physics

Projector

Projector is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Projector rather than just read about it. In short: A projector or image projector is an optical device that projects an image (or moving images) onto a surface, commonly a projection screen. Most projectors create an image by shining a light through a small transparent lens, but some newer types of projectors can project the image directly, by using lasers.

Key takeaways

Projector belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Projector to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Projector from memory before moving on to harder problems.

Reference excerpt

A projector or image projector is an optical device that projects an image (or moving images) onto a surface, commonly a projection screen. Most projectors create an image by shining a light through a small transparent lens, but some newer types of projectors can project the image directly, by using lasers. A virtual retinal display, or retinal projector, is a projector that projects an image directly on the retina instead of using an external projection screen.

The most common type of projector used today is called a video projector. Video projectors are digital replacements for earlier types of projectors such as slide projectors and overhead projectors. These earlier types of projectors were mostly replaced with digital video projectors throughout the 1990s and early 2000s, but old analog projectors are still used at some places. The newest types of projectors are handheld projectors that use lasers or LEDs to project images. Movie theaters used a type of projector called a movie projector, nowadays they are mostly replaced with digital cinema projectors.

Different projector types Projectors can be roughly divided into three categories, based on the type of input. Some of the listed projectors were capable of projecting several types of input. For instance: video projectors were basically developed for the projection of prerecorded moving images, but are regularly used for still images in PowerPoint presentations and can easily be connected to a video camera for real-time input. The magic lantern is best known for the projection of still images, but was capable of projecting moving images from mechanical slides since its invention and was probably at its peak of popularity when used in phantasmagoria shows to project moving images of ghosts.

Real-time Camera obscura Concave mirror Opaque projector Overhead projector Document camera Shadow projector

Still images Slide projector Large-format slide projector Magic lantern Magic mirror Steganographic mirror (see below for details) Enlarger (not for direct viewing, but for the production of photographic prints)

Moving images Movie projector Mini portable home theatres projector Video projector Handheld projector Virtual retinal display Revolving lanterns (see below for details)

History There probably existed quite a few other types of projectors than the examples described below, but evidence is scarce and reports are often unclear about their nature. Spectators did not always provide the details needed to differentiate between for instance a shadow play and a lantern projection. Many did not understand the nature of what they had seen and few had ever seen other comparable media. Projections were often presented or perceived as magic or even as religious experiences, with most projectionists unwilling to share their secrets. Joseph Needham sums up some possible projection examples from China in his 1962 book series Science and Civilization in China.

Prehistory to 1100

Shadow play

The earliest projection of images was most likely done in primitive shadowgraphy dating back to prehistory. Shadow play usually does not involve a projection device, but can be seen as a first step in the development of projectors. It evolved into more refined forms of shadow puppetry in Asia, where it has a long history in Indonesia (records relating to Wayang since 840 CE), Malaysia, Thailand, Cambodia, China (records since around 1000 CE), India, and Nepal.

Camera obscura

Projectors share a common history with cameras in the camera obscura. Camera obscura (Latin for "dark room") is the natural optical phenomenon that occurs when an image of a scene at the other side of a screen (or for instance a wall) is projected through a small hole in that screen to form an inverted image (left to right and upside down) on a surface opposite to the opening. The oldest known record of this principle is a description by Han Chinese philosopher Mozi (ca. 470 to ca. 391 BC). Mozi correctly asserted that the camera obscura image is inverted because light travels in straight lines. In the early 11th century, Arab physicist Ibn al-Haytham (Alhazen) described experiments with light through a small opening in a darkened room and realized that a smaller hole provided a sharper image.

Chinese magic mirrors The oldest known objects that can project images are Chinese magic mirrors. The origins of these mirrors have been traced back to the Chinese Han dynasty (206 BC – 24 AD) and are also found in Japan. The mirrors were cast in bronze with a pattern embossed at the back and a mercury amalgam laid over the polished front. The pattern on the back of the mirror is seen in a projection when light is reflected from the polished front onto a wall or other surface. No trace of the pattern can be discerned on the reflecting surface with the naked eye, but minute undulations on the surface are introduced during the manufacturing process and cause the reflected rays of light to form the pattern. It is very likely that the practice of image projection via drawings or text on the surface of mirrors predates the very refined ancient art of the magic mirrors, but no evidence seems to be available.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Projector

Start with the simplest possible case. Write down what Projector claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Projector before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Projector ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Projector

In research: Projector appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Projector in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Projector is common in secondary-school and first-year university syllabi. It links to neighbouring topics Display technology, Optical devices, Projectors, so understanding it makes those chapters shorter.
In everyday life: Look for Projector outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Projector in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Projector means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Projector out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Projector in simple terms?

Why does Projector matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Projector?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Projector.