Name________________________________
Fill in the blanks: The two main categories of waves are _______________ and _________________.
____________ waves require a medium and ______________ do not. The two types of waves are _____________
and ______________. In ____________ waves, the particles move at a right angle (perpendicular) from the
direction of energy transfer. In ______________ waves, they move parallel to (in the same direction of) the energy
transfer. ______________ is the wave behavior when the light wave (which is a straight line called a ______)
bounces off an object in another direction. In ______________, the waves appear bent because they travel through
more than one medium. Waves travel at different ________ in different mediums. In ____________, the waves
collide with and bend around an object that is in its way. ___________ is the amount of wavelengths that pass a
point in a certain amount of time, usually 1 second. A _____________is the amount of time it takes one wavelength
to pass a certain point.
Application Activity: EM Spectrum Waves in Motion Detectors
A motion detector is a device that can sense when a person or object moves through a room. Read the passage and study
the figures. Then use a separate sheet of paper to answer the questions that follow.
Many businesses have burglar alarms that are connected to motion detectors. One type of motion detector can sense the
infrared rays given off by a person’s body. An alarm is set off when infrared rays hit a sensor on the detector. This type of
device is called a passive infrared (PIR) motion detector.
Another type of PIR motion detector involves two sensors in different parts of an office. This type of detector sounds an alarm
only after both sensors have been hit by infrared rays from a person’s body.
PIR motion detectors only sense infrared rays; they do not transmit them. By contrast, part of an active infrared (AIR) motion
detector gives off infrared rays. A second part of the detector in another part of the room reflects these rays back to the first
part where they hit a sensor. If someone walks through the path of the invisible infrared rays given off by the detectors, his or
her body will prevent the rays from hitting the sensor. When this happens, the alarm is set off.
1. Label each of the motion detectors shown above as one-sensor PIR, two-sensor PIR, or AIR.
2. Suppose a business has a nighttime alarm system with a one-sensor PIR motion detector. What do you think would
happen if a lamp close to the sensor was left turned on overnight?
3. Suppose a business has a nighttime alarm system with a two-sensor PIR motion detector. What do you think would
happen if a lamp close to one of the sensors was left turned on overnight?
4. Do you think that a motion detector that used visible light instead of infrared rays would work very well? Explain your
answer in terms of both passive and active detectors.
Refraction Basics
When scientists talk about refraction, they use a formula. "n = c / v" "c" is the speed of light in a vacuum, "v" is the speed of
light in that substance and "n" is the index of refraction. According to the formula, the index of refraction is the relation
between the speed of light in a vacuum and the speed of light in a substance. But what is refraction? When light moves from
one substance to another it changes speed and direction. That change in direction is called refraction. Some indexes of
refraction are diamond (2.419), glass (1.523), and water (1.33).
Let's make a jump here. Since our formula says n=c/v and we know that the value for c is a constant, we can figure out that
light has different speeds when it is in different substances. It goes at full speed in a vacuum, and slower everywhere else.
Using our examples, we can discover that light moves faster in water than it does in a diamond. Light moves at its fastest
speeds when it is in a vacuum. Light moves at about 124,000,000 meters per second (less than half the speed in a vacuum) in a
diamond compared to 299,792,458 meters per second in a vacuum.
Light is refracted only when it hits a boundary at an angle, so if light goes straight down into a substance it will continue to
move straight down. You need to understand that the speed of light changes in different substances. If a light ray slows down
when it hits a substance, it bends towards the normal. The normal is the line that is perpendicular to the surface of the
substance. If a light ray speeds up when it hits a substance, it moves away from the normal.
Summarize the five most important pieces of information above:
 ________________________________________________________________________________________________________________________________
 ________________________________________________________________________________________________________________________________
 ________________________________________________________________________________________________________________________________
 ________________________________________________________________________________________________________________________________
 ________________________________________________________________________________________________________________________________
Additional Questions
1. It is very important to note that light travels slower as the medium gets denser. Therefore, which state of
matter would light travel the fastest through? _________________ How do you know? _______________________________
_____________________________________________________________________________________________________________________________
2. Sound travels faster as the medium gets denser because it needs to compress the particles to be able to
transfer its energy. Therefore, which state of matter would sound travel the fastest through? _________________
How do you know? _______________________________________________________________________________________________________
3. When we see a color, we are seeing that color’s wavelength of the white light being reflected off of the
object. All of the other colors of white light are __________________ by the object.
4. How does light relate to us seeing white? __________________________________________________________________________
How does light relate to us seeing black? ______________________________________________________________________________
5. A ___________________ is an optical illusion caused by atmospheric conditions, especially the appearance of a
sheet of water in a desert or on a hot road caused by the refraction of light from the sky by heated air due to
the different ____________________________ of the mediums.
Answer Key – Seat 3 Review Questions
Fill in the blanks: The two main categories of waves are __mechanical___ and ___electromagnetic___.
Mechanical___ waves require a medium and __EM___ do not. The two types of waves are _transverse___ and
_compressional/longitudinal_. In _transverse___ waves, the particles move at a right angle (perpendicular) from
the direction of energy transfer. In ___longitudinal___ waves, they move parallel to (in the same direction of) the
energy transfer. _reflection___ is the wave behavior when the light wave (which is a straight line called a _ray_)
bounces off an object in another direction. In __refraction__, the waves appear bent because they travel through
more than one medium. Waves travel at different _speeds__ in different mediums. In __diffraction__, the waves
collide with and bend around an object that is in its way. _frequency_ is the amount of wavelengths that pass a
point in a certain amount of time, usually 1 second. A __period__is the amount of time it takes one wavelength to
pass a certain point.
Application Activity:
1. A = one-sensor PIR ; B = two-sensor PIR ; C = AIR
2. Infrared rays given off by the lamp might be detected by the motions detector’s sensor and cause the alarm to go
off.
3. Infrared rays given off by the lamp might be detected by the sensor close to the lamp. However, the alarm would
not go off because the infrared rays would hit only one of the two sensors.
4. Answers will vary. Sample: A passive motion detector would not work because a person’s body does not give off
visible light. An active motion detector would not be very effective either because a burglar would be able to see the
beam of light given off by the detector and would be able to avoid it.
Refraction Basics:
Ms. Wasson will read these summaries and grade them.
Additional Questions:
1. gas (although it travels fastest, at light speed, in no medium/a vacuum) – because the particles are the most
spaced out and least dense (there are less particles to show down the wave’s energy)
2. solid – because the particles are the most dense/more tightly compact, so they can vibrate each other and
compress much easier and faster
3. absorbed
4. when we see white, all of the colors of white light (the rainbow) are being reflected back to our eyes
when we see black, all of the colors of white light are being absorbed by the object (none are reflected – black is the
absence of color)
5. mirage - densities