Primary Science – Light and Shadow Topic

Tricky Question on Light and Shadows

This science revision note aims to help your child:

–  identify how different positions affect the size of the shadow.

– Which part of the shadow to look out for?

 

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Arif had 3 different cardboard shapes of identical height and width, as shown below.

He created a setup by placing the different cardboard at different positions (E, F and G) as shown in the diagram below to make a shadow.

 

The shadow he observed is as follows:

(a) Identify the cardboards (“X” shape, Ring shape and Plus shape) at positions E, F and G respectively.

E:                                                                                           

F:                                                                                           

G:                                                                                          

 

 

Arif wants to rearrange the cardboard shapes such that he will be able to obtain this shadow

(b) How should he arrange the cardboard? Write your answer in the table below.

 

Let’s answer part (a)

 

STEP 1

 

Identify which cardboard corresponds to which shape.

To do this, we have to look at the shadow Arif observed.

As we can see, the 3 shapes form shadows of 3 different sizes.

 

Take note of this concept: The bigger the shadow, the closer the cardboard is to the light source.

 

STEP 2

Let’s analyse this first. Arrange the 3 shapes from the smallest to the biggest shadow.

The plus shape shadow is the biggest.  That means the plus shape cardboard is the closest to the torch.

However, what do you notice about the ring shape shadow and the “X” shape shadow?

Both shadows look like they are of the size.

Aha!

That is the trap laid out by the examiner!

When you look at the ring shadow and the “X” shadow carefully, one of the shadows is actually larger than the other!

 

How come?

Let’s consider this:

When both the X-shaped cardboard and the ring-shaped cardboard were at the same height and width (AND at the same position away from the torch), the edges of the “X” would be able to be seen outside the circumference of the ring (see the diagram below to help visualise it).

Did you see the part of the “x” shadow juts out from the ring shadow? 

This happens when the ring-shaped and X-shaped cardboards are placed at the SAME position (in other words, both cardboards are at the same distance away from the torch.)

 

Now, what happens when I put the X-shaped cardboard further away from the torch (we did not change the position of the ring-shaped cardboard? How will the shadow look like now?

It will look like this:

There’s no jutting out of the “X” shadow!

Tricky isn’t it?

We need to visualise and understand this before we can proceed.

So, pause your reading here…

Digest the information above…

And make sure you can visualise how the shadow changes when we move the objects.

Once you are ready, let’s move to the next step!

 

STEP 3

From the question

Looking back at the 1st shadow Arif formed, the ring shadow is larger than the “X” shape shadow.

The ring shadow is larger than the “X” shadow.

Thus, arranging from smallest to biggest shadow: “X” shadow à Ring shadow à Plus shadow.

The “X” shadow is the smallest. That means the X-shaped cardboard is furthest away from the torch. The X-shaped cardboard is at position G.

Plus shadow is the largest. That means the Plus-shaped cardboard is closest to the torch. The Plus-shaped cardboard is at position E.

Finally, the Ring-shaped cardboard is at position F.

 

FINALLY, put your answer together.

E: Plus-shaped cardboard

F: Ring-shaped cardboard

G: X-shaped cardboard

Let’s answer part (b)

From the question

Arif wants to rearrange the cardboard shapes such that he will be able to obtain this shadow:

(b) How should he arrange the cardboard? Write your answer in the table below.

STEP 1

Looking at the shadow Arif wants to obtain, identify the sizes of the shapes from largest to smallest.

The smallest shape is the plus.

 

STEP 2

Similar to the previous part analysis, since the edges of the “X” shadow do not exceed the circumference of the ring in the shadow Arif wants to make, the ring shadow is larger than the “X” shadow.

Hence, from biggest shadow to smallest shadow: ring-shaped shadow à  X-shaped shadow à Plus-shaped shadow.

 

Arrange the cardboard so that they can form the shadow Arif wants to obtain.

 

FINALLY, put your answer together.

Question: How should he arrange the cardboard? Write your answer in the table below.

Answer:

 

Additional learning material for this topic

Do read another tricky question on the light and shadow (apply the correct set of rules!) >>> https://scienceshifu.com/light-and-shadow-apply-the-correct-rules/

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