Video Transcript
Which row of the table correctly
shows how the wavelengths of various types of electromagnetic waves compare with
each other?
In this question, we’re given a
table. Each row of the table gives five
different types of electromagnetic waves in a certain order. We need to work out which row of
the table gives these waves in the correct order: from longest wavelength in the far
left-hand column to shortest wavelength in the far right column.
Before we try and answer this
question, let’s remind ourselves about the electromagnetic spectrum. As the name suggests, the
electromagnetic spectrum is just a spectrum of different types of electromagnetic
waves. The different types of
electromagnetic wave are defined by their wavelengths. At one end of the spectrum, we have
electromagnetic waves with very long wavelengths. And at the other end of the
spectrum, the waves have very short wavelengths.
To help us understand this, we can
sketch a wave that has a long wavelength at one end and a short wavelength at the
other end. Notice how the wavelength decreases
smoothly. There are no sudden changes or
jumps, just a nice continuous wave. Then, we can divide this into
regions, where each range of wavelengths corresponds to a different type of
electromagnetic wave. Keep in mind that this wave is just
a way of showing whether wavelengths are shorter or longer. In reality, the differences in
wavelengths for these wave types will occur over several orders of magnitude, unlike
this visual aid we are using. So, let’s now label these
regions.
Recall that radio waves have the
longest wavelength of any electromagnetic wave. Next, it’s microwaves, then
infrared, then visible light, which contains all the colors we can see, then we have
ultraviolet, and then X-rays. Finally, the type of wave with the
shortest wavelength is 𝛾. Now we know how the wavelengths of
the different types of electromagnetic waves compare, we can go back to the table
given to us and see which row is correct.
Let’s start with row i. This row tells us that radio waves
have the longest wavelength and 𝛾 rays have the shortest, which we know is
correct. However, let’s look at these types
in between. This row suggests that X-rays have
a longer wavelength than visible light and that visible light has a longer
wavelength than infrared. This is not correct. These three types are not in the
correct order. So, we know row i cannot be the
correct answer.
Now let’s look at row ii. Out of the five types of wave in
this row, we know radio waves must have the longest wavelength because they have the
longest wavelength of any electromagnetic wave. However, this row suggests that
radio waves have shorter wavelengths than visible light, ultraviolet light, and
X-rays. This is definitely not correct. This is enough for us to rule out
row ii.
Row iii suggests that infrared
waves have the longest wavelength of these types. Although we know that radio waves
and microwaves have longer wavelengths, they are not included anywhere in this
row. So, it is correct to say that of
these types, infrared has the longest wavelength. This row also suggests that 𝛾 rays
have the shortest wavelength, which we know is true. And these other three types —
visible, ultraviolet, and X-rays — are also in the correct order.
So, everything in row iii seems to
be correct. Row iii could definitely be the
correct answer to this question. But just to double check, let’s
also think about the final two rows.
Row iv suggests that X-rays and 𝛾
rays have the two longest wavelengths. We know this isn’t true; these
waves actually have the two shortest wavelengths. So, row iv is not the correct
answer.
Row v correctly lists the order of
radio waves, microwaves, and infrared. However, visible light and
ultraviolet are in the wrong order. Visible light has a longer
wavelength than ultraviolet. So, row v can be ruled out.
This leaves us with row iii. Row iii is the only row that lists
five types of electromagnetic wave in the correct order, from longest wavelength to
shortest. So, the row of the table that
correctly compares the wavelengths of various electromagnetic waves is row iii.
