How would you know if the light emitted is of one wavelength or if it is a mixture of two or more wavelengths?
The green light from copper flame is sadly not a single wavelength. It is very good question though because not many students even think about it. In order to &quot;see&quot; what wavelengths are present in a light source, you either need a spectroscope (old school way) or a monochromator with a detector (modern way). Of course you can search about Bunsen-Kirchoff spectroscope. It uses a prism to disperse the light falling on the prism.
Depending on how much interest and money you have you can easily construct a spectroscope in a shoebox (several published articles out there). Ten years ago or so I used to make CD diffraction spectroscopes in shoeboxes or cardboard boxes for seeking answers to queries like yours.
If you want spent some money ($400-500) you can buy a high quality pocket spectroscope with a wavelength scale . Then you can enjoy all light sources (except the Sun). It is fascinating to see and explore so many phenomena. You will find out by looking at the moon that it has some dark lines (never look at the Sun from a spectroscope). I am not suggesting to buy third rate products from Amazon because twice I purchased from them and the wavelengths were all wrong!
The monochromator is a fancier product. I never had the courage to buy one. They are very expensive. The most common design is a Czerny-Turner design. I am sure Prof. EdV had even fancier products which we have never fathomed :-)
This picture shows white light entering a slit, it falls on a mirror which makes all the rays parallel. The mirror sends the light to a high quality diffraction grating, the reflection diffraction grating separates wavelengths in space. The light from reflection grating falls on a focusing mirror, and the mirror focuses lies on the exit slit. The exit slit has a detector. You would slowly rotate the grating. If there are multiple wavelengths in the light source (say a flame) the detector will record a spike every time a different wavelength reaches the detector.