If you have to write an introduction to your lab report it should give the reader enough background information so that the context and purpose of your experiment are clear. Here are some tips about writing your Introduction:

Read Read your laboratory notes, textbooks and, in some cases, other sources such as journal articles.
Draft Draft and re-draft your introduction a number of times. Leave the final draft until you have written up the rest of your report and you have a clearer understanding of how your results and your interpretation of your results relate to your aim and your introduction as a whole.
Write Two paragraphs is usually enough - you don’t need to write everything that is known about the topic. Your lecturer wants to see that you understand the topic, so try to use your own words combined with the technical language of your field.
Avoid Be careful not to copy your laboratory notes and textbooks.  Also avoid quotations.

The next screen will show a diagram of the typical stages of the Introduction.

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Peter explains the importance of the purpose of the report in structuring the report (33s):

Peter talks about how to reduce the information in a report (31s):

John discusses planning issues in report writing (27s):

John talks about researching for report writing (1m 25s):