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Humans can see only a part of the larger electromagnetic spectrum, specifically a range called the visible light spectrum. This part of the spectrum includes the colors red, orange, yellow, green, blue, indigo, and violet. Each color corresponds to a different wavelength, with red having the longest wavelength and violet having the shortest.
Visible light wavelengths range from about 400 to 700 nanometers (nm). Light wavelengths shorter than 400 nm include ultraviolet light, which is not visible to humans. On the other end, wavelengths longer than 700 nm involve infrared light, also not visible to the human eye.
Understanding this range is crucial because it shows that what we see is only a tiny portion of the electromagnetic spectrum, making many other forms of electromagnetic waves invisible to us.

• Red light: ~700 nm
• Violet light: ~400 nm

Light is unique in that it does not need a medium to travel. Unlike sound, which requires air, water, or another medium, light can travel through the vacuum of space. This property allows sunlight to reach Earth across the vast emptiness of space.
The speed of light in a vacuum is approximately 299,792 kilometers per second (km/s). This speed slows down when light passes through materials like glass or water. However, in a vacuum, it remains constant and is considered a fundamental constant of nature.
Because light can travel through a vacuum, this enables us to see distant stars and galaxies. If light required a medium to travel, space would appear completely dark to us.

• Mediums affect light speed
• Vacuum speed: ~299,792 km/s

The electromagnetic spectrum encompasses all types of electromagnetic radiation, categorized based on their wavelengths and frequencies. The spectrum ranges from gamma rays, which have the shortest wavelengths, to radio waves, which have the longest.
The visible light spectrum that humans can see is only a tiny fraction of the entire electromagnetic spectrum. Other parts include:

• Gamma rays
• X-rays
• Ultraviolet light
• Infrared light
• Microwaves
• Radio waves

The wavelength and frequency of these waves determine their properties and applications. For instance, X-rays can penetrate the human body and are used in medical imaging, while radio waves are used for communication.

Human vision operates by detecting light that falls within the visible light spectrum. Photoreceptor cells in the retina, called rods and cones, are sensitive to different wavelengths of light.
Rods are more sensitive to low light levels and are crucial for night vision, but they do not detect color. Cones, on the other hand, are responsible for color vision and work best in bright light conditions. There are three types of cones, each sensitive to either red, green, or blue light.
When light enters the eye, it is focused by the lens onto the retina where the photoreceptor cells are located. The photoreceptors then convert the light signals into electrical signals, which are sent to the brain via the optic nerve, allowing us to perceive images.

• Rods: low light, no color
• Cones: bright light, color
• Colors detected: red, green, blue