The Mirror - beam, angle, angle of incidence, critical angle
The Mirror
beam, angle, angle of incidence, critical angle
Light Unit
Did you brush your teeth in front of the mirror this morning? Did you see yourself in the glass? Light
played an important part in making that picture. Without light, you could not see yourself at all. To check
this, turn off the bathroom light and close the door. Light must bounce off the mirror, like a ball bounces off
the ground, to make that picture of you.
Light moves in a straight line from its starting point and spreads out. Think of sunlight. At sunrise, it
spreads out across the land, but if you look at sunlight that's going through clouds, you can sometimes see
the straight lines. You can see these lines because light is made of tiny packets of energy that shoot
straight out from the light's starting point, like the Sun or a light bulb. A laser is a kind of light that moves in
a straight line. Because the light is so focused, doctors can use lasers to cut and do surgery. A focused,
straight line of light is called a beam. That's why you hear people use the term laser beam.
Light moves in straight lines until something stops it, like that bathroom mirror. Light bounces back from
the mirror, just like your basketball bounces off the court. This bounce back, or change in direction, is
something you can measure. You can measure the space between two make-believe lines: the ground
and the ball's path after it hits the ground. An angle is the difference in direction between two lines that
started at the same point. The word comes from the word for "corner,"• so think of an angle as two lines
or walls coming together to form a corner.
If you look around your classroom, you will find angles everywhere: the corners of your desk, the
chalkboard and even your school books. Now look carefully at your science book; when your book is
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The Mirror - beam, angle, angle of incidence, critical angle
closed there is no space between the lines formed by the front cover and the back cover of the book. This
is the same as if you bounced the ball straight down at the ground, and it bounced right back into your
hands. Now open your book just a bit, and the space between the covers -- the angle -- is larger. It is as if
you bounced a basketball at the ground in front of your feet, and it bounced even farther away from you.
Finally, if you throw the ball at the ground very far away from you, it will bounce at a large angle and land
very far away from you, just like when you open your book wide to read a page.
When light bounces off of a mirror and changes direction, two angles are made. The first is formed when
the light hits the mirror, while the second is formed when the light bounces away from it. This idea might
be easier to see with the ball again. If you bounce the ball at the ground, it hits the ground making one
corner. Then the ball bounces off the ground making another. The first corner, made when the ball hits
something, is called the angle of incidence. The angle at which light hits or bounces off of something is
called the angle of incidence.
Have you ever wondered how the Internet works at the speed of light? Well, it is because it IS light.
Information is turned into light that moves through bending fiber optic cables which are a lot of tubes as
thin as your hair. The tubes have a coating on the outside that bounces light back inside itself, allowing
the light to bend and bounce and never escape until it reaches your house. They can send light for 60
miles or more while keeping the light's brightness.
The cables that carry most of what you see on the Internet are one example of light bouncing without
escaping, but the same thing can also happen inside of water. Sometimes light bounces at a special
angle, so it never escapes a pool of water. Instead the light bounces back inside the water. If you were
under water and this happened, the top of the water would look like a mirror; you would not be able to see
anything above the surface. The angle where the light bounces back inside without escaping is called
the critical angle.
If light did not bounce, you could not see yourself in the mirror or see anything else. Light moves in a line,
just like your ball, and bounces off of things. What we see when we look at most things is the light that is
bouncing off of them, or the light that it is coming out of them. People use light's straight path to make
tools like lasers to help doctors. People use light's bounce to carry light a long way through thin tubes.
Say "Thank you light!"• Because without it, you could not chat with your friends on the Internet or watch
your favorite shows.
References:
Allday, Jonathan. Light and Sound. New York: Oxford University Press, 2002. Print. The Young Oxford
Library of Science Vol. 6.
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The Mirror - beam, angle, angle of incidence, critical angle
Lerner, Brenda Wilmoth & Lerner Ed. K. "Angle." The Gale Encyclopedia of Science. 4th ed. Detroit: Gale,
2008. Gale Student Resources In Context. Web. 11 Sep. 2012.
"Angles."• KS2 Bitesize: Maths. BBC, 2014.
<http://www.bbc.co.uk/bitesize/ks2/maths/shape_space/angles/read/1/>
Hamilton, Gina L. Light. New York: Weigl Publishers Inc., 2009. Print. Science Q & A.
"Fiber optics." World of Invention. Gale, 2006. Gale Science In Context.
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Herwig Kogelnik. "Reflection of electromagnetic radiation." AccessScience. McGraw-Hill Companies,
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"Light, reflection and refraction of." U*X*L Science. U*X*L, 2008. Gale Science In Context. Web. 11 Sep.
2012.
"Specular and Diffuse Reflection."• Kahn Academy.
<https://www.khanacademy.org/test-prep/mcat/physical-processes/reflection-and-refraction/v/specular-anddiffuse-reflection> N.d. Web. 11 Sep. 2012.
Whinnery, John R. "Waveguide." AccessScience. McGraw-Hill Companies, 2012. Web. 11 Sep. 2012.
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