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The light that human beings can see is called visible light . Visible light is actually just a small part of the large spectrum of electromagnetic radiation which you will learn more about in [link] . We can think of electromagnetic radiation and visible light as transverse waves. We know that transverse waves can be described by their amplitude, frequency (or wavelength) and velocity. The velocity of a wave is given by the product of its frequency and wavelength:
However, electromagnetic radiation, including visible light, is special because, no matter what the frequency, it all moves at a constant velocity (in vacuum) which is known as the speed of light. The speed of light has the symbol and is:
Since the speed of light is , we can then say:
Our eyes are sensitive to visible light over a range of wavelengths from 390 nm to 780 nm (1 nm = m). The different colours of light we see are related to specific frequencies (and wavelengths ) of visible light. The wavelengths and frequencies are listed in [link] .
Colour | Wavelength range (nm) | Frequency range (Hz) |
violet | 390 - 455 | 769 - 659 |
blue | 455 - 492 | 659 - 610 |
green | 492 - 577 | 610 - 520 |
yellow | 577 - 597 | 520 - 503 |
orange | 597 - 622 | 503 - 482 |
red | 622 - 780 | 482 - 385 |
You can see from [link] that violet light has the shortest wavelengths and highest frequencies while red light has the longest wavelengths and lowest frequencies .
A streetlight emits light with a wavelength of 520 nm.
We need to determine the colour and frequency of light with a wavelength of nm = m.
We see from [link] that light with wavelengths between 492 - 577 nm is green. 520 nm falls into this range, therefore the colour of the light is green.
We know that
We know and we are given that m. So we can substitute in these values and solve for the frequency . ( NOTE: Don't forget to always change units into S.I. units! 1 nm = m.)
The frequency of the green light is Hz
A streetlight also emits light with a frequency of 490 Hz.
We need to find the colour and wavelength of light which has a frequency of 490 Hz and which is emitted by the streetlight.
We can see from [link] that orange light has frequencies between 503 - 482 Hz. The light from the streetlight has Hz which fits into this range. Therefore the light must be orange in colour.
We know that
We know and we are given that Hz. So we can substitute in these values and solve for the wavelength .
Therefore the orange light has a wavelength of 612 nm.
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