Saturday X-tra
X-Sheet: 6
Electromagnetic Spectrum
Key Concepts
This lesson will focus on the following;
 Electromagnetic spectrum
 Characteristics of electromagnetic spectrum
 Addition of light
 Subtraction of light
Terminology & definitions
 Electromagnetic waves – waves that can travel through a vacuum.
 Electromagnetic spectrum – a continuous range of frequencies of electromagnetic
waves.
 Frequency – number of vibrations/oscillations per second in a wave.
 Wavelength – the distance between any two points that are in phase in a wave.
 Photon – energy package of a specific electromagnetic radiation, also called a
quantum of energy.
 Electromagnetic radiation – Radiation that consists of wave-like electric and
magnetic fields in space, including white light (red, orange, yellow, green, blue,
indigo, violent), microwave, radio signals, x-rays, infrared, ultraviolet, X-rays and
Gamma Rays.
X-planation of key concepts and terminologies
Unlike other types of waves such as sound and water waves, electromagnetic waves
do not need a medium to travel in. This type of waves is able to travel in a vacuum at
a constant speed, c = 3x108m∙s-1. Like any other types of waves, electromagnetic
waves possess energy. The energy carried by an electromagnetic wave is discrete
and its magnitude depends on the frequency of the electromagnetic wave. Planck’s
equation, E = hf, is used to calculate for the energy carried by an electromagnetic
wave. The particles responsible for carrying the energy in an electromagnetic wave
are called photons. If an electromagnetic wave is generated by a number of photons,
the total energy carried by such a wave can be found by using Planck’s equation as
follows; E = nhf, where n is the number of photons in the wave.
The following are characteristics of electromagnetic (EM) waves;
 EM waves are caused by the acceleration of charged particles.
 EM waves are transverse waves made up of changing electric and magnetic
fields vibrating at right angles to each other.
 EM waves travel in a vacuum at a constant speed of 3x108m∙s-1.
 EM waves carry specific amount of energy which is directly proportional to their
frequencies according to Planck’s equation; E = hf.
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The electromagnetic spectrum is made up of frequencies ranging from low to high or
wavelengths ranging from long to short. The lowest frequency end of the
electromagnetic spectrum is occupied by radio waves whilst the highest frequency end
is occupied by the Gamma Rays as shown in the diagram below
The wave equation, c = fλ is applicable to electromagnetic waves and therefore can
be used when solving problems.
The visible section of the EM spectrum is made up of the white light. The white light
section of the EM spectrum can be broken up into seven colours. This is known as the
spectrum of white light. Each colour is associated with a specific frequency. The
colours of the spectrum of white light in increasing frequency are as follows; Red,
Orange, Yellow, Green, Blue, Indigo, Violet. The following table gives the seven
colours of the white light spectrum with their associated frequencies and wave lengths:
Red, Green and Blue are known as the primary colours. Adding the three primary
colours together produces white light. Secondary colours are produced when any two
of the primary colours are added together as shown below:
Red + Green → Yellow
Red + Blue → Magenta
Green + Blue → Cyan
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Yellow, Magenta and Cyan are secondary colours since they produced by the
combination of two primary colours. Any two colours that would produce white light
when they combine are called complementary colours, for example; Red + Cyan →
White. Therefore, Red and Cyan are complementary colours.
Secondary colours are used as pigments in making paint. Pigments help in giving an
object its unique colour through subtraction of other colours. An object would appear
yellow because it is able to absorb every other colour of light and reflect the yellow
light. Thus the object would have absorbed blue and reflected Red and Green which
will combine to produce yellow. This is called subtraction of colours. In case of filters,
certain colours of light are allowed to pass whilst others are absorbed.
X-sample Questions
1.
2.
2.1
2.2
What is the wavelength of orange light which has a frequency of 5,0 x 10 14 Hz?
A light source has a wavelength of 520 nm.
What is the frequency of this light?
What colour is the light?
3.
3.1
3.2
3.3
In a television set, three beams of light are used to make all the colours you see.
What are the colours of these beams of light?
How do these beams make a black image?
What combination of light beams is used to make yellow light?
4. A beam of cyan light is shone from a light source onto a white screen.
4.1 What are the primary colours of light that produce the beam of cyan?
4.2 Transparent filters are placed in the path of the cyan beam. Predict what colour of
light will appear on the screen when the following filters are used:
4.2.1 blue filter
4.2.2 red filter
4.2.3 green and red filter
4.3 What colour combination would you use if you wanted to block the cyan beam?
X-ercise
1. What colour of light is associated with the frequency 5,1 x 1014 Hz?
2. What is the frequency of light with a wavelength of 660 nm?
3. Which colour of light is most refracted by a prism and why?
4. A magenta light source shines a beam on an object containing yellow pigments.
Predict the colour in which the object will appear.
Answers
1. Yellow
2. 4,54 x 1014 Hz
3. Violet. Because violet has the highest frequency.
4. Object will absorb blue colour from magenta and reflect Red. The object will,
therefore, appear red.
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