Digital representation: everything is bits
Computers have tiny switches that are either off or on. We write off as 0 and on as 1. One 0 or 1 is a bit. Eight bits together are a byte.
Pictures: a screen is made of tiny squares called pixels. Each pixel is stored as a number. In a black-and-white picture, one bit per pixel is enough: 1 = dark, 0 = light. A 4 × 4 picture needs 16 bits.
Numbers: bits can make any number. With 4 bits there are 16 patterns (0000 to 1111). The places count 8, 4, 2, 1 from the left, so 1011 = 8 + 0 + 2 + 1 = 11.
Sound, text and video are also saved as long rows of bits. This is called digital information. Digital copies can be copied again and again without getting worse.
How networks work
A network is a group of devices joined together so they can share data. Your home Wi-Fi is a small network. The internet is a huge network of networks.
- Device: phone, laptop, or a server (a computer that stores web pages).
- Router: a box that sends each packet towards the right place.
- Packets: big data is cut into small pieces. Each packet has the sender's address, the receiver's address and its number, like a numbered parcel.
- Address (IP address): a number that tells the network which device to deliver to.
Packets may take different roads. At the end the receiver puts them in order using the numbers. If a packet is lost, it is asked for again.
Information security
Information security means keeping data private (only the right people read it), correct (nobody changes it) and available (you can use it when needed).
- Strong password: long, with letters, numbers and symbols. Never share it. Use a different one for each account.
- Two-step check: a password plus a code sent to your phone.
- Encryption: data is scrambled with a secret key, so a spy sees nonsense. The padlock in a web address shows this.
- Phishing: fake messages that ask for your password or money. Do not click unknown links.
- Backup and updates: keep a second copy of important files and update your apps.
Try it
Try it in the 3D: tap four squares to make a new picture. First guess: what will the bits of row 1 be? Then look at the bits shown below the picture.
Try it at home: draw a 5 × 5 grid on paper. Shade a letter, then write 1 for shaded and 0 for blank in each row. Give the numbers to a friend and ask them to draw your letter. You just sent a picture as bits.
Key formulas and definitions
- 1 byte = 8 bits
- Bits for a black-and-white picture = width × height
- Binary place values (4 bits): 8, 4, 2, 1
- n bits give 2ⁿ patterns (4 bits = 16)
Worked examples
1. A black-and-white picture is 8 pixels wide and 8 pixels tall. How many bits does it need?
8 × 8 = 64 bits. That is 64 ÷ 8 = 8 bytes.
2. Write 1101 as an ordinary number.
Place values 8, 4, 2, 1. So 8 + 4 + 0 + 1 = 13.
3. A message is cut into 5 packets. 1 packet is lost on the way. How many packets must be asked for again?
Only the lost one: 1 packet. The other 4 are kept and the new one is put in its place by its number.
Common mistakes
- Thinking a computer stores a picture as a picture. It stores numbers (bits).
- Mixing up bit and byte: 1 byte is 8 bits.
- Thinking the internet is one big computer. It is many networks joined together.
- Using the same easy password everywhere, like a name or 1234.