Should I use 40 or 80 MHz frequency for a 5 GHz WiFi?

Choose 80 MHz definitely, wider bandwidth equals less interference.

As for the channel, 5 GHz band doesn't usually suffer from interference, though for the best results try to choose a channel that is not overused by nearby networks, I really recommend you to use any WiFi analyzer app to do so, especially this WiFi Analyzer app, it’s free and helped me figure the best channel to use for my dual band router.

Is it really possible to hack WiFi using CMD?

Interesting question. Normally I would use Kali Linux for this task. Kali has built in tools that make WiFi cracking easier. But if you are forced to use a Windows OS for this task, I would use the application Wireshark. Wireshark is built for network reconnaissance. But, that is not what the question wanted. You want to know how to hack Wifi using cmd.exe

First of all, cmd is a little outdated. For this task, I would use Powershell. If you are working on Windows 7 or later OS, you should have Powershell pre-installed. While there are some wifi network using cmd. You my find using cmd is cumbersome.

Secondly, Microsoft has already written a document describing what you want to do. Read ‘Get Wireless Network SSID and Password with PowerShell’. This document should point you in the right direction.

Finally, know that what you are attempting to you is most likely illegal in your area. You want to be well versed in your country’s laws. In my area, passively monitoring a wifi network is usually legal, attempting to access that network is illegal. Do not send deauth attacks. Do not attempt to log onto a network that you do not have formal access to. Seriously, do not do it.

Have fun and be safe!

What is the difference between wifi router and wifi modem?

Router and Modem are two different technologies, but often come in one equipment. 

A MODEM is MOdulator DEModulator. Your Internet or network on TCP/IP protocols with Ethernet cabling & standards. If your ISP is using a different connectivity type e.g. Coaxial Cable (Internet and Cable TV together) or its on a telephone line (Internet + Phone) you will have to use a MODEM for converting your PC to Internet traffic digital signalling to ADSL etc... and back. 

Routers function is to connect different networks (Layer 3 of OSI Network Model) together. When you take a internet connection from ISP, you get only single IP. So to use multiple devices like phone etc, you can put a Wireless Router. Your devices will connect to the router and you can enjoy internet on all devices. If you have a modem based internet, then router uplink can be connected to this modem.

Is Wi-Fi harmful to the human body?

No. Wi-Fi and Bluetooth radiation is much too low energy to cause any damage. And by "too low energy", I mean the photons are low frequency so that the quanta of energy is too small to interact in any meaningful ways with any biomolecules. It's referred to as "non-ionizing radiation". In contrast, UV radiation (e.g. from the sun), is a form of "ionizing radiation" and can (and does) actually directly damage DNA, which is why too much tanning can lead to skin cancer. So, the sun is much more dangerous, because there is a lot more high-energy photons than Bluetooth.

Who is the inventor of WiFi?

First, let's take a look at the history of the 802.11 standards (radio frequency) used for broadcasting a WiFi signal. Secondly, we have to look at the electronic equipment involved in sending and receiving a WiFi signal. Not surprisingly, there are many patents connected with WiFi technology, though one important patent stands out.

Vic Hayes has been called the "father of Wi-Fi" because he chaired the IEEE committee that created the 802.11 standards in 1997. Before the public even heard of WiFi, Hayes established the standards that would make WiFi feasible. The 802.11 standard was established in 1997. Subsequently, improvements to the network bandwidth were added to the 802.11 standards. These include 802.11a, 802.11b, 802.11g, 802.11n and more. That's what the appended letters represent. As a consumer, the most important thing you should know is that the latest version is the best version in terms of performance and is the version you would want all your new equipment to be compatible with.

Who Owns the WLAN Patent?

One key patent for WiFi technology that has won patent litigation lawsuits and does deserve recognition belongs to the Commonwealth Scientific and Industrial Research Organisation (CSIRO) of Australia.

CSIRO invented a chip that greatly improved the signal quality of WiFi.

According to the tech news site PHYSORG, "The invention came out of CSIRO's pioneering work (during the 1990s) in radio astronomy, with a team of its scientists (led by Dr. John O'Sullivan) cracking the problem of radio waves bouncing off surfaces indoors, causing an echo that distorts the signal. They overcame it by building a fast chip that could transmit a signal while reducing the echo, beating many of the major communications companies around the world that were trying to solve the same issue."

CSIRO credits the following inventors for creating this technology: Dr. John O’Sullivan, Dr. Terry Percival, Mr. Diet Ostry, Mr. Graham Daniels and Mr. John Deane.