This post comes unashamedly on the tails of Pritesh Raichura’s excellent series on teacher explanation which you can read here. I’ve written recently on dual coding and the multimedia effect because, like Pritesh, I’m worried that dual coding is in danger of lethally mutating beyond its evidence base. For me, dual coding is a process that is best used when explaining difficult material, not when making jazzy posters or the like. To summarise my previous article:

  • The working memory includes two channels: verbal (language) and visual (things you see that don’t have anything to do with language)
  • Utilising both increases the capacity of working memory and allows for a greater number of entities to be processed at once
  • This is called dual coding
  • When dual coding is carried out effectively, there is a boost to processing and learning, known as the multimedia effect

Teachers have been using diagrams to support instruction for a long time though, and the above is probably not hugely new (even if it is refined through a strong cognitive model). In my opinion, there are two powerful ideas that go with the above which can make a huge difference in your classroom:

  • Redundancy is when you include information that is superfluous to your explanation – without it the explanation would be just fine
  • Split attention is when students have to split their attention between different things/spaces/areas/ideas/processes too much

Generally you want to avoid these. The way I do it is by using “live drawing” – constructing an image from scratch. This allows me to make sure that I am in complete control of the flow of information and that my students’ attention does not wander away from where I am directing it. It’s a pretty simple technique, but it does take some practice. Below is an example of how I use it to teach core concepts around microscopes and magnification to year 7.

Introduction to cells

First, I introduce my students to the idea of a cell. I do a bit of hinterland here by discussing people like van Leeuwenhoek and using the scale of the universe tool to establish some frames of reference.


At this point I jump straight into using microscopes. I do this as a Slow Practical so I can show students exactly how to carry the microscope, how not to carry it, how to set up the specimens etc etc. Works quite nicely.

Come on, we came here for live drawing not 18th century Dutch biologists

I’ll do the rest as a kind of script, not because I literally script it but because I think the blog will flow better. There are various points at which I would pause for students to write down a definition or whatever, but broadly this is the process:

Look at these small squares.


They are quite small, only just big enough for me to see. But I really want to see them bigger. What can I use to make them look bigger? Yes, a microscope. And when I look down my microscope I see those same squares but bigger.


This is called magnification.


We can break this big word into two smaller words. Magna is from Greek and it means great or big. The -ification bit just means to make: so magnification means to make big.


It’s about taking something really little and making it look bigger. We don’t actually make it bigger, it just looks bigger.

Ok, so does anyone remember from the practical, if I want to make something look even bigger, what do I do?

[student answers use the focusing knobs]

I’m glad you said that, because it is wrong. That’s not what we use the knobs for. We use the knobs to do something called focusing. Do you remember when you first looked through the microscope, it looked a bit like this:

3.1Right. So it’s not really that clear, is it? kind of blurry. So now I turn the focusing knobs and it looks more like this:


To focus something means to make it look clearer. But how can you tell by looking at the picture – and this is a hard question – how can you tell by looking at it that I have not changed the magnification?

Excellent. The magnification hasn’t changed because it’s exactly the same size, it’s just clearer.

So lets go back to our first diagram

[construct the diagram again, but ask lots of questions of the students about how you should do it and why – this is your check for understanding]


Let’s imagine that the thing I’m looking at is 1mm in length. I put it under the microscope which makes things look bigger.

5.1Let’s say it just makes things look twice as big. We’ll call that 2x, because it means “two times as big.”


So in this case, if I’m making it look two times bigger, how big will it look to me? Yes, it will look 2mm big.

That thing that you see, the bigger thing, that’s called the image. And the “image size” here is 2mm. The first thing, the actual thing, we call that the “actual size” and in this case its 1mm.

7.1Ok let’s do another one. If I’ve got something with an actual size of 1mm, and a magnification of 100x, how big will it look to me?


Excellent, yes. It will look 100mm. Now let’s do some more examples [have students do a couple more examples either together or with mini-whiteboards or whatever]

Ok, now let’s go back to one of our first diagrams


I want to be able to work out how big something will look, but I don’t think I can be bothered to draw it all out every time. So what we do is write an equation. An equation is like a recipe: it tells me what to do. Now, mathematically, can someone remind me how I used 1mm and 100x magnification to get the image size? Yes – I multiplied them together. I could write that like this:


Mission accomplished. You have explained what focus is, what magnification is, what the magnification equation is and where it comes from. You’ll probably want to model maybe one more example, perhaps with 2mm and x100 magnification just to vary a bit, but asking lots of questions as you go. Once you’ve done that you can use whatever equation/formula method you use to give students shed loads of practice and you can go home and feel confident that you’ve totally bossed this explanation. 

UPDATE: I was asked why I use a visualiser instead of just drawing straight onto the board. Good question, few answers:

  1. Drawing on the board hurts my shoulder – constant writing, rubbing out etc etc
  2. With the visualiser I can see the class while I write
  3. My handwriting tends to be better