Study app for Class 5 to 10
CBSE Class 9 and 10 exam prep with virtual labs, mock tests and audio revision, plus 30-minute AI basics from Class 5.
The Light chapter, open in the app
Drag the angle and the ray bends by Snell’s law. Move the object and the lens shows where the image lands.
- sin i / sin r
- 1.520
- angle r
- 25.0°
- critical angle
- none
Into a denser medium, so the ray bends towards the normal.
Question bank, mock tests and revision audio
Every answer is written out step by step, in the order a CBSE marking scheme gives marks.
Solves each question in marked steps
Long answer · 5 marks
A ray enters water (n = 1.33) from air at 45°. Find the angle of refraction.
- Step 1 sin r = sin i / n
- Step 2 sin r = 0.7071 / 1.33 = 0.5317
- Step 3 r = sin⁻¹(0.5317) ≈ 32.1°
Towards the normal, as expected for air into water.
Names the topic you got wrong
3/5
- Lens formula
- Power of a lens
Reads the chapter aloud in 4 minutes
No ads, no feed and no autoplay. KnullVoid opens on the chapter you left, and the screen holds only that chapter.
AI basics for Class 5 and up, 30 minutes a day
Day 1 is “How machines learn patterns”. The child teaches a small machine with real examples and sees it make guesses.
Open the Day 1 sessionRead the cards
8 min
Five short cards: what a pattern is, and how a machine spots one.
Teach the machine
10 min
Label six fruits. The machine guesses three it has never seen.
Check your understanding
5 min
Three questions, marked at once.
Ask the AI teacher
7 min
It answers with a question back, so the child does the thinking.
Prices for students and schools
Start free with no card. Upgrade only when you want the full syllabus.
Free Starter
₹0
- Class 10 Light chapter
- Both virtual labs
- Question bank with steps
- Day 1 of AI basics
Student Plus
₹299
a month, or $5 outside India
- Every chapter as it goes live
- Unlimited timed mock tests
- Audio revision for each chapter
- AI teacher (beta)
School licence
Price on request
for schools and coaching institutes
- Student accounts for a whole batch
- Class-wise mock test results
- The same app students use at home
Null on the screen, no void in the chapter
The name is two promises. Null: nothing on the screen except the lesson. Void: the gap in what you understand, which the mock test finds so you can fill it before the exam.
Questions from students and parents
Yes. Chapters follow the current NCERT textbooks for Class 9 and 10, and question types match the CBSE pattern: MCQ, assertion-reason, case-based, short and long answer.
Class 10 Science, Light: Reflection and Refraction, and Day 1 of AI basics. KnullVoid is in public beta, and chapters are added as each one is finished and checked.
No. It asks you a question back, gives a hint, and checks your next step. It stays on the lesson you have open.
Yes, with a school licence: student accounts for a whole batch and class-wise mock test results. Email us for a price.
During the beta, on the device you study on. Moving to a new phone or browser starts a fresh record.
Yes. The labs, tests and audio work on phones, tablets and laptops in the browser. Nothing to install.