1, use PS to open the electron microscope picture to which the ruler needs to be added (Note: the ruler length of 1cm is equivalent to 1? m).

2, new layer, through the menu bar layer, new layer.

3. Click and select the box selection tool in the left toolbar, and box a suitable rectangle. When selected, an ant line will be displayed.

4. Select the paint bucket tool in the left toolbar.

5. Use the paint bucket tool to fill the selected area with white. Here, you need to set the foreground color to white first.

6. Create a new floor and draw a 3 cm long black line. Here is a clever way to draw the thickness of black, that is, use a rectangular toolbox to select an area with a length of 3cm, and fill it with black after the width is appropriate.

7. Finally, add the size, that is, create a new layer (of course, you can also write directly on it), and write with a text tool. At this point, the ruler has been made.