Mummyâ€™s birthday is around the corner and youâ€™re planning a surprise party for her! đźŽ‰

Youâ€™ve arranged with your closest relatives and family friends to hide in the dark living room and catch her unaware when she arrives home from work. However, as the living room has to be dark and you have to be hidden from sight in order for Mummy to not be suspicious, how will you be able to tell when she is back and successfully surprise her?

You bring up this issue to your Science teacher, Ms. Joanna, after your lesson. She suggests using a light sensor connected to a voice recorder & speaker to help solve your problem. How will this device work and how can it help you with your party?

## Building the Device

First, what items do we need to build this device?

1. Light Sensor With Data Logger X 1
2. Light Source X 1
3. Some Wires
4. Voice Recorder With a Speaker and Playback Function X 1

## Setting Up the Device

Here is how the set-up should look like:

What is the function of a light sensor?

A light sensor is able to measure the intensity of light from a light source. When a light sensor is connected to a meter (or commonly known as a data logger), it records the amount of light that is received from the light source (torch).

When an object (X)Â  is placed in between the torch and the light sensor, the amount of light received by the light sensor depends on the transparency of the object.

Do you recall the 3 properties of materials that relate to light?

## Testing the Accuracy of the Device

Letâ€™s test the accuracy of the device by placing 3 different materials (cardboard, tracing paper and glass) one at a time between the torch and the light sensor (Position X).

Here are the results from the experiment:

What are we able to interpret from the results in the table above?

No Object

When no object was placed at Position X, the light sensor received 80 lux of light. This shows that the torch emits 80 lux of light.

Cardboard

When a cardboard piece was placed at Position X, the light sensor did not receive any light (0 lux). This shows that no light from the torch passed through the cardboard, causing light from the torch to be blocked completely. This is considered to be opaque.

Glass

When the glass was placed at Position X, the light sensor received 79 lux of light. Receiving 79 units of light shows that most of the light from the torch passed through the glass piece, indicating that very little light was blocked by it. This confirms our understanding that glass is transparent.

Tracing Paper

When tracing paper was placed at Position X, the light sensor received 40 lux of light. What happened to the other 40 lux of light? It must have been blocked by the tracing paper. Hence, tracing paper allowed some light to pass through and blocked some light, which indicates that it is a translucent object.

Here is another way we are able to present the experiment results:

In order to analyse the above graph most effectively, we should identify the amount of light that is blocked by the object. Letâ€™s indicate the amount of light being blocked by the different objects in green.

No Object

When no object was placed in between the light sensor and the light source, no light is being blocked.

Cardboard

When the cardboard was placed in between the light sensor and the light source, all the light from the light source was being blocked by the cardboard. This shows that the cardboard did not allow any light to pass through it.Â Thus, the cardboard is opaque.

Glass

When the glass is placed in between the light sensor, very little light was blocked by the glass. This shows that most of the light passed through the glass. Thus, the glass is transparent.

Tracing Paper

When the tracing paper was placed in between the light sensor and the light source, some light from the light source was being blocked by the tracing paper. This shows that the tracing paper only allowed some light to pass through it.Â Thus, the tracing paper is translucent.

## How Does the Device Work?

Now weâ€™ve determined that the light sensor can detect transparent, translucent and opaque objects, how will this help us with surprising Mummy? Hereâ€™s where the voice recorder comes in!

We will record a welcome message â€śSURPRISE! HAPPY BIRTHDAY!â€ť and connect the voice recorder to the light sensor. The recording will be played when Mummy (who is opaque) walks in through the door and blocks light from the light source! Once everyone hears the recording, they will know that the guest of honour has arrived and will be able to jump out with party poppers!

Now that you know how a light sensor works, do you think it can also count the number of guests entering the living room as well? Yes, the light sensor will be able to do so!

Here is how we will be able to use the light sensor to count the number of guests entering,

The data collected by your light sensor is recorded in the graph below. Are you able to tell how many people arrived to surprise Mummy in the first 60 minutes?

If your answer is 7, you are correct! If you did not get the right answer, let us take a look at the graph again.

When a guest walks past the light source, ALL light will be blocked by the guest. Thus, no light will be detected by the light sensor. In order to count the number of guests that have entered, we just have to look at the points where the amount of light is 0 (as shown by the red cross (x) on the graph above). Therefore, we can determine that 7 guests entered within 60 minutes.