CAPTCHAs | A Detailed Description

What’s it? 👀

I use the internet a whole lot. I’d say I spend 90% of my day with it. I could either be streaming my favourite tracks on Spotify, engage in battle royale games with friends, studying or downloading files. I’m sure you do the same.

Engaging in these activities can be fun, but most times, we’re greeted by this:

Which then leads to this:

They can sometimes, prove to be annoying

What Is It?

It is known as CAPTCHA. Pronounced as kap-chuh, it is an acronym for (Completely Automated Public Turing test to tell Computers and Humans Apart).

It is a program used to verify or determine if an online user is either a computer or maybe, an alien.

Fun fact: Even though they’re developed to block computerised computers or bots, they are also programmed.

Why Are They Used?

Most websites and services hold sensitive information, which could be your registered account or credit card details. To prevent automated bots to access such info, and also to ensure that only a human being with the right credentials can access it, they implement CAPTCHA.

A good use case is form-filling on a website. Website admins prevent bots from filling forms, which could result to spamming. They’re also being utilised to help prevent/reduce rapidly repeated requests, which could bog down a site’s server.

How do They work?

First off, there’re two famous variants

Traditional captcha

They’re quite old but are still seen on some websites today. As seen above, they consist of two components, a distorted text, and an input box.

To pass the test, users have to interpret the text, and type them correctly in the input box, and submit. If there’s a mismatch, they’re prompted to try again. Usually, bots face difficulties in interpreting such images. Therefore, they fail the test and are blocked.

This approach is now less reliable. This is because advanced bots implement machine learning to identify these distorted images and can pass the test.

Google’s reCAPTCHA

It’s a free service and serves as a better alternative to the traditional one. It is more advanced. It also has it’s variants which are:

a. Image recognition

Users are presented with either 9 or 16 squared images. The user is to identify images that contain objects that are specified by the program

b. Single checkbox

This is more subtle and effective. The test lies in the user’s previous activity, and not the actual clicking of the checkbox. What do I mean?

The program studies the movement of the user’s cursor as it approaches the box. If the movement contains some level of randomness or unpredictability, the tests conclude that the user is human. If it still fails to determine the movement, it then presents another test, which is that of image recognition.


a. Bad User Experience

It takes an average time of 5–6 seconds to run a test. This could be frustrating for users who are in a hurry, or on a time-crunch

b. Inaccessibility

Since they’re developed with advanced techniques, it makes it difficult for those who are visually impaired or deaf. Few CAPTCHAs contain an audio option.


Although CAPTCHAs could be annoying for most of us, we should be aware of what they are and the advantages they offer towards a healthy online experience

Au revoir 👋

Software Engineer && Technical Writer