Where does this come from, and can I check it?

A short barrel of lenses shows a whole landing through a small hole. The figure is taken from US Patent and Trademark Office, which is linked in full on this page. Every fact on Mondo Knows names its source rather than repeating an unsourced claim.

Is this really a everyday life fact?

Yes. It sits on the Everyday Life shelf, one of 2,600 sourced facts grouped into space, animals, the human body, history, science and more. Category pages collect every fact in the same field.

What else can I read about it?

The links under the fact point to a quiz, game, puzzle or calculator on the same subject, and the shelf below shows related facts. The wording here is original, and quoted phrases are marked as quotations.

Mondo Knows Facts

A door viewer is a fisheye lens

A short barrel of lenses shows a whole landing through a small hole The viewer is a tube of glass elements, with a strongly curved front lens that gathers a field of view of roughly 160 to 200 degrees. The rear lens shrinks that wide image to something an eye can take in from a few centimetres away. A hinged cover on the inside stops light and draughts passing through when it is not in use. It sits in Mondo's everyday life collection, one of 200 facts that each trace back to the reference below.

Where this comes from

US Patent and Trademark Office

Facts that can change over time are reviewed and updated. If something here looks wrong, Mondo would rather know than be confidently incorrect.

Go further

Technology trivia General Knowledge trivia

More facts like this

A deadbolt has no spring, unlike a door latchThe latch slides back by spring and bevels to close, but the bolt does not Long door cylinders can be snapped at the middleThe weak point sits where the lock body meets the handle plate A letter plate hinges inward over a hoodThe flap sheds rain while the hood keeps water off the opening itself Public doors often open outward for escapeA door swinging with the escape route empties faster than one pulled inward Door closers slow the swing with oilA spring shuts the door while hydraulic fluid meters how fast it moves Cylinder pin-tumbler door locks align split pins along a shear lineKey ridges lift spring-loaded split pins to unlock door cylinders

Tags: doors, optics, security

A door viewer is a fisheye lens: the short answers

Where does this come from, and can I check it?: A short barrel of lenses shows a whole landing through a small hole. The figure is taken from US Patent and Trademark Office, which is linked in full on this page. Every fact on Mondo Knows names its source rather than repeating an unsourced claim.
Is this really a everyday life fact?: Yes. It sits on the Everyday Life shelf, one of 2,600 sourced facts grouped into space, animals, the human body, history, science and more. Category pages collect every fact in the same field.
What else can I read about it?: The links under the fact point to a quiz, game, puzzle or calculator on the same subject, and the shelf below shows related facts. The wording here is original, and quoted phrases are marked as quotations.