Refraction is a phenomenon that happens when a light wave that is incident at an angle different from the normal passes through a medium and experiences a change in velocity.
When light transitions from the faster-moving air to the slower-moving glass, it is refracted.
Although the path of the light changes as it travels through a material, it still does so.
When passing through anything other than a vacuum, light can only move at its maximum speed of the speed of light.
The pace at which different materials slow down light varies. Light doesn't alter its frequency as it refracts, but because its speed does, the wavelength must also alter.
Therefore, when light undergoes refraction, its frequency remains the same.