Lesson Plan: The Beam

Subject areas: Physics, technology and engineering, history of science Recommended level: Ages 13 to 18 (lower and upper secondary), adaptable for introductory undergraduate Duration: One to two 50 minute lessons

Learning objectives

By the end of this lesson, students will be able to:

1. Explain how Bell's photophone encoded sound onto a beam of light and decoded it again (modulation and detection).

1. Describe photoconductivity and the role of selenium in converting light back into an electrical signal.

1. Explain why the photophone failed to find use in 1880 and what later technologies (laser, optical fiber) made optical communication practical.

1. Summarise the 2026 photonic chip that can generate, steer, and read light-based information on a single device, and why computing with light is attractive.

1. Reflect on why a correct idea can look like a failure if it arrives before the supporting technology exists.

Background for the teacher

In 1880 Alexander Graham Bell, already famous for the telephone, and his assistant Charles Sumner Tainter built the photophone. A beam of sunlight was reflected off a thin mirror; speaking at the mirror made it vibrate, which brightened and dimmed the reflected beam in time with the voice. This is modulation: the sound information is carried by changes in the light. At the receiving end, the beam fell on a selenium cell. Selenium is photoconductive: its electrical resistance falls when light hits it, so the flickering beam produced a varying electric current that drove a telephone receiver. On 3 June 1880 the system carried a voice about 213 metres across the rooftops of Washington, the first wireless voice transmission, on light.

Bell thought it was his greatest invention, greater than the telephone. But it had almost no practical use. Sunlight is unreliable, there was no strong controllable light source, and no way to guide the beam through weather or over distance. The idea was about a century early. Only with the laser (1960) and low-loss optical fiber did light become the backbone of communication. Today most of the world's data travels as pulses of light through glass fibers, including the cables crossing the oceans. In June 2026 a Monash University team led by Dr Chi Li, publishing in Nature Photonics, reported a chip that can generate, steer, and read information carried by light on one integrated device, using atomically thin materials and a quantum property of light called the valley degree of freedom (valleytronics). Computing with light promises speed and low energy use, important for AI and quantum machines. The photophone, in a sense, finally arrived.

Warm up (10 minutes)

Shine a torch at a small mirror taped to a balloon or a stretched piece of plastic, reflecting the spot onto a far wall. Have a student hum or speak against the membrane and watch the reflected spot quiver. Ask: how could that quiver carry the sound of a voice to someone far away?

Core activities

Activity 1: Modulating light (15 minutes). Diagram the photophone: sunlight, vibrating mirror, beam, selenium cell, receiver. Students label where the sound is encoded onto the light and where it is decoded. Define modulation in their own words.

Activity 2: Light to electricity (15 minutes). Introduce photoconductivity with a simple light-dependent resistor (LDR) in a circuit: cover and uncover it and watch a meter or a buzzer respond. Connect this to what selenium did in 1880.

Activity 3: Right idea, wrong decade (15 minutes). Students list what the photophone needed but did not have (a strong steady beam, a guiding medium, fast detectors) and match each to the later invention that supplied it (laser, optical fiber, modern photodetectors and chips).

Discussion questions

1. Why is modulation necessary to send a voice on a beam of light?

1. The photophone worked, yet it was useless for decades. Can an invention be both a success and a failure? What decides?

1. Most of the internet now runs on light through glass. How does that vindicate Bell?

1. The 2026 chip computes with light at room temperature. Why might using light instead of electrons save energy, and why does that matter for AI?

Extension

• Research how fiber-optic communication works and why glass fibers lose so little light.

• Investigate other inventions that arrived ahead of their supporting technology and were only useful much later.

Glossary

• Photophone: Bell's 1880 device that transmitted speech on a beam of light.

• Modulation: varying a carrier (here, a light beam) to encode information such as sound.

• Photoconductivity: the property by which a material's electrical resistance changes when light falls on it.

• Selenium: the photoconductive element used in the photophone's receiver.

• Optical fiber: a thin glass thread that carries light over long distances with very little loss.

• Photonics: the science and technology of generating, controlling, and detecting light.

• Valleytronics: encoding information in the 'valley' quantum property of electrons or light in certain crystals.