Parts of a Flashlight – Lens, Reflector, Bulb and Switch

All 10 parts on the diagram

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Outside 5

Lens The clear cover on the front that protects the bulb.
Head The wide front end that holds the bulb and the reflector.
Switch Sliding it on completes the circuit, and the bulb lights.
Case The tube that holds the batteries and serves as the handle.
Tail cap The cap that screws onto the back end of the case.

Inside 5

Spring A small spring at the back that the batteries rest on.
Batteries The power source; they sit end to end inside the case.
Contact strip A thin metal strip that carries electricity between the parts.
Reflector A shiny cup round the bulb that sends its light forward.
Bulb The light source: a filament bulb or an LED.

How this list was compiled: Every name checked on 2026-09-30, a name kept only where two independent sources use it. (1) Wikipedia, Flashlight, https://en.wikipedia.org/wiki/Flashlight, text only (the light source set in a reflector, a clear cover or lens, battery and switch, all in a case; threads for the head and tail cap). (2) How Products Are Made, volume 6, Flashlight, http://www.madehow.com/Volume-6/Flashlight.html (batteries in a line inside the handle, the threaded head that houses the bulb, the reflector, the slide switch). (3) A battery maker's page, How Does a Flashlight Work?, https://energizer.com/about-flashlights/how-does-a-flashlight-work/, names and what each part does only (case, contacts, switch, reflector, lamp, lens, batteries; the batteries rest on a small spring joined to a contact strip). (4) US Patent 2,599,254 (1952), Angular head flashlight casing, https://archive.org/details/us_patent_2599254 (barrel, head, cap screwed onto the barrel, lens, reflector, bulb, conductor strips, spring, switch). Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; no maker's flashlight, nothing traced. Licence: own work

How it works

The parts of a flashlight you can see are the tail cap, the case, the switch, the head and the lens. The second drawing shows the inside of a flashlight: a spring, batteries, contact strips, the bulb and its reflector.

Read the full guide

Outside

Numbers 1 to 5. The lens is the clear front of the head. The switch sits on the case, and the tail cap closes the back end.

Inside

Numbers 6 to 10. The batteries rest on the spring. A contact strip runs along the wall to the switch, and another runs on to the bulb, which sits in the reflector.

The bulb may be a filament bulb or an LED; the drawing shows a filament bulb.
In this drawing the raised positive end of each battery points toward the bulb.

Questions and answers

How does a flashlight work?

Sliding the switch on joins two contact strips and completes a circuit. Electricity flows from the batteries through the strips to the bulb, which glows. The reflector sends the light forward through the lens.

What does the reflector in a flashlight do?

It is a shiny cup round the bulb. It redirects the light rays from the bulb forward, so they leave the flashlight as a steady beam.

What is the spring in a flashlight for?

The batteries rest on it, and it is part of the circuit. The spring touches the end of the back battery and is joined to a contact strip that runs along the case to the switch.

What is the glass on the front of a flashlight called?

The lens. It is the clear cover on the front of the head, often plastic, and it protects the bulb and the reflector behind it.