Question Video: Defining Magnification Biology

Which of the following best defines magnification? [A] Magnification is how much bigger the image is than the actual object. [B] Magnification is the number of different structures that can be identified in an image. [C] Magnification is calculated as a magnifying power of the objective lens divided by the magnifying power of the eyepiece. [D] Magnification is the minimum distance apart two objects can be in order for them to appear as separate items.

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Video Transcript

Which of the following best defines magnification? Magnification is how much bigger the image is than the actual object. Magnification is the number of different structures that can be identified in an image. Magnification is calculated as a magnifying power of the objective lens divided by the magnifying power of the eyepiece. Or magnification is the minimum distance apart two objects can be in order for them to appear as separate items.

This question is asking about magnification using a microscope, like the one in the image here. Let’s start by looking at the stem of the word magnification. If you think of the other ways you’ve used the word “magnify,” they always refer to making something appear bigger than it actually is. For example, this magnifying glass is making these very small ants look a lot larger than they actually are. The same thing happens when looking through a microscope at a specimen, for example, bacteria, which here we’ve shown in pink on a slide. In the orange circle, we’ve shown an image of what the bacteria might look like when you look through a microscope. If we increase the magnification of the microscope to up to times 400, the bacteria in the image will appear larger. When we look through a microscope, we look through what we call the eyepiece lens, which has been labeled here.

The objective lenses which can change the magnification, as we just did when looking at our bacteria, are labeled here. These two lenses work together to get the total magnification of the microscope. The calculation for total magnifying power of a microscope is the power of the objective lens multiplied by the power of the eyepiece lens. While the power of the eyepiece lens cannot be changed on most light microscopes, the objective lens magnification can be changed, like we did here to view the bacteria more closely at a higher total magnification.

Resolution is the degree to which structures can be distinguished from each other on the minimum distance apart at which you can do this. So, resolution is a measure of distance. The two bacteria drawn here at the top are further apart than the two that have been drawn below them. So, the two at the top can be distinguished as two objects, whereas the two on the bottom are too close to each other and are therefore resolved as one object. As it is a measure of distance, resolution is normally measured in nanometers, written as nm.

Let’s use this information to work out our best definition for magnification. In the answer choice that says magnification is the number of different objects that can be identified in an image, it is important to remember that the number of structures you can distinguish using a microscope will depend on how magnified it is and how easily the different objects can be resolved. The key point of magnification, however, is that it makes structures appear larger than they actually are, like we’ve seen in our bacterial example. Therefore, this option referring to the number of different structures which can be identified is missing the key point of magnification and cannot be our best definition.

Let’s look at the answer choice telling us how to calculate total magnifying power of a microscope. It does not do this correctly, and it also doesn’t even define magnification itself. The equation written in green at the bottom here is the actual calculation for total magnifying power. If you spot that a calculation is incorrect, it is an easy way to eliminate an option in multiple-choice questions. So, our best definition can’t be the option referring to the calculation.

By recalling our definition of resolution as the degree to which different objects can be distinguished from each other and the minimum distance apart that they must be for this to happen, we can eliminate the option referring to the minimum distance apart two objects must be for them to appear as separate items, as this is not defining magnification but resolution instead. Even without knowing the definition of resolution, magnification is not a measure of distance, helping us confirm our elimination of this option.

Our best definition is, therefore, that magnification is how much bigger the image is than the actual object.

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