23) C

When a wave travels from a less dense medium to a more dense medium (such as from air to glass), its speed decreases and it refracts. So, we can eliminate options E, F, G and H.

Speed = wavelength x frequency

FREQUENCY NEVER CHANGES DURING REFRACTION.

Since frequency does not change but speed decreases, therefore, its wavelength must also decrease.

Since the wavelength in air was 600nm, its new wavelength must be lower than 600nm.

Hence, we can eliminate options B and D.

Frequency remains unchanged.
Frequency of light in air = speed/wavelength = 3 x 10⁸ / 600 x 10-9 = 5.0 x 10¹⁴