Getting Started with Light

Use the Ambient Light sensor to investigate the light around you.

Try this:

1. Find the Light icon and press to open the Ambient Light sensor.

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2. Your phone has a light sensor—can you find it?

It’s usually located on the front face of the phone near the top. Try slowly moving your finger across the phone while you watch the sensor card on Science Journal. The sensor card is measuring lux, a unit of illumination. When the sensor card reading is close to 0 lux, you’re covering the sensor.

3. Move your hand near and far from the sensor.

What do you notice? Make observations in both meter and graph modes.

4. Investigate the light in your environment. If you’re inside, explore all corners of the room:

How do artificial light sources compare to natural light from a window? What happens near a wall, under a table? Where do you find the lowest reading? Where do you find the highest reading? What happens if you place the light sensor at different angles to the light source? Does tilting the phone make a difference?

If you’re outside, explore the natural light from the sun and the different shadows it creates.

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What’s Going On?

The ambient light sensor measures light in lux, a measure of illumination that depends on the amount of incoming light and the area over which it is spread.

A full moon provides about 1 lux of illumination, a typical lamp-lit living room about 50 lux, classroom lighting and sunrise and sunset about 400 lux, daylight (indirect sun) over 10,000 lux, and direct sun over 30,000 lux.

Lux is an unfamiliar unit for most of us. When we buy lightbulbs, we often shop by watts—but the wattage of a bulb doesn’t tell you how bright a bulb is, only how much energy it uses. Lumens are a better measure for light shopping; they tell you how much light a bulb will actually produce. Lux, meanwhile, tells you how much of that light arrives at a particular area.

Since light spreads out as it travels, the number of lux dwindles as you back away from a light source, even though the source is still emitting the same amount of light in lumens. An area that is tilted away from a light sources also receives less illumination—which happens to be why the equatorial regions here on Earth are so much warmer than the chilly poles.

By the way, the purpose of the light sensor in your phone is to control the light level of the screen, adjusting accordingly to dim and bright environments. Inside the light sensor, a tiny semiconductor responds to incoming light by producing a small but measurable electrical current—a phenomenon known as the photoelectric effect. Similar sensors are used in some streetlights to turn them on when it gets dark outside.

Going Further

Here are a few of the many experiments you can do with your light sensor.

Experiment with light from different sources (incandescent, compact fluorescent, and LED bulbs), or with light reflecting off of opaque surfaces and traveling through translucent and transparent materials.

Multiple light sources shining on a single object will create multiple shadows—an umbra or total shadow where all the light is blocked, and a penumbra or partial shadow where one source is blocked and another source fills in. How do the light measurements in the various shadows compare?

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Make observations over a longer period of time: What happens if you record the natural light, either through a window or outdoors, in the same place at the same time every day? How does the light change over the course of an hour? A day? A week? A month? A year?

For maximum efficiency, solar panels must be tilted at an angle to maximize their illumination by the sun over the course of the day. Can you use the light sensor to determine exactly what this angle would be for your location? Would your result be different at a different time of year?


If your light meter is maxing out in full sun, try putting sunglasses or another light filter in front of the light meter.

Put a single point source of light in a darkened room and measure the illumination at various distances from the light. (A candle makes a good point source, as does a Mini MagliteTM if you unscrew the cap so the small halogen bulb is exposed.) How does doubling or tripling the distance affect the measurement?

Light obeys a mathematical relationship known as the inverse square law —as your distance from the source increases, the amount of light arriving at an area decreases in proportion to the square of the distance. Mathematically, the relationship is expressed as 1/r2, where r is the distance from the source. Not just light, but sound, gravity, electric force, and even spray paint obey the inverse square law. Do your measurements seem to confirm the law? Body Conductivity