In today’s article, I will be analysing an examination question on Light from the 2019 CHIJ St. Nicholas Girls’ School (SNGS) P6 SA1 Examination Paper.

I will also be sharing three common Light questions that you can familiarise yourself with ahead of your Science examinations.

You may also choose to watch the video here.

## Let’s Get Started On This Question!

Source: CHIJ St. Nicholas Girl’s School — P6 SA1 Examination Paper [Q36]

Let us first define what the question means by transparency. Transparency refers to how much light is able to pass through the material.

There are three different types of transparency of a material:

1. Opaque: Opaque materials do not allow any light to pass through.
2. Translucent: Translucent means that it allows some light to pass through.
3. Transparent: Transparent means that the object allows most light to pass through. Take note that we say ‘most’ light, not ‘all’ light, as a bit of light will still be blocked.

When no material is placed at position P, the light sensor shows a reading of 100 units. The 100 units in this question represent the brightness of the torch.

Materials X, Y and Z are placed at Position P, one at a time, and the readings recorded by the light sensor are shown in the table below.

Let us examine the results of the experiment. The numbers indicate the amount of light recorded.

Bearing in mind that the total amount of light emitted from the torch is 100 units, we can deduce that:

• Material X allows 95 units of light to pass through it. This means that is it a transparent material.
• Material Y allows 0 units of light to pass through it. This means no light at all, so Material Y is opaque.
• Material Z allows 50 units of light to pass through it, so it is translucent.

## Let’s Analyse Part (A)

“Based on the table, compare the transparency of materials X and Z.”

Some students will answer this question as “X is transparent, while Z is translucent.”, simply stating the properties of each material.

Unfortunately, that is not the answer that teachers are looking for.

## Suggested Answer For Part (A)

X allows more light to pass through it than Z.

## Let’s Analyse Part (B)

Source: CHIJ St. Nicholas Girl’s School — P6 SA1 Examination Paper [Q36]

What property of light allows this set-up to work?

Objects move between the light sensor and the light source. The light sensor registers the amount of light it receives. The brightness detected rises and falls regularly as Object A passes between them.

Most students are quick to conclude that the light is blocked because Object A is opaque and does not allow light to pass through.

💡 However, the term “opacity” is a property of the material and not a property of light!

The correct answer is light travels in a straight line.

If light could go around any obstruction, the opacity or transparency of any material would be completely irrelevant to the set-up as light would simply bend to reach the sensor!

However, as light travels in a straight line, any object passing between it and the sensor would be blocked.

This is shown on the graph by the sensor readings dropping to 0 units. If light could bend around any obstruction…

Then the sensor would register as 100 forever, making the set-up redundant.

⭐️ Tip 1: One trap that students tend to fall for is the duration of the time stated in the question.

Some questions will include a graph that reads 20 seconds, but only requires students to derive 8 seconds’ worth of data.

Thankfully, this particular question is quite straightforward. Let’s examine the graph!

Source: CHIJ St. Nicholas Girl’s School — P6 SA1 Examination Paper [Q36]

In order to read this graph, we must first understand how the reading is derived from the sensor.

As mentioned above, whenever an object passes between the light and the sensor, the light is blocked, causing the sensor to detect 0 units of light. This means that the line on the graph will similarly drop to 0.

Therefore, you will need to count the number of times the line touches the bottom of the graph.

On the timeline, it will be as follows: (2 to 4 seconds), (6 to 8 seconds), (10 to 12 seconds), (14 to 16 seconds).

That’s 4 times, so the answer is 4 objects.

4 objects.

## Let’s Analyse Part (C)

“The light source and the sensor are placed 5 cm above the belt. State whether an object less than 5 cm in height can be counted by the light sensor. Give a reason for your answer.”

Let’s visualise this scenario:

## Suggested Answer For Part (C)

No, because the object is shorter than 5 cm, the light from the torch, which travels in a straight line, cannot be blocked by the object.

⭐️ Tip 2: It is extremely important that students include the property of light (i.e. light travels in a straight line) in the answer.

## Let’s Analyse Part (D)

“When identical objects B made of material Y were placed on the moving belt, the number of objects counted in 18 seconds were fewer. The data recorded is as shown below.”

Source: CHIJ St. Nicholas Girl’s School — P6 SA1 Examination Paper [Q36]

“Based on the above graph, state a difference between object A and B.”

Object B is made of Material Y. This is the same material used to make Object A. We can immediately rule out a difference in opacity.

Next, let us count the number of objects that passed through the sensor in 18 seconds:

• 2 to 6 seconds;
• 8 to 12 seconds; and
• 14 to 18 seconds

That’s 3 objects. Not only are there less objects passing through, but the amount of time it takes for the object to pass through the sensor has increased!

What does this mean? It means that Object B is larger than Object A. Its larger width causes it to block the light for a longer period of time as it takes a longer time for it to pass through the pathway of the light.

## Suggested Answer For Part (D)

Object B is larger than Object A.

## Common Examples Of How Light May Be Tested During Examinations

### 1) The Light Source Is Also The Object To Be Seen

Instead of describing how light is reflected off the object, you need to describe how the light from the object/light source travels into their eyes directly.

A typical item used for this scenario is a handphone since the screens are backlit and do not require an additional light source to be visible.

### 2) The Light Source Is NOT The Object To Be Seen

In this scenario, you must describe how the light from the lamp reflects off the object into the eyes, allowing the person in the question to see the object.

You may also be expected to define shadows and explain how shadows are formed.

💡 Shadows are formed when light from a light source, which travels in a straight line, is blocked by an object that is translucent or opaque.

You will need to be able to relate these shadows to three properties: shape, size and shade.

Many of such questions will describe a torch shining on an object (or a combination of objects) from different angles or distances.

You will be expected to predict the likely shape and size of the shadow formed from a particular angle of the torch and vice versa.

For the last property, shade, it is determined by the opacity of the material. A material that is opaque will cast a darker shadow than a material that is translucent, and a material that is transparent will cast no shadow at all.

## Conclusion

I have covered the essential concepts on the topic of Light that you will need to be familiar with in order to do well in your Science examinations. I hope that you also spend some time looking through the three other common questions that I have highlighted so that you will not be caught off guard during your Science examinations, but can tackle them confidently and accurately.

Check out our other Light articles and stay tuned for more articles! 🙂

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