Sorting Transparent, Opaque (and Translucent) Objects
• Each group is given a basket of objects, with examples of each of the categories mentioned above.
• You might include: clear jars/bottles/containers; plastic lunch-boxes or tumblers (you could find examples of all 3 categories among these); coloured clear plastic (e.g. wrapping on Lucozade bottles); wood; cloth; cling-film; foil containers; cooking foil; thick paper; thin paper!; card; paper plates; tracing paper; grease-proof paper.
Introduction
• Give the pupils a few minutes to explore the objects/materials which they've been given.
Then ask each group to come up with a way in which they might sort the objects into 2 sets or categories.
Ask them to call you over when they've agreed on a way of doing this.
At this stage, the pupils might choose criteria such as shape, colour, texture, etc.
• As you visit each group of children, ask them if they were all in agreement on the categorisation of the materials.

• Find out whether there were any materials which they found hard to assign to a category.
Then ask them to come up with another way of sorting the objects into 2 sets
You could tell them that it may sometimes be necessary to invent a 3rd category.
Development
• Once each group has sorted the materials along a couple of different lines, get the attention of the whole class and ask them to list the various ways in which the objects might be sorted. These could be written on the board. They may include
Shiny/not shiny
Circular/not circular
Dark/bright
Rough/smooth
Hard/soft
Paper/not paper
etc.
• If the pupils don't suggest 'see-through'/ 'not see-through' as an option, suggest it yourself at this point. Ask each group of pupils to sort the objects into 2 sets: those through which they can see and those through which it is impossible to see.
• By this stage, the pupils should have warmed to the skills involved in the task. They should also have had to deal with objects/materials which were difficult to fit into the given categories.  There may not be unanimous agreement in every group with regard to the sorting process.  Encourage pupils to discuss the differences of opinion which emerge.
• Sorting might take place along the following lines:
 See-Through Not See-Through Difficult to Classify Glass jar Glass bottle Clear plastic bottle Cling-film Tin foil Thick paper Paper plates Foil plates Cloth Wood Grease-proof paper Tracing paper Coloured clear plastic (e.g. wrapping on Lucozade bottle) Some children may even be confused by the foil
• If you haven't done so already, you could now introduce the torches and tell pupils that they may use them to help with the sorting activity.

• As you go from group to group, introduce the terms transparent and opaque (or elicit these from the children).  Get the pupils to write each word at the top of a column in their notebooks and to list the objects which fall under each heading.
By now, the pupils will have become aware of a third category. There are some materials through which you could see the torch shine, or through which daylight might be visible. Yet these aren't transparent.
If you feel that your class is up to it, you could introduce the term translucent at this stage and ask pupils to list the items which fall into this category. Closure
• Bring the whole class together again (withdraw the materials if necessary) and recap on what has been learned.
• If the pupils are up to it, check if they see any significance in the fact that the materials which didn't let the torch light through are also the ones through which we can't see. This might nudge them towards a more scientific idea about how our sense of sight works.  Or it might confuse them totally!
The class teacher can call that one.