of light that can
exist here on Earth. It ranges from very low frequency waves like radio waves, to very high
frequency waves like gamma rays. All forms of light transfer ​energy​.
What happens when light strikes an object? To understand why objects have different colors, you need to know how light can interact with an
object.​ When light strikes an object, the light can be reflected, transmitted, or absorbed.
Think about a pair of sunglasses. If you hold the sunglasses in your hand, you can see light that
reflects off the lenses. If you put the sunglasses on, you see light that is transmitted by the lenses.
The lenses also absorb some light. That is why objects appear darker when seen through the
lenses. Lenses, like all objects, are made of one or more materials. Most materials can be
classified as transparent, translucent, or opaque based on what happens to light that strikes the
material.
Transparent Materials A ​transparent material​ transmits most of the light that strikes it. The light passes right through
without being scattered. This allows you to see clearly what is on the other side. Clear glass, water,
and air all are transparent materials.
Translucent Materials A ​translucent material​ (trans ​LOO​ sunt) scatters light as it passes through. You can usually see
something behind a translucent object, but the details are blurred. Wax paper and a frosted glass
are examples of translucent materials.
Opaque Materials
An ​opaque material​ (oh ​PAYK)​ reflects or absorbs all of the light that strikes it. You cannot see
through opaque materials because light cannot pass through them. Wood, metal, and tightly woven
fabric all are opaque materials. Materials like mirrors, aluminum foil can reflect light. Because the
light doesn’t pass through them, they are considered opaque, too.
The picture below shows examples of each type of material.
You can see the differences in how the giraffe is viewed, or not viewed by the different materials.
You can see the giraffe’s head because the light is going through a transparent material. In the
middle, the image is blurred because the material the light goes through is translucent. You are
not able to see the rest of the giraffe, because the light is being absorbed and therefore, the light
can not go through. The material there is opaque.
No Light, No Sight!
What do you see when you turn off the lights in your room at night? Not much, probably. But flick
the lights on and everything comes back into view. No surprise there. You know you need light to
see, but do you know why?
You see most objects only if light ​reflects​, or bounces, off of them and into your eyes. You are
able to read these words only because light waves from the Sun or light bulb are reflecting off this
page and into your eyes. Your eyes are light detectors!
Reflected light not only lets you see, it lets you ​see yourself​. A mirror is a sheet of glass with a
smooth silvery coating on the back. Light passes through the glass and reflects off the coating.
You see your reflection because the waves bounce off the smooth surface at the same angle at
which they hit it. Rougher surfaces, like cloth, rocks and most everything else, reflect light in all
different directions. So you don’t see a clear reflection.
Bending Light Did you ever look at your reflection in a store window? Some of the sunlight hitting the glass
reflects off the smooth surface, so it acts like a mirror. But some of the light goes right through. It
reflects off things inside, so you can see what’s in the store. Some of the light that shines through
is reflecting off of you. So people in the store can see you, too.
The light shining through the window doesn’t exactly go ​straight​ through. When light passes at an
angle from one material (or medium) to another, like from air to glass, the light waves change
speed. That makes the light change direction, or ​refract​, which is the bending of light.
Think about riding a skateboard. Suppose you veer off the sidewalk and hit the grass at an angle.
The front wheel that hits the grass and first slows down. But, the front wheel still on the sidewalk
continues at its regular speed. The difference in speeds makes the skateboard change direction and maybe makes you almost lose your balance! The skateboard’s change in direction is like what
happens with light waves. You saw an example of light bending, when we put a pencil in a cup of
water. The side of the light wave that first entered the glass from the air slowed down. The other
side of the wave went faster, so the whole wave changed direction. It bent. All of this bending of
light can play tricks with your eyes. It can make objects look “broken” or larger than they are.
Seeing Color Speaking of light playing tricks, how can you explain the rainbow of dazzling colors coming from a
glass prism? After all, the light going into the glass is white. As light passes through the glass
prism, the different wavelengths of light slow down by different amounts - so they bend at different
amounts. Light of shorter wavelengths, like blue and violet, bends the most. Long wavelengths,
like orange and red, bend the least. These differences in bending, separate light into its different
colors.
Have you seen a shiny red apple? What gives the apple its red color? The answer is that objects
absorb​ some wavelengths of light and reflect others. The wavelengths that are reflected are what
you see. Some apples look red because the skin of the apple reflects wavelengths of red light to
our eyes and absorb the other wavelengths. Green or yellow apples reflect green or yellow and
absorb all other colors.
A black object looks black because it absorbs all of the light hitting it. Most of the light turns into
heat, which is energy you can feel if you wear a black shirt on a sunny day! A white object reflects
all of the light. It doesn’t absorb any colors, so a white shirt keeps your cooler.