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Lesson 9

The Observer Effect


To perceive that light acts as a wave or a particle depending on how you observe it. 


1. Using what you learned in Lesson 8: Young's Interference, choose the correct conclusion to the following statement:

 If a photon is shot through two slits and hits a screen, it:

a.     cannot hit the middle because it is blocked by the material between the slits

b.     is completely random and has equal chance of hitting the screen anywhere

c.     must hit at the maximum of the interference pattern

d.     has some chance of being anywhere, but on average has a better chance of being where the interference is brightest

e.     will hit anywhere it has a straight shot from either slit

Watch the following clips:

2. Using the new information presented in the videos answer the following question:

During the last lesson, you learnt,  using Young's Double Slit experiment,  that light displays wave properties. If we were to repeat Young’s Experiment with really small objects such as electrons instead of light, what results would you expect to observe?


View the Observational Dependency slides, summarise the slides into your workbook and discuss with your teacher. 

The Observer Effect

Watch the following video:


(25 mins)

Use the PhET Interactive Program to complete the PhET activity worksheet - Quantum Wave Interference (see below) modelling wave-particle behaviour during Young's Double slit experiment.

This applet is designed so that you can observe when photons, electrons, and atoms behave like particles and when they behave like waves, and that the act of measuring/detecting can change the waves and the patterns they produce on the screen.


(5 mins)

Select topic A or B:

A: Justify how the double slit experiment explains that matter and light behave as waves.

B: Describe the behaviour of a wave function in the presence and absence of a detector.

Think about your understanding (1 min) and share it with another student (1 min each student) and then reflect (1 min).


(40 mins)

Observational Dependency Activity on qCraft. (You will need to discuss and negotiate this activity with your teacher.)

Log into qCraft on a computer with the qCraft Curriculum map loaded (see the How to install qCraft with MinecraftEdu section on the Appendix of the Teacher Guide). Complete the PLAY challenge activity from Lesson 1 in the qCraft Teacher guide.