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

The idea

The electromagnetic spectrum is a continuous family of transverse waves that all travel at the same speed in a vacuum, yet differ in wavelength and frequency. Human eyes contain light-sensitive cells that respond only to the narrow band of wavelengths from about 400 nm to 700 nm, which we call visible light. Longer waves such as radio, microwave and infrared, and shorter waves such as ultraviolet, X-rays and gamma rays, carry energy but produce no visual sensation. For example, a television remote emits infrared that a camera may record but the eye cannot see, while an X-ray image forms only on a detector, not on the retina. Because vision samples just one small region of the spectrum, our eyes detect a limited range of electromagnetic waves.

What earns the marks

• State that the electromagnetic spectrum includes radio, microwave, infrared, visible, ultraviolet, X-rays and gamma rays, and that visible light is only one region.
• Identify the approximate wavelength range of visible light as about 400 nm to 700 nm, or the frequency range about 4 × 10¹⁴ Hz to 7.5 × 10¹⁴ Hz.
• Explain that the retina contains cells sensitive to visible wavelengths, so other electromagnetic waves do not cause a visual response.
• Use an example such as infrared from a remote control or ultraviolet from a black lamp being detected by a sensor or camera rather than by the unaided eye.
• Recognise that all electromagnetic waves transfer energy, so undetected waves can still have effects such as heating or ionising.

Common mistakes and how to put them right

• Thinking that invisible means no energy is transferred.

  Put it right by stating that infrared, ultraviolet and other waves still carry energy and can heat or damage materials.

• Believing that the eye can see all colours of the rainbow but no other electromagnetic waves.

  Put it right by noting that the rainbow is simply the visible band spread out, not the whole spectrum.

• Confusing wavelength order with detection.

  Put it right by separating the continuous spectrum order from the narrow visible window that human eyes respond to.

Exam tips

• Link the answer to the structure of the eye: mention the retina and light-sensitive cells when explaining why detection is limited.
• Give a named wave outside the visible range and state what detects it instead of the eye, such as a thermometer for infrared or a photographic film for X-rays.
• Use comparative language such as longer wavelength than red light or shorter wavelength than violet light to place a named wave precisely.