Video Transcript
The diagram shows three different
angles at which the temperature shown on a thermometer could be read. From which angle is it best to read
the thermometer to get an accurate reading?
All right, so we see in this
diagram these three potential angles A, B, and C for positioning one’s eye to read
off the temperature on this analogue thermometer. From angle A, our eye is looking
down on the level of the liquid in the thermometer. From angle B, our eye is on level
with that liquid. And from angle C, our eye is below
that liquid level. So we’re looking up at it. Our question asks, from which of
these three angles is it best to read the thermometer in order to get an accurate
reading?
To see which angle of observation
will give us the most accurate temperature reading, let’s take a very close up,
zoomed-in view on the level of the liquid in our thermometer. That view looks something like
this, where the level of liquid in our thermometer is on line with a smaller hash
mark. And that’s one hash mark below
20. And if we look carefully at the top
of our thermometer, we see that this thermometer measures temperature in degrees
Celsius. Therefore, this 20 that we see here
indicates a temperature of 20 degrees Celsius. Now, we don’t know the particular
liquid that this thermometer is filled with. It might be mercury; that’s one
common type. Or it might be ethanol, a type of
alcohol. Or it could be something else.
But the important thing to notice
is that whatever the liquid is, it doesn’t run up to the edge of our
thermometer. That is, there’s this gap that
we’ve just drawn in between where the liquid actually rises and falls in a column
through the center of the thermometer and the place on the outside of that
thermometer where the temperatures are marked in. This distance, this gap, is very
important because it can affect the accuracy of our reading.
For example, consider what would
happen if our eye was at the position marked out as A. In that case, our line of sight
drawn in on our zoomed-in diagram might look very much like this. That is, our eye is over here
looking down on the liquid in our thermometer. And this would mean that our line
of sight, when it looks at the level of the liquid in the thermometer, passes
through the marking identified as 20 degrees Celsius. So if our eye was up here, if it
was around position A, then we might say the thermometer is reading a value of 20
degrees Celsius. But from our up-close view, we can
see that that’s not accurate.
Now, let’s consider putting our eye
at the lower position at angle C. In this case, again using our
zoomed-in view, our line of sight might look very much like this. That is, our eye is positioned
here, looking up at the level of the liquid. And from this position, we see the
liquid level crossing here on our thermometer. Now, if we look at the big scale
view of our thermometer, we see that each one of these smaller tick marks indicates
a change in temperature of one degree Celsius. Which means that this tick mark
here represents 18 degrees Celsius. It’s two tick marks below 20.
So if our eye was in position A, we
might record the temperature as 20 degrees Celsius. And if it was in position C, we
might record 18 degrees Celsius. But neither of these we can see is
accurate. The better reading would be on
level with the liquid in our thermometer. That is, it’s best for our eye to
be here on that level. So that the distance gap between
the liquid in the center of our thermometer and the thermometer markings on the
outside doesn’t lead to a distorted or inaccurate reading. So we’ll say it’s from angle B that
this thermometer should be read in order to get an accurate reading.
