* Decreasing Wavelength: When the wavelength of spectral lines decreases, it means the light is shifting towards higher frequencies.

* Visible Light Spectrum: The visible light spectrum goes from red (longest wavelength, lowest frequency) to violet (shortest wavelength, highest frequency).

* Movement: This shift towards higher frequencies is called a blueshift. It indicates that the object emitting the light is moving towards Earth.

Explanation:

The Doppler Effect applies to light waves just as it does to sound waves. When a source of light is moving towards an observer, the waves are compressed, resulting in a shorter wavelength and a shift towards the blue end of the spectrum. Conversely, if the source is moving away, the waves are stretched, resulting in a longer wavelength and a shift towards the red end (redshift).