What the eye can detect
Definition
Visible light
The narrow band of electromagnetic waves, with wavelengths from about 400 nm400\ \text{nm}400 nm to 700 nm700\ \text{nm}700 nm, that the human eye can detect.
- Your eyes respond to only one narrow band of the electromagnetic spectrum, called visible light.
- Visible light has wavelengths from about 400 nm400\ \text{nm}400 nm to about 700 nm700\ \text{nm}700 nm, where 1 nm=1×10−9 m1\ \text{nm}=1\times10^{-9}\ \text{m}1 nm=1×10−9 m.
- These wavelengths match frequencies from about 4.3×1014 Hz4.3\times10^{14}\ \text{Hz}4.3×1014 Hz to about 7.5×1014 Hz7.5\times10^{14}\ \text{Hz}7.5×1014 Hz.
- Light-sensitive cells in the retina at the back of the eye absorb these wavelengths and send signals to the brain.
- The brain sees each wavelength in this band as a different colour, from red at about 700 nm700\ \text{nm}700 nm to violet at about 400 nm400\ \text{nm}400 nm.
- The cells of the retina do not respond to any wavelength outside this band, so those waves are invisible to you.
- The visible band is a tiny fraction of the whole spectrum, which stretches from radio waves more than a kilometre long to gamma rays smaller than an atom.
Just beyond the visible band
Definition
Infrared
Electromagnetic waves with wavelengths slightly longer than red light, which the human eye cannot detect.
Definition
Ultraviolet
Electromagnetic waves with wavelengths slightly shorter than violet light, which the human eye cannot detect.
- Waves with wavelengths a little longer than red light are called infrared, a name that means "below red" because their frequency is lower than that of red light.
- Waves with wavelengths a little shorter than violet light are called ultraviolet, a name that means "beyond violet" because their frequency is higher than that of violet light.
- You cannot see either of them, even though your skin can feel the infrared from a radiator as warmth.
- Every other group of the spectrum, from radio waves to gamma rays, is also invisible to the human eye.
- Some animals detect a slightly different range, for example bees can see some ultraviolet, but the human range stays at roughly 400400400 to 700 nm700\ \text{nm}700 nm.
Detecting invisible waves
- A wave being invisible does not mean that it is absent or weak.
- Most of the electromagnetic waves around you at any moment, such as radio waves, microwaves from phones and infrared from warm objects, are invisible.
- To detect waves outside the visible band you need a detector that responds to those wavelengths.
- For example, a radio receiver detects radio waves, a thermal imaging camera detects infrared and an electronic sensor or photographic film detects X-rays.
Example
Deciding whether a wave is visible
- A lamp emits a wave with a frequency of 3.0×1014 Hz3.0\times10^{14}\ \text{Hz}3.0×1014 Hz.
- Find its wavelength from the wave equation:
- This is longer than 700 nm700\ \text{nm}700 nm, the red end of the visible band, so the wave is infrared and you cannot see it.
Common Mistake
- Do not say that you cannot see infrared or ultraviolet because they are too weak, because the real reason is that your eyes do not respond to those wavelengths.
- Do not call the whole electromagnetic spectrum "light", because only the band from about 400400400 to 700 nm700\ \text{nm}700 nm is visible.
Exam technique
Describing the limits of sight
- A full description says that the eye detects only a limited range of wavelengths, names this range as visible light, and says that waves outside it cannot be seen.
- If a question gives a wavelength or frequency, convert it and compare it with 400400400 to 700 nm700\ \text{nm}700 nm before you decide whether it is visible.
- When you name the invisible neighbours, put infrared beyond the red end and ultraviolet beyond the violet end.
Self review
- What range of wavelengths can the human eye detect?
- Which colour of visible light has the longest wavelength?
- What are the invisible waves just beyond the violet end of the visible band called?
- Why can you not see radio waves, even when a strong signal is present?
- Is a wave with a wavelength of 900 nm900\ \text{nm}900 nm visible?
