Objects smaller with distance (again)

[Benjamin Fergus-grey]

Hi all,
Ok so the forum and internet are littered with the answer to the question of why objects appear smaller with distance but - can anyone explain this to me as if I'm 5 years old please? My son came out with this question the other day and I have no idea how to break it down for him.
Thanks in advance...

 

[pixel]

You see objects by the image that they form on your retina. As the distance of an object increases, its image on your retina gets smaller. To explain that further would require some simple ray trace diagrams that you probably don't want. But if you have a lens handy, you can form the image of, say, a lamp on a piece of paper and notice how the size of the image varies as you move the lamp further away.

Equivalently, the size of an object is judged by the angle that the object subtends at your eye. As an object recedes into the distance, that angle gets smaller.

 

[A.T.]

 

[Benjamin Fergus-grey]

Thanks guys, but i need to put those answers to a five year old. This is the challenge! I understand the concept, but lack the ingenuity of a teacher.

 

[Benjamin Fergus-grey]

This is getting close:
But if you have a lens handy, you can form the image of, say, a lamp on a piece of paper and notice how the size of the image varies as you move the lamp further away

Thanks!

 

[sophiecentaur]

Thanks guys, but i need to put those answers to a five year old.

One thing you need to remember about five year olds is that, with very few exceptions, they do not think at the same level as an 'educated' adult.
Piaget, an early and well respected educational psychologist, identified a number of different stages in cognitive development. This link describes them and gives the age ranges that are associated with the different cognitive levels. For five year olds, we are talking in terms of the Preoperational stage. They are still having problems in estimating relative sizes and have not sorted out many of the basics that come later in the Concrete Operational stage.
Your problem with 'explaining' the perceived size of an object is actually more of a Formal Operational level, which can take until late teenage, early adult - or even for ever. Many people are too busy to bother with some of the intellectual processes that academics love. Many people function at a very high level, judging by the results of running a successful company or playing chess - even without thinking about the why's and wherefores.
So your five year old is more than adequately served with the 'fact' that things look smaller as they get further away. I would suggest that being able to hide several bricks at a distance behind a single nearby brick would be enough and easily demonstrated. (Concrete) It's the what happens more than the how it happens. Steer clear of "angle subtended" etc. they are more likely to look out of the window at a passing cow. Father Ted explains

There's always a caveat with statements about psychology, of course ; many of the theories are not really falsifiable like good old Physics theories all are. But Piaget is still highly regarded and I have confirmed a lot of his ideas from personal experience of teaching and parenthood.

 

[Benjamin Fergus-grey]

Thank you. You are quite right of course. My son asks many questions about how things work. We recently had a great conversation about how clouds are formed, he was happy with the mechanisms behind evaporation and thermal behaviour, etc. This one is tricky precisely because of the explanation being mathematical in nature.
I shall present him with a practical demonstration and see if it satisfies his curiosity.

 

[sophiecentaur]

Good luck. I have been there, of course, but we don't remember just how we, ourselves learned this stuff. It is a big help for your son, to have someone who is prepared to spend time in answering these questions. It's amazing how many adults just aren't interested in such stuff and can't / won't discuss it with kids. Worse still are the adults who churn out nonsense in the "nature abhors a vacuum" style. That absolutely kills any investigative spirit and propagates the 'anti boffin' attitude.

because of the explanation being mathematical in nature.

Perhaps you can present it in a less 'mathematical' way. At least, a diagrammatic and less demanding way with just a hint of the word "angle" involved.

 

[Tom.G]

Try placing him in front and up close to a sofa or an easy chair. Then ask him to turn his head so he is looking at the end or edge of it.

Then move him several feet away and try ther same head-turn experiment.

Explain that things look a particular size according to how much of the world, or view, they cover up.