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Study Guide
Grade 8 Science: Chapter 4
Light and the Wave Model
Things to Know
1. What were the early theories on the nature of light?
Some of the early theories of light were that beams
of light were made up of tiny particles and that
these beams came from objects and reached the
eye. Others believed that the eye sends out fibres
or threads allowing the eye to touch or sense an
object.
2. Know the role of the following people regarding the
study of light:
i)
ii)
Pythagoras – made first attempts to explain
the relationship between light and vision. He
believed that beams of light were made of
tiny particles. These beams came from
objects and reached the eye.
The Janssens – experimented with the lenses
and learned how to magnify things even more.
They made tubes with lenses in the end that
would slide inside one another.
iii) Van Leeuwenhoek- famous for building the
microscope and discovering micro organisms.
He discovered that he could increase the
magnifying power of the lens by increasing the
curvature of the lens.
iv) Galileo – made the first attempts to measure
the speed of light. As well invented the first
telescopes to study space. Discovered four of
Jupiter’s moons.
v)
Michelson- first person to accurately measure
the speed of light.
3. Who was the first person to accurately measure the
speed of light? How did they achieve this?
Albert Michelson was the first person to measure
the speed of light. He used a strong light source,
an eight sided mirror, and another large mirror
about 35 km away. He shone a light on the rotating
mirror which reflected on a larger mirror. The
returning beam reflected off another face of the
rotating mirror into the eye of the observer. By
measuring the speed of the mirror and the distance
to the far away mirror, he could calculate the speed
of light.
4. What is the speed of light?
The speed of light is 300 000 km/s. It would circle
the earth seven and a half times in one second.
5. Explain why there is a delay between when you see
lightening and then hear thunder.
Thunder and lightening happen at the same time.
However the speed of sound is slower that the
speed of light. So you see the lightening right away
but there is a delay in the thunder because it takes
longer for the sound to travel. You can tell how far
away lightening is by timing the difference between
when you see the flash and then hear the thunder
clap.
6. Describe how microscopes and telescopes were
developed using knowledge of the properties of light.
Both were developed as early scientists discovered
the relationship between vision and light. The
Janssen’s developed the first compound microscope
when they discovered multiple lenses could magnify
objects. Microscopes were further developed as
Leeuwenhoek discovered that increasing the
curvature of the lenses would increase the
magnifying power of a lense. When Galileo heard
about the experiments with lenses, he built a
telescope to magnify objects in space.
7. Describe and /or accurately label a wave, noting such
things as wavelength, frequency, and what is carried as
a wave moves forward.
8. Describe the relationship between wavelength and
frequency.
As wavelength increases (the distance between the
crests/troughs) , the frequency decreases. As
wavelength decreases, frequency increases. There is
an inverse relationship between wavelength and
frequency.
Hertz: a measure of frequency. As hertz increases
that means the frequency increases. The greater
the frequency ,the greater the hertz.
9. List the 6 properties of light and give an example:
i)
ii)
iii)
iv)
v)
vi)
rectilinear propagation
reflection (specular and diffuse)
refraction
dispersion
travels in a vacuum
travels through different material to
different degrees
10. What is the visible light spectrum?
This is light waves that you can see. It is the ROY
G. BIV
11. List the colours in the visible light spectrum and
describe each in terms of wavelength, frequency and
refraction
red- longest wavelength, lowest frequency, least
refraction (700nm)
orange
yellow
green
blue
indigo
violet – shortest wavelength, highest frequency,
most refraction (400 nm)
12. Define and give an example of wavelengths longer
than visible light:
i)
Radio waves – a type of electromagnetic
radiation that have the longest wavelength and
lowest energy and frequency compared to all
other types. Used for MRIs.
ii)
Microwaves- a type of radio wave. The radio
wave that has the shortest wavelength and
highest frequency.
iii) Radar – shorter wavelength microwaves used in
remote sensing. Waves are beamed out
through the air. The waves reflect from an
object and show the location and speed.
iv) Infrared waves- are a type of
electromagnetic radiation that, relative to
light, has a longer wavelength and lower
energy and frequency. Used for remote
controls/heat lamps.
13. Define and give an example of wavelengths shorter
then visible light:
i)
UV rays – these are waves beyond violet.
Radiation energy like UV waves, are very
energetic and potentially dangerous. Needed
for your body to make vitamin D. They have a
higher frequency and shorter wavelength than
visible light.
ii)
X rays – much shorter wavelength and higher
frequency than UV rays. Discovered in 1895
by Roentgen. Can have harmful health
effects with prolonged exposure.
14. Compare wavelengths longer AND shorter then
visible light to visible light.
Visible light can be found between waves longer
than and wave shorter than visible light.
Waves like
wavelength
Waves like
wavelength
radio waves, radar etc have a longer
and lower frequency that visible light.
x-rays and UV rays have a shorter
and higher frequency than visible light.
15.
Give an example of positive and negative
aspects of electromagnetic radiation.
Positive and Negative Effects of Electromagnetic
Radiation
 Exposure to high frequency and short wave length
electromagnetic radiation is dangerous.
Type of
Radiation
X-rays
Positive Effect
Ultraviolet
Used to treat
jaundice in babies
Improved
Uncertain of
telecommunications long term
exposure
effects
Radio waves
Medical detection
Negative
Effect
Overexposure
can lead to
cancer
Skin cancer