What things (factors) could affect the angle at which the ray travels in the second medium (glass or perspex in this case), or the point where it exits the second medium, or the amount it is displaced by travelling through the second medium, or whatever variable (the dependent variable) you are intending to investigate?
Which of these would it be possible and sensible for you to investigate? (These are the key factors.) You are, of course, going to choose to vary the angle of incidence.
How are you going to make sure that any changes are only due to the factor you have chosen to vary? (A fair test)
What are you going to do to make your results accurate? (Not just "repeat"... repeat what? Not just "take care"... take care to do what? etc)
How exactly are you going to measure the angles or distances? What precautions are you going to take to make sure that this is really accurate? Does the ray exiting the other side make a "repeat reading"?
What do you think is going to happen to the dependent variable when you change your angle of incidence? Can you be as numerical as possible? (If you say "doubling one causes the other to double" you need to support this with a model (see below) or a preliminary experiment which gives you good reason to suggest your prediction)
Do you have any scientific knowledge (models of the behaviour of waves in various situations) which could help you to support your prediction? (Demonstrations you have seen, or information you have read. Always quote sources: "My notes"; "Cutnell & Johnson Pg 114" etc)
Why not have a look at these Applets?
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