The spectrum has seven groups
Definition
Electromagnetic spectrum
The continuous range of electromagnetic waves arranged according to wavelength or frequency.
- From longest wavelength and lowest frequency to shortest wavelength and highest frequency, the order is radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
- The groups meet without gaps, so the spectrum is continuous even though named groups make it easier to describe.
- Moving from radio waves to gamma rays, wavelength decreases while frequency increases.
- Because v=fλv=f\lambdav=fλ and every group has the same speed in a vacuum, a higher frequency corresponds to a shorter wavelength.
Example
Linking frequency and wavelength
If one wave has twice the frequency of another in a vacuum, its wavelength is half as large because λ=vf\lambda=\dfrac{v}{f}λ=fv.
Visible light is a narrow range
Definition
Visible light
The range of electromagnetic radiation that can be detected by the human eye.
- Within visible light, the order from longest to shortest wavelength is red, orange, yellow, green, blue, indigo and violet.
- Red light has the longest wavelength and lowest frequency in the visible range.
- Violet light has the shortest wavelength and highest frequency in the visible range.
- Infrared lies beyond the red end and ultraviolet lies beyond the violet end, but neither can be detected by the eye.
Common Mistake
Do not reverse the trends: from radio waves to gamma rays, frequency increases while wavelength decreases.
Ordering answers need both trends
- An order based on decreasing wavelength runs from radio waves to gamma rays.
- An order based on increasing frequency also runs from radio waves to gamma rays.
- If the direction is reversed, both trends reverse, so gamma rays to radio waves means increasing wavelength and decreasing frequency.
Exam technique
- Write all seven group names rather than initials in a final answer.
- For visible colours, anchor the order with red at the long-wavelength end and violet at the short-wavelength end.
Self review
- What are the seven electromagnetic groups from longest to shortest wavelength?
- How do frequency and wavelength change from radio waves to gamma rays?
- Why is the electromagnetic spectrum continuous?
- Which visible colour has the longest wavelength?
- What part of the spectrum can the human eye detect?
