Spectrum

The Sunlight is white. White is not just a plain color (like other colors). It consists of a variety of colors. When these colors are coming simultaneously to our eye, we perceive it as white. Newton was the first to analyze the light. He Analyzed each color by pouring the sunlight on a prism.

Light Spectrum

The whole range of colors in which the light is analyzed is called light spectrum. This spectrum is continuous in other words it contains every color from red to violet. Each color is characterized by a number. This number is called wave length. Red color's wave length is λ=700 nm while violet has wave length λ=400 nm.

There seems to be no relation between what mentioned above and atoms. Yet, there is a certain relation. The light is created inside atoms. These little and invisible structures are responsible for whatever we see.

Bohr’s model

A model for the Hydrogen atom (the simplest one) which can explain how the light is created is the Bohr’s model. The Hydrogen atom consists of one proton and one electron. The electron revolves in certain circular stable orbits called shells. Each shell has a number (n). The first shell has n=1, the second one has n=2 etc. The electron cannot be found in a middle position but only in specific orbits.

The electron ends up on the first orbit n=1, the orbit with the lowest energy (ground state). The electron can absorb energy with various ways e.g. collision. When this happens it may be excited and jump to a higher orbit. The electron does not remain in an excited state for long (approximately for ten nanoseconds) and drops to the lowest energy state emitting light by releasing a particle which is called photon. The color of the light that is emitted is related to the initial and final orbit and it is visible only when the electron drops to the orbit n=2. In any other case the light is not visible. You may wonder how there can be light if we cannot see it. In Physics, light may be rays that warm us or can be any radio or television broadcast signal.

Linear spectra of gas emission

The spectrum of Hydrogen emitted consists of lines and differs from a solid being in high temperature (lamp) or the Sun. When atoms are very close to each other, like in solid bodies, the lines are close to each other as well and the spectrum becomes continuous. Each gas has a unique set of emitted lines. This set identifies the gas.

A few words about the filters

Filters consist of substances that can absorb a specific part of the spectrum. The light that comes to our eyes is called the complementary of the absorbed light. For example, the yellow filter absorbs the blue light since if blue is removed from the white light, the remaining light seems yellow to us. When you see on your computer screen a yellow color, it means that two pixels, a green, and a red one, send light and as a result yellow is created.

Excercise

Lab part