Visible Light

## Smartphone activities

Practical Activity for 11-14 14-16 16-19

There is a dizzying array of microscopic sensors inside the latest smartphones. Even a bog-standard mobile will have at least 10 sensors packing enough punch to allow anyone to transform their device into a powerful mobile physics lab. In this collection, all the practical activities use smartphones and all of them can be done at home or in school.

Visible Light
Light, Sound and Waves

## Transmitting light

Practical Activity for 11-14

In this activity students explore how the number of tracing paper sheets changes the amount of light that is transmitted.

## Equipment

Each students will need:

• A smartphone with phyphox installed
• A light source
• 10 sheets of tracing paper (large enough to cover the phone camera)

## Perparation

Close blinds or curtains and turn off any background light sources to ensure you are in a dark room.

Consider pairing or grouping students to ensure each student has access to a device with the light sensor. This can be checked in the phyphox app (if it doesn’t have a light sensor, “light” will be greyed out).

## Instructions

1. Open the phyphox app on their phone, select ‘light’ and select ‘simple’.
2. Identify whether the app is gathering data from the front or back camera on their phone by each camera respectively while the app is taking readings. The experiment will be easier to perform if the front camera is taking readings.
3. Place their phone face up on a table and set up the bright light source above their phone.
4. Press the play button, let the illuminance reading settle and record their reading for 0 sheets of paper (the background light level).
5. Place a sheet of tracing paper on top of the phone, covering the light sensor but leaving the display visible, and record the reading for 1 sheet.
6. Repeat as 9 more sheets are added.
7. Plot a graph of illuminance vs number of sheets.

## Teaching notes

Students will have seen that light can pass through some materials and recognise from everyday experiences that more translucent materials will let more light pass through. This activity allows them explore transmission in an experimental context.

Typical results are shown below:

As an extension activity, you could ask the students to plot illuminance vs distance using the same set-up, but the students now measure the light level at increasing distances. It is also possible to repeat the experiment for other papers (tissue) or even transparent/translucent materials such as coloured plastic film.

## Learning outcome

Students describe how the amount of light transmitted by a material depends on thickness.

Simple Harmonic Motion
Light, Sound and Waves

## Period of a spring (SHM)

Practical Activity for 16-19

In this activity students use the phyphox app to determine the spring constant, k, using a mass-spring system.

## Equipment

• A phone with phyphox installed
• A mass balance
• A set of springs
• A set of masses
• A clamp and clamp stand
• A tight elastic band or homemade holder

## Preparation

Make sure the phones have protective cases to protect them should students accidentally drop their phones.

Consider pairing or grouping students to ensure each student has access to a device with the “spring” function. This can be checked in the phyphox app (if it doesn’t have the “spring” function it will be greyed out).

## Instructions

1. Open the phyphox app and click on “Spring”.
2. Record the mass of the phone in kg.
3. Suspend the spring from a clamp and attach the phone securely to the bottom of the spring (e.g., using a tight elastic band wrapped around the midline of the phone) and secure the stand so that it does not move as the phone and spring oscillate (either clamp the stand or put a large mass on the base).
4. Pull the phone vertically downwards a few centimetres, press play and release.
5. Make a note of the period once the oscillations have settled, which takes approximately 15 seconds.
6. Suspend a 50 g mass from the bottom of the phone.
7. Repeat steps 3-4 adding 50 g each time up to 300 g.
8. Plot a straight-line graph to determine the spring constant, k.

## Teaching notes

If students are unfamiliar with the relationship for the period T of a spring introduce it:

T = 2π    m k

They can determine k by plotting T 2 against m, where each value of m is the mass of the phone plus the standardised mass set the students are using. The gradient of the straight-line graph will be equal to  2/k. The line of best fit should pass through the origin.

The results should look similar to to the graph below:

As an extension activity, students can investigate adding more springs in series and/or parallel to see how this changes the period and effective spring constant.

## Learning outcome

Students describe an experiment to determine the spring constant of a spring.

Simple Harmonic Motion
Light, Sound and Waves

## Period of a pendulum

Practical Activity for 16-19

In this activity students use the phyphox app to determine the gravitational field strength using a pendulum.

## Equipment

Each student will need:

• A phone with phyphox installed
• A length of string
• A small clear plastic bag which can be sealed (optional)

## Procedure

1. Open the phyphox app and click on Pendulum.
2. Click on the top right part of the screen and switch on do a timed run (use the default settings of 3 seconds for start delay and 10 seconds for experimental duration).
3. Securely attach the phone to one end of the string. For example, you could put it in a clear plastic bag and then attach the string.
4. Hold the other end of the string and pull the phone-pendulum to one side. Press play and let the pendulum go.
5. Keeping your hand steady, let the phone swing for more than 13 seconds to enable the app to record for at least 10 seconds. Make a note of the period from the reading.
6. Measure the length of the pendulum l (length of string + distance to centre of phone).
7. Repeat steps 4 to 6 for more pendulum lengths.
8. Plot a straight-line graph to determine the acceleration of free-fall g.

## Teaching notes

If students are unfamiliar with the relationship for the period T of a pendulum introduce it:

T = 2π    l g

They can determine g by plotting T 2 against l (the gradient of the straight-line graph will be equal to  2/g).

## Learning outcome

Students describe an experiment to determine gravitational field strength.

### Resources

Speed of Sound
Light, Sound and Waves

## Speed of sound

Practical Activity for 14-16

In this activity students work with a partner at home (or in school) and use phyphox app to estimate the speed of sound.

#### Equipment

Each pair of students will need:

• Two phones with phyphox installed

#### Procedure

1. Place the phones 3 m apart.
2. Stand next to their phone and open the phyphox app. Click on the Acoustic stopwatch and press play. Their partner should do the same.
3. Clap once. When their partner hears the clap they should also clap.
4. Subtract the times displayed on the phones to find the difference. This gives the time delay for the sounds to travel 6 m (twice the distance between the phones).
5. Divide 6 m by the time delay to work out the speed of a sound wave.
6. Repeat for two more distances and find an average value for the speed of sound.

#### Teaching notes

The acoustic watch starts counting when it hears a noise above a certain threshold and stops when a second noise is detected. The phone closest to the source of the noise will detect it before the other phone. If the phones are a distance d apart and the speed of sound is v, the further away phone will detect a sound wave a time dv after the nearby phone. In this experiment there are two claps, and so the total distance travelled is 2d. Finding the difference between the times t1 and t2 displayed on the phones eliminates your partner’s reaction time and gives the following relationship for the speed of sound:

2dt1  −  t2

Students should be able to obtain an estimate for the speed of sound of between 300 and 400 ms -1. Echoes can cause erroneous readings. Students could try to identify sources of possible echoes and eliminate them, change location or increase the threshold on the acoustic stopwatch if this proves to be an issue.

#### Learning outcome

Students describe an experiment to estimate the speed of sound.

### Resources

Sound Wave
Light, Sound and Waves

## Whistling waveforms

Practical Activity for 11-14 14-16

Students download an oscilloscope app onto their phones to investigate pitch and loudness of sounds.

## Equipment

Each student will need:

• A smartphone
• Two different sized bottles with long necks (optional)

## Preparation

Students will need download an oscilloscope app with a pause function.. For example, they could use Oscilloscope (xyz apps) from the Play store (tap screen to stop trace) or Oscilyzer from the App Store (use pause button to stop trace).

## Procedure

2. Whistle and observe the trace on the screen. If they can’t whistle, blow over a bottle to make a sound.
3. Whistle a loud, steady note pause the trace. If they missed it, repeat to try to catch the waveform mid-whistle.
4. Repeat, but this time whistle more quietly.
5. Whistle with a high pitch and then a low pitch.

## Discussion prompts

• How does the waveform change with loudness?
• How does the waveform change with pitch?

## Teaching notes

If students can’t whistle, they can blow over bottles or use a musical instrument such as a guitar or a recorder. They should see that when the sound is louder the peaks of the waveform are bigger, and that when the pitch is higher the peaks are closer together.

## Learning outcome

Students sketch waveforms for sounds with different volumes and frequencies.