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H-alpha is a red spectral line, at a wavelength of 6563 (656.3 nanometers), emitted by a hydrogen atom when its electron falls from the third lowest energy level (n=3) to the second lowest energy level (n=2). The same line appears in absorption when electrons are raised from n=2 to n=3.

Many solar features, such as prominences, show up best in H-alpha, so that, for observation of the Sun, filters are often used that allow only light close to the H-alpha wavelength to pass.

H-alpha is the first member of the Balmer series.

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