explain xkcd - User contributions [en]

3097: Bridge Types

2026-01-04T12:50:13Z

2001:4450:812D:4300:6877:F10D:1B22:92F9: /* Explanation */

{{comic
| number = 3097
| date = June 2, 2025
| title = Bridge Types
| image = bridge_types_2x.png
| imagesize = 740x581px
| noexpand = true
| titletext = Pontoon bridges are just linear open-sided waterbeds.
}}

==Explanation==
This comic shows, in a four-by-four grid of images, a series of bridge types. As with [[1714: Volcano Types]] and [[1874: Geologic Faults]], the comic starts off with real bridge types, and then swiftly proceeds to make things up for absurdity. That said, real-life examples of some of the joke bridges exist, as shown in the table below. The joke lies in the progression of bridge types from the simple and straightforward to the complex and ridiculous.

{| class="wikitable sortable"
! Name given
! style="width:7em;"| Status
! Type
! Explanation
|-
! Plank
| {{Yes|Real}}
|{{w|Beam bridge}}
|A straightforward piece of solid material (in this case, made of solid wood, but there are {{w|Clapper bridge|other materials}}) is the most basic form of bridge, and generally the easiest to construct, but also the weakest. Consequently, such bridges are only suitable for small spans and light weights (such as a footbridge over a stream).
|-
! Rope
| {{Yes|Real}}
|{{w|Simple suspension bridge}}
|Rope bridges consist of several lengths of rope anchored on both sides of the span. Typically, one or more ropes will be intended to support the crossing load (possibly with boards or some other walkway between them), and additional ropes will act as handrails, reducing the risk of falling. These are typically only intended for foot traffic, due to their light construction and lack of rigidity. Because of the simple materials and relative ease of construction, they're often used as improvised bridges. However, a ridiculous design for this bridge type was explored in [[3048: Suspension Bridge]], in which the rope functions as a road.
|-
! Truss
| {{Yes|Real}}
|{{w|Truss bridge}}
|A truss is a common type of framework consisting of supports connected in a series of triangles which provide support for a load. This design provides significant strength and rigidity with minimal material and weight. A truss bridge can either have the truss above the bridge platform (as in the drawing) or underneath it (also known as a deck truss). This is the first bridge type on this list which is commonly used for vehicle traffic.
|-
! Trestle
| {{Yes|Real}}
|{{w|Trestle bridge}}
|A trestle bridge is held up by supports reaching all the way to the ground beneath. Typically at least some of the supports will slope outward to give a larger base of support. Once common for railroads, these are less popular nowadays, but are still seen in certain areas and applications.
|-
! Arch
| {{Yes|Real}}
|{{w|Arch bridge}}
|Arches are one of the oldest kinds of bridges for carrying significant loads. They can be made out of rock or metal. Each span consists of an arch resting on supports. Simple arch bridges rest on both sides of a river or other gap, but longer bridges (as in the drawing) will have intermediate pillars to support multiple arches. The arches distribute the load, allowing a relatively small number of pillars to support weight across the entire deck of the bridge.
|-
! Suspended Arch
| {{Yes|Real}}
|{{w|Through-arch bridge}}, possibly {{w|Tied-arch bridge}}
|A through arch bridge uses a similar concept as an arch bridge, but the deck is entirely or partially suspended below the top of the arch (in this case fully suspended, at the bottom of the arch). The tie of a tied arch bridge refers to using the deck as a tension member to restrain the horizontal spreading of the arch; without a tie, the arch's foundation must resist that horizontal loading. From the picture, it is not clear whether the deck is acting as a tie. Such bridges may use a single arch (as in the drawing) or multiple arches in succession.
|-
! Draw
| {{Yes|Real}}
|Drawbridge (more precisely a fixed-trunnion {{w|Bascule_bridge|bascule bridge}})
|Drawbridges are used to allow ships to pass through obstacles like bridges. They use various methods to raise one or multiple sections of the bridge to create enough height clearance for vessels to pass through, in this case using a cable to rotate the bridge about a pivot point (trunnion).
|-
! Suspension
| {{Yes|Real}}
|{{w|Suspension bridge}}
|A suspension bridge suspends its deck with cables or rods from a cable linked to a pillar and a point a certain distance from each pillar
|-
! Filler
|{{Maybe|Real}} method of maintaining {{w|Grade (slope)|grade}}, not really a 'bridge'
|{{w|Embankment (earthworks)|Embankment}}, {{w|Causeway}} or even a {{w|Dam}}
|Serves the purpose of allowing travel across the gap, but by removing (or {{w|Culvert|mostly removing}}) passage through the gap itself. By filling the gap with hard, irregular material (most commonly rocks), support can be provided, while still allowing water to flow through the gaps. Due to the generally small size of the gaps, generally only slow-flowing water can reliably get through.
|-
! Budget Overrun
| {{Yes|Real}} (with an absurd name)
|{{w|Cable-stayed bridge}}
|Specifically, the pictured bridge is a {{w|cantilever spar cable-stayed bridge}}, similar in appearance to the {{w|Samuel Beckett Bridge}} in Dublin or the {{w|Erasmusbrug}} in Rotterdam. Many bridges in this category suffer severe cost overruns. Randall may be drawing upon his local knowledge of the {{w|Leonard P. Zakim Bunker Hill Memorial Bridge|Zakim Bridge}} in downtown Boston's {{w|Big Dig}}, also strongly associated with cost overruns. Also, while not a bridge, the budget overrun and diagonally inclined tower supporting cables may be a reference to {{w|Montreal Tower}} and {{w|Olympic Stadium (Montreal)|Olympic Stadium}}, which has suffered infamous budget overruns, costing over 6.7 times the initially planned amount.
|-
! Jump
| {{No|Not real}}
|Similar to a pair of small {{w|cantilever bridge}}s, constructed at an incline.
|A "bridge" that appears to be being used by a skateboarder, though in a manner far more dangerous than any jump in a typical skatepark. While not {{w|London Buses route 78#History|normally}} a feature of the highway, jump ramps can be used for gap-crossing stunts by almost any vehicle with sufficient speed. Partial bridges, which allow ''some'' vehicles using them to safely cross the gap, iconically featured in {{w|The Dukes of Hazzard}} TV show, as well as common in various {{w|The Man with the Golden Gun (film)#Car stunts|action}} {{w|Speed (1994 film)#Filming|films}}, though typically less easy to use correctly than the setting implies.
|-
! Halfhearted
|{{Maybe|Not}} real under this name, but with real analogs
|[https://www.archdaily.com/184921/moses-bridge-road-architecten Moses bridge]
|The diagram shows that there was barely any attempt to bridge the gap in the landscape at all, just take the 'deck' down into it and back up out again. The concept may have been inspired, in part, by [https://www.fastcompany.com/90186315/the-strange-art-of-the-melting-bridges-of-google-earth an artifact in Google Earth software].
Structures exist, at the {{w|Fort de Roovere}} in Halsteren, Netherlands and elsewhere, that resemble this 'solution', though these would have involved much thought and commitment in their building, possibly more 'hearted', even, than any more conventional bridge design, especially in the provision of stairs to allow easier ingress/egress (at least for foot traffic) than in the comic's version.
|-
! Waterbed
| {{No|Not a bridge}}
|{{w|Waterbed}}
|Rather than a bridge, it is more like another version of a causeway (see 'Filler') using trapped water to maintain the upper surface.
Named for a 'mattress' type, which is usually a raised surface ''on top of'' a piece of bedframe, with an unusual approach to padding and comfort.
|-
! L'Engle
| {{No|Not real}}
|[https://www.refinery29.com/en-us/2018/03/192728/tesseract-definition-wrinkle-in-time-space-dimension Tesseract AWIT] not {{w|Tesseract|Tesseract (geometry)}} cf. {{w|Einstein–Rosen bridge}}
|References {{w|A Wrinkle In Time}} by Madeleine L'Engle. Characters cross great distances by "tessering", moving via a tesseract through a higher dimension which essentially brings the two ends of the journey together from the perspective of the traveler. The image shows the two ends of the gap being brought together, with the gap apparently crumpled in between them.
|-
! Fun
| {{No|Not real}}
|{{w|Vertical loop}}/{{w|Rollercoaster}}
|It is a loop-de-loop, not normally a practical or necessary way of bridging a gap. Something previously seen, in an arguably even more impractical manner, in [[2935: Ocean Loop]].
|-
! Repurposed Elevator
|{{Maybe|Real}}, but not as displayed
|Horizontal {{w|elevator}}/{{w|People mover}}
|There are various implementations of such designs, but the best-known one is probably the {{w|Schmid Peoplemover|Schmid Peoplemover}}. However, unlike a regular peoplemover, where the door stays upright, the image shows a regular elevator that has been rotated 90 degrees. This not actually its own type of bridge, but just a type of plank bridge where the elevator shaft (or one side of the elevator shaft) is used as a plank - closing the circle to the first image. This could also be a reference to Wellington Station on the Boston Subway, {{w|Wellington_station_(MBTA)#People_mover|where there was a vertical people mover}} over the yard but was later repurposed into a walkway (bridge).
|-
! (Title text)
| {{Yes|Real}}
|{{w|Pontoon bridge}}
|Pontoon bridges are described as a series of fictitious "waterbed bridges", as shown above, but constructed without sides. This would mean that that the 'bed'-supporting water flows in one side and out the other, if there is any passage or tidal flow of water. It may technically mean that you cannot cross {{w|The Same River Twice|the same bridge twice}}.
Pontoons rely upon buoyancy, either of the whole deck or distinct floating elements, whereas an enclosed "waterbed" bridge would rely upon the strength of the membrane to keep the mass of water within it, and thus the deck above that mass.
|}

==Transcript==
:Bridge Types

:[A 4x4 matrix of 16 ways to cross the same rectangular hole in the ground]

:Plank [A plank laid over the hole, with Cueball standing on the left side of the plank, just past the edge of the hole.]

:Rope [A rope bridge with rope guardrail. Cueball is standing in the middle, causing the bridge to sag about one Cueball-length down.]

:Truss [A truss bridge with a triangular truss above the bridge deck. Cueball is visible in the center of the third truss from the left.]

:Trestle [A trestle bridge, slightly higher than the edges of the hole, with small ramps at either end. Cueball is standing about a third of the way from the left.]

:Arch [A straight deck, supported by what looks like three stone or brick arches. Cueball is notably absent.]

:Suspended Arch [A single large arch spanning the entire gap, with the bridge deck suspended from the arch by nine support cables. Cueball is standing approximately in the middle, between the fourth and fifth cables from the left.]

:Draw [A truss bridge split into two parts, the right half supported by a single straight pillar in the hole and the left half being pulled upwards by a large winch. One Cueball is standing on the outermost part of the left half and another is standing on the innermost part of the right half, directly over the pillar.]

:Suspension [A bridge deck supported by cables hung from two pillars and going to the edges of the hole, with smaller cables linking those to the deck. Cueball is standing just left of center on the bridge.]

:Filler [The hole is filled with rocks, which are larger than Cueball at the bottom but diminish in size the further up they are. Cueball is standing a third of the way from the left edge of the hole.]

:Budget Overrun [A bridge deck suspended from a single pillar that starts in the bottom left corner of the hole, then angles to the right such that the top of it is above the middle of the deck. Nine support cables are visible, but another one is likely behind the pillar itself. Cueball is standing between the thoitd and fourth cables from the right.]

:Jump [Ramps are present on both sides of the hole. Cueball, riding a skateboard, is in the air approximately a third of the way from the left, indicating that he has jumped off of the left ramp.]

:Halfhearted [A simple bridge deck, sagging so much that the majority of it is on the floor of the hole. Cueball is standing in the hole, about two-fifths of the way from the left.]

:Waterbed [The hole is now full of water, with two fish and an octopus visible in the water. Two Cueballs are crossing, and seem to be struggling to maintain their balance as the bridge warps and undulates under their feet.]

:L'Engle [There is no bridge; instead, the terrain itself is warped such that the top corners appear to be pulled together to join in the center, with arrows illustrating their movement, although there is still a void in the lower center of the hole. Cueball is hopping from the left protrusion to the right.]

:Fun [The bridge does a rollercoaster-style loop in the center. The entirety of the bridge is supported by vertical struts. Cueball, on a skateboard, is halfway to the top of the loop.]

:Repurposed Elevator [Instead of a bridge, there is an elevator tower rotated on its side, with 7 evenly spaced exits. One Cueball is standing to the left of the elevator tower, one Cueball is on top of it and appears to be hugging it, and one Cueball is in the elevator carriage on the "seventh floor," on the right side of the hole.]

{{comic discussion}}<noinclude>

[[Category:Engineering]]
[[Category:Comics featuring Cueball]]
[[Category:Multiple Cueballs]]
[[Category:Animals]]

3182: Telescope Types

2026-01-04T11:30:19Z

2001:4450:812D:4300:6877:F10D:1B22:92F9:

{{comic
| number = 3182
| date = December 17, 2025
| title = Telescope Types
| image = telescope_types_2x.png
| imagesize = 517x680px
| noexpand = true
| titletext = I'm trying to buy a gravitational lens for my camera, but I can't tell if the manufacturers are listing comoving focal length or proper focal length.
}}

==Explanation==
{{incomplete|This page was created recently ACCORDING TO A TELESCOPE POINTING BACK IN TIME. Don't remove this notice too soon.}}
This comic shows diagrams of a number of different types of {{w|telescope}} — some real, while others are other objects, or made up by [[Randall]]. It includes both refracting and reflecting designs; see [[1791: Telescopes: Refractor vs Reflector]] for the important (according to Randall) differences between them.

{| class="wikitable"
! Type !! Telescope? !! Refractor? !! Reflector? !! Description
|-
| {{w|Reflecting telescope#Prime_focus|Prime focus}} || Yes || No || Yes || A telescope design where the observer/receiver is situated at the focal point of a single mirror. Rare in optics, but a common design in radio telescopes.
|-
| {{w|Herschelian telescope|Herschelian}} || Yes || No || Yes || A telescope design much akin to Prime Focus but with the mirror tilted so that the observer does not block incoming light. Named after astronomer William Herschel.
|-
| {{w|Newtonian telescope|Newtonian}} || Yes || No || Yes || Newtonian telescopes employ a second, flat mirror along with the primary parabolic mirror.
|-
| {{w|Galilean telescope|Galilean}} || Yes || Yes || No || What might usually come to mind when picturing a telescope. A long tube that uses lenses rather than mirrors (making it a refracting telescope) to magnify images.
|-
| {{w|Keplerian telescope|Keplerian}} || Yes || Yes || No || An improvement on Galilean telescopes, using a convex lens rather than a concave one at the eyepiece (as shown in the diagram). It does however invert images.
|-
| {{w|Gregorian telescope|Gregorian}} || Yes || No || Yes || Uses two concave mirrors, the secondary being placed beyond the primary's focal point. The image is reflected back through a hole in the primary mirror. Unique among reflectors in that the image is not inverted.
|-
| {{w|Cassegrain telescope|Cassegrain}} || Yes || No || Yes || Similar to prime focus, but uses a secondary mirror to reflect light through a hole in the primary mirror to the observer (situated at the rear)
|-
| {{w|Cardboard}} tube || No, does not magnify || No || No || Children may sometimes use tubes, particularly the cardboard middles from paper rolls, as a play 'telescope'. Looking through a tube can give an illusion of magnification by removing distractions and focusing your attention on the object in view, but it doesn't actually magnify the object being viewed. It will still cause a minor optical effect due to {{w|diffraction}} on the edges of the tube.
|-
| {{w|Kaleidoscope|Kaleido''(scope)''}} || No, just used to create decorative patterns || No || Yes || A {{w|kaleidoscope}} is similar in form to the stereotypical 'ship's telescope', being a tubular object that you look in to one end of. However, it isn't really a telescope, because you can't use it to magnify arbitrary objects of interest. The non-viewing end is closed, and you view patterns created by many fragmented reflections of tiny objects contained at the end, rather than remote objects. The mirrors (set lengthways and angled to each other) are also usually flat, so provide no magnification.
|-
| {{w|Liquid mirror telescope|Liquid mirror}} || Yes || No || Yes || A telescope with the same design as Prime Focus, using a rotating pool of reflective liquid (most commonly mercury) as a mirror, the steady rotation creating a parabolic lens. The diagram adds a straw so that someone can drink the liquid. This would not improve telescope performance or end well for the drinker (if the liquid was to be anything but water).
|-
| Narcissian || No, observers view themselves || No || Yes || This is like a prime focus telescope, but the focus is outside the end of the telescope where the viewer is located, so they can only see themselves, magnified by the concave mirror. This is inspired by the myth of {{w|Narcissus}}, who fell in love with his reflection in a pool of water. A {{w|house of mirrors}} (a typical attraction at a funfair) might feature such a 'telescope', because it is basically a concave mirror.

Furthermore, a narcissist, someone who is inordinately self-centered and arrogant (named for Narcissus), would likely appreciate this kind of mirror, as a narcissist considers self-viewing more worthwhile than viewing the outside world.
|-
| {{w|Gravitational lens|Gravitational}} || Not as shown || Yes || No || Using the gravitational effect of very large objects on the light passing around them to gain a magnified (if distorted) view of objects beyond them. These are formed naturally by large stars (particularly {{w|black holes}}) and galaxies, but can't be constructed on Earth{{cn}}. There are proposals to launch missions to the very far reaches of the Solar System to "construct" a {{w|Solar gravitational lens}} telescope, but the masses and distances involved are not compatible with consumer camera hardware. In the title text, Randall is concerned about whether the listed focal length of a gravitational lens is measured in the {{w|comoving and proper distances|comoving or proper}} reference frame — that is, whether the expansion of the universe (between the place and time of the lens's creation or construction and Randall's decision to purchase) has been factored out or not. At the cosmological scales between stars and galaxies, where gravitational lensing is most relevant, this is a useful distinction to make, but [https://iauarchive.eso.org/public/themes/buying_star_names/ stars are not for sale] (by any legitimate commercial entity) and so nobody would be advertising any focal length in either reference frame for any purchaser.
|-
| Geological || No || No || Yes || This 'telescope' employs a single mirror to show the observer the 2003 movie {{w|The Core}}, which was broadly derided by science-minded people. As a telescope it would not be useful, not least because it cannot be pointed at an arbitrary object. Its relevance to real geology is also dubious.
|}

==Transcript==
{{incomplete transcript|This page was created recently ACCORDING TO A TELESCOPE POINTING BACK IN TIME. Don't remove this notice too soon.}}
This comic shows diagrams of a number of different types of {{w|telescope}} — some real, while others are other objects, or made up by [[Randall]]. It includes both refracting and reflecting designs; see [[1791: Telescopes: Refractor vs Reflector]] for the important (according to Randall) differences between them.

{| class="wikitable"
! Type !! Telescope? !! Refractor? !! Reflector? !! Description
|-
| {{w|Reflecting telescope#Prime_focus|Prime focus}} || Yes || No || Yes || A telescope design where the observer/receiver is situated at the focal point of a single mirror. Rare in optics, but a common design in radio telescopes.
|-
| {{w|Herschelian telescope|Herschelian}} || Yes || No || Yes || A telescope design much akin to Prime Focus but with the mirror tilted so that the observer does not block incoming light. Named after astronomer William Herschel.
|-
| {{w|Newtonian telescope|Newtonian}} || Yes || No || Yes || Newtonian telescopes employ a second, flat mirror along with the primary parabolic mirror.
|-
| {{w|Galilean telescope|Galilean}} || Yes || Yes || No || What might usually come to mind when picturing a telescope. A long tube that uses lenses rather than mirrors (making it a refracting telescope) to magnify images.
|-
| {{w|Keplerian telescope|Keplerian}} || Yes || Yes || No || An improvement on Galilean telescopes, using a convex lens rather than a concave one at the eyepiece (as shown in the diagram). It does however invert images.
|-
| {{w|Gregorian telescope|Gregorian}} || Yes || No || Yes || Uses two concave mirrors, the secondary being placed beyond the primary's focal point. The image is reflected back through a hole in the primary mirror. Unique among reflectors in that the image is not inverted.
|-
| {{w|Cassegrain telescope|Cassegrain}} || Yes || No || Yes || Similar to prime focus, but uses a secondary mirror to reflect light through a hole in the primary mirror to the observer (situated at the rear)
|-
| {{w|Cardboard}} tube || No, does not magnify || No || No || Children may sometimes use tubes, particularly the cardboard middles from paper rolls, as a play 'telescope'. Looking through a tube can give an illusion of magnification by removing distractions and focusing your attention on the object in view, but it doesn't actually magnify the object being viewed. It will still cause a minor optical effect due to {{w|diffraction}} on the edges of the tube.
|-
| {{w|Kaleidoscope|Kaleido''(scope)''}} || No, just used to create decorative patterns || No || Yes || A {{w|kaleidoscope}} is similar in form to the stereotypical 'ship's telescope', being a tubular object that you look in to one end of. However, it isn't really a telescope, because you can't use it to magnify arbitrary objects of interest. The non-viewing end is closed, and you view patterns created by many fragmented reflections of tiny objects contained at the end, rather than remote objects. The mirrors (set lengthways and angled to each other) are also usually flat, so provide no magnification.
|-
| {{w|Liquid mirror telescope|Liquid mirror}} || Yes || No || Yes || A telescope with the same design as Prime Focus, using a rotating pool of reflective liquid (most commonly mercury) as a mirror, the steady rotation creating a parabolic lens. The diagram adds a straw so that someone can drink the liquid. This would not improve telescope performance or end well for the drinker (if the liquid was to be anything but water).
|-
| Narcissian || No, observers view themselves || No || Yes || This is like a prime focus telescope, but the focus is outside the end of the telescope where the viewer is located, so they can only see themselves, magnified by the concave mirror. This is inspired by the myth of {{w|Narcissus}}, who fell in love with his reflection in a pool of water. A {{w|house of mirrors}} (a typical attraction at a funfair) might feature such a 'telescope', because it is basically a concave mirror.

Furthermore, a narcissist, someone who is inordinately self-centered and arrogant (named for Narcissus), would likely appreciate this kind of mirror, as a narcissist considers self-viewing more worthwhile than viewing the outside world.
|-
| {{w|Gravitational lens|Gravitational}} || Not as shown || Yes || No || Using the gravitational effect of very large objects on the light passing around them to gain a magnified (if distorted) view of objects beyond them. These are formed naturally by large stars (particularly {{w|black holes}}) and galaxies, but can't be constructed on Earth{{cn}}. There are proposals to launch missions to the very far reaches of the Solar System to "construct" a {{w|Solar gravitational lens}} telescope, but the masses and distances involved are not compatible with consumer camera hardware. In the title text, Randall is concerned about whether the listed focal length of a gravitational lens is measured in the {{w|comoving and proper distances|comoving or proper}} reference frame — that is, whether the expansion of the universe (between the place and time of the lens's creation or construction and Randall's decision to purchase) has been factored out or not. At the cosmological scales between stars and galaxies, where gravitational lensing is most relevant, this is a useful distinction to make, but [https://iauarchive.eso.org/public/themes/buying_star_names/ stars are not for sale] (by any legitimate commercial entity) and so nobody would be advertising any focal length in either reference frame for any purchaser.
|-
| Geological || No || No || Yes || This 'telescope' employs a single mirror to show the observer the 2003 movie {{w|The Core}}, which was broadly derided by science-minded people. As a telescope it would not be useful, not least because it cannot be pointed at an arbitrary object. Its relevance to real geology is also dubious.
|}

{{comic discussion}}<noinclude>

[[Category:Science]]
[[Category:Astronomy]]
[[Category:Geology]]
[[Category:Telescopes]]
[[Category:Movies]]
[[Category:The Core]]
[[Category:Comics featuring Cueball]]