Retold at your level
Keep every formula, or none. You choose how deep it goes.
Any book · your level · read aloud
Hard books, retold as lectures you can follow
Upload a textbook or any difficult book. Get it back explained at your level, read aloud, with a drawing for every idea.
- EPUB · FB2 · PDF · DjVu
- Five levels of depth
- Free sample first
- 1Upload the book
- 2Make your choices
- 3Try a free sample
- 4Generate what you need
Drop your book here
EPUB, FB2, PDF, DjVu or TXT, up to 200 MB
Your file stays private
The focal length f′ of a system is the distance from its rear principal plane H′ to the rear focal point F′. For a thin lens in air it is determined by the radii of curvature r₁, r₂ of its surfaces and the refractive index n of the glass: 1/f′ = (n − 1)(1/r₁ − 1/r₂).
A lens is stronger when its glass is denser and its surfaces are more curved. Take ordinary glass, in which light travels one and a half times slower than in air, and curve both sides like a ball twenty centimetres across. Work it through and the focal length comes out at about ten centimetres: sunlight gathers into a point a palm's width behind the lens.
What you get
What happens to your book
Read aloud
Your device reads each lecture aloud, free. The sentence being read stands out in the text.
Drawn as it is explained
Schemes and graphs build up step by step, in step with the voice.
With your own examples
Say what you work with, and the examples use your tools, not someone else’s.
Who it is for
One book, read from where you stand
The same hard book is a different problem for a student before an exam and for an engineer on a deadline. Pick yours.
Students
The textbook the course is built on, at a level you can follow tonight.
See howEngineers and specialists
The reference book of your field, at the depth today’s task needs.
See howResearchers
Read into the neighbouring field without losing a month to its textbook.
See howTeachers
See a hard chapter explained five ways before you explain it yourself.
See howCurious readers
The great hard books you always meant to read, finally followed to the end.
See howLevels
Choose how deep it goes
The same paragraph at five levels. Before anything is made, you choose a level on a paragraph from your own book.
Formulas told in words, with a worked example.
A lens is stronger when its glass is denser and its surfaces are more curved. Take ordinary glass, in which light travels one and a half times slower than in air, and curve both sides like a ball twenty centimetres across. Work it through and the focal length comes out at about ten centimetres: sunlight gathers into a point a palm’s width behind the lens.
An example paragraph. Yours will come from your book.
Study aids
Then make it stay
Add questions after each part, worked examples and a pointer back to the page. Each is optional. Try them here.
Question 1 of 3
You swap a lens for one of the same shape made of denser glass. What happens to the point where sunlight gathers?
Where the sunlight gathers From the book
Ordinary glass, in which light travels one and a half times slower than in air. Both sides are curved like a ball twenty centimetres across. How far behind the lens does sunlight gather?
Try it yourself
The same glass, but now one side is flat and only the other is curved. Where does the light gather?
Chapter 2. The thin lens · pages 41–44
Starts at: «The focal length f′ of a system is the distance from its rear principal plane H′ to the rear focal point F′…»
Every part says which chapter and pages of your book it retells. Cite the book, not the retelling, and open the original where it matters.
An example. Yours are written from your own book, and a second model answers every question before you see it. Available in the player
Styles
One style of drawing, for the whole book
Five hard passages, five styles, every drawing made by our stage. Pick one style and every scene of the book follows it.
Ways to use it
Read, watch or listen
Voice, drawings and text in any combination. Switch whenever you like.
LecturizedPhotographic Optics
VoicePicturesText
A thin lens forming an image
The first thing to know about a lens is how hard it bends light. Hold a magnifying glass in the sun: the rays arrive side by side, leave it leaning towards one another, and a little way behind the glass they meet in a bright dot. The distance from the glass to that dot is the focal length, and nearly everything else about the lens follows from it.
A lens is stronger when its glass is denser and its surfaces are more curved. Take ordinary glass, in which light travels one and a half times slower than in air, and curve both sides like a ball twenty centimetres across. Work it through and the focal length comes out at about ten centimetres: sunlight gathers into a point a palm's width behind the lens.
The book writes this as a formula, and we will come to it in a moment. It says no more than what we have just seen: denser glass or more curved surfaces, and the focal length gets shorter.
IntroductionPart 1 of 17
1:238:22
1×
Introduction
A thin lens forming an image
The distance from the glass to that dot is the focal length, and nearly everything else about the lens follows from it.
A lens is stronger when its glass is denser and its surfaces are more curved.
1:238:22
1×
On a computer
The drawing on the left page, the text on the right, the sentence being read in full ink.
On a phone
The drawing on top and the current sentence below it. Or just the voice, on a walk.
As files
Take it with you: MP3, MP4, EPUB, FB2, PDF.
How it works
How it works
- 1
Upload the book
EPUB, FB2, PDF or DjVu. Scanned pages are recognised.
- 2
Make your choices
Level, language, manner of the lecturer, drawing style, voice, your own examples.
- 3
Try a free sample
The first ten pages, complete with voice and drawings.
- 4
Generate what you need
The whole book or the first hundred pages. You see the price before anything is made.
Pricing
Pay for what you make
The sample is free. After that the price depends on how much text is retold and how many drawings are made, and you see it before you start. Listening with your device’s voice costs nothing; studio narration is an extra. Top up a balance and spend it book by book.
Upload a bookQuestions
Questions
Which books work?
Textbooks, science and engineering, and any demanding non-fiction: philosophy, economics, history. Formulas are welcome.
Which formats?
EPUB, FB2, PDF, DjVu and plain text. Scanned books go through text recognition.
Which languages?
Lectures can be in many languages, and a book in one language can be told in another.
Is my book private?
Yes. Your uploads and lectures are visible only to you. You can share a lecture by link if you want to.
Can I come back later?
Every book is saved as a session. Return any time to change settings, continue further into the book or add a download format.
Is it the author’s text?
At the first level, yes: the book is read as written. At the other levels it is retold in new words, keeping the order, the conclusions and the results.