Recall that our eyes can only detect a limited range of frequencies of…

The idea

The human eye responds to only a small slice of the electromagnetic spectrum, the band we call visible light. Radiations such as radio waves, microwaves, infrared, ultraviolet, X-rays and gamma rays carry energy but produce no visual sensation, so we detect them only with instruments such as aerials, thermal cameras or Geiger counters. Within the visible band, different frequencies are seen as different colours, with red at the lower-frequency end and violet at the higher-frequency end. A useful check is to ask whether a named radiation lies inside the visible band; if it does not, the eye cannot detect it directly.

What earns the marks

• States that the human eye detects only a limited range of frequencies of electromagnetic radiation.
• Identifies the detectable range as visible light.
• Recognises that radiations outside the visible band, such as radio waves, infrared, ultraviolet, X-rays and gamma rays, are not seen by the eye.
• Explains that different frequencies within the visible range are perceived as different colours.

Common mistakes and how to put them right

• Claims the eye can detect all electromagnetic radiations.

  Put it right by stating that detection is limited to the visible range.

• Confuses infrared with visible light because both feel warm.

  Put it right by stating that infrared lies outside the range the eye detects.

• Assumes instruments such as thermal cameras show what the unaided eye sees.

  Put it right by stating that such devices detect radiations the eye cannot.

Exam tips

• Remember the key phrase 'limited range' and link it directly to visible light.
• If a question names a radiation, decide first whether it lies inside or outside the visible band.
• Use the colour order red to violet to show that frequency varies within the visible range.