Our eyes only detect visible light and so detect a limited range of…

The idea

The retina responds only to the narrow band from about 400 nm to 700 nm, roughly violet to red, which is a tiny slice of the whole spectrum. Everything else is arriving all the time and produces no visual sensation at all: radio waves crossing the room, infrared radiated by every warm object, ultraviolet from the Sun. That is why detectors are needed, an aerial for radio waves, a thermal camera for infrared, a fluorescent material or photographic film for ultraviolet and X-rays. It also has consequences you can feel. You cannot tell by looking whether ultraviolet is present, so sunburn can happen on a cool or overcast day, and an infrared heater warms you strongly without looking bright.

What earns the marks

• Stating that our eyes detect visible light.
• Saying that this is only a small part of the whole electromagnetic spectrum.
• Naming a detector needed for a group we cannot see, such as an aerial or an infrared camera.
• A consequence, such as not being able to see that ultraviolet is present.

Common mistakes and how to put them right

• Saying the other groups are not present because we cannot see them.

• Claiming the eye detects infrared because we can feel warmth on the skin.

• Giving the visible range in metres without converting from nanometres.

• Writing that our eyes only detect light without specifying visible light.

Exam tips

• One mark questions here want the phrase our eyes detect visible light, so keep it short.
• If asked for a consequence, give a concrete one, such as needing a camera to see an infrared source.
• Do not spend time on the biology of the eye; the marks are for the range of waves.