Here's why:
* Refractive Index: Perspex has a higher refractive index than air. This means light travels slower in Perspex than in air.
* Relationship between Wavelength and Speed: The speed of a wave is directly proportional to its wavelength. So, if the speed decreases, the wavelength must also decrease.
In other words: As light enters Perspex, it slows down, and to compensate for this, the wavelength gets shorter.
Important Note: While the wavelength changes, the frequency of the light wave remains constant. Frequency is determined by the source of the light and doesn't change as the light passes through different mediums.
