2 Planning for effective demonstrations

Miss Bulsara asked another science teacher to review her plans for a demonstration of images formed by a convex lens (Activity 10.12).

This is the first time I have taught this topic. There are a lot of points to cover in the practical activity about images in lenses, and I was worried that my students might lose interest or become distracted if the demonstration took too long. I wanted to get someone else’s opinion on what I was planning to do, so I asked my colleague Mrs Gupta if we could meet after school in the room where I will teach the lesson, and talk through the demonstration.

The room has a nice, large demonstration bench, so I put my equipment tray down on the bench and began to set up the demonstration.

Mrs Gupta asked ‘Which lens are you planning to use first? It will affect how you set up the lines and the lens holder.’

I hadn’t decided, so I spent a few minutes checking the focal length of three different convex lenses, then putting one of them in a lens stand and drawing the parallel lines on the table. I made a note on my plan about which lens to use for the demonstration and how far apart the lines needed to be. I would need this information to help me set up quickly in the lesson.

I was starting to mark the F and 2F positions when Mrs Gupta commented, ‘This is a very quiet class today, isn’t it?’

We laughed, because I was so absorbed in following the instructions that I had forgotten about how it would appear to my students. I was about to say, ‘I’m marking this line as F’ when I stopped myself. Perhaps it would be better to ask one of my students why they thought I had lines drawn at these particular distances, and why I was labelling them F and 2F? Another quick note to myself on the plan!

I set up the screen and lit the candle, then made a few adjustments to get a nice sharp image on the screen with the candle as far away as possible.

‘Where will your students be? Will everyone be able to see the image?’ asked Mrs Gupta. It was worth checking. It would have been difficult for some of my students to see the image from where they would normally stand. Another quick note on my plan!

I drew up a table like Table 10.4 on the blackboard, with all the different object positions in the first column but with everything else left blank. After showing everyone the first example, for all the other object positions I planned to ask a student to show me where I should place the candle each time, and someone else where they thought the image would be. I would ask someone else to describe the image they could see on the screen.

I was going to tell my students to fill in the information themselves during the demonstration, so they would need to keep paying attention. But Mrs Gupta suggested that this might be more of a distraction, especially when everyone would know that the table with all the answers was in the book. Instead, I decided to fill in the table on the board as we worked through all the different positions, so students could see how we had arrived at the completed table. Another note on the plan!

By the end of our discussion, I felt I had a plan that would really help me to deliver a more effective demonstration. Our discussion had also made me think about what I could do to improve my demonstrations for other topics, too.