Visible light is a small portion of the electromagnetic spectrum
Light is represented as an arrow. The two vectors of light, electric and magnetic, vibrate at right angles to each other. NASA has this link describing the EM spectrum in the context of visible light.
Ultraviolet; 365nm

Blue; 480nm
Green; 520nm
Orange; 590nm
Red, 620nm
The speed of light is a function on the medium through which it travels.
Medium
C
n
Vac
1.00
1.00
Air
1.00
1.00
Water
0.75
1.33
Glycerol
0.68
1.47
Immersion Oil
0.66
1.515
Glass
0.66
1.515
Diamond
0.42
2.42
Refraction, or the bending the light path, occurs at the interface between two media having different n values. e.g., the air/glass interface of an objective lens or the top of a microscope slide coverslip.
Refraction of light is described by Snell-Descartes Law, where n=refractive index; i=angle of incidence; r=angle of refraction.
Corollaries
Glass/Air 41.3°
Glass/Water 61.7°
Water/Air 48.6°
Here's a link describing Critical angle
Go To Lens Types
Refractive index
Optical path: The product of the refractive index (n) and the geometric path length (in µm).
OPL = n • l(µm)
e.g., for a 10µm thick cell in water

OPL = 1.33 • 10µm = 13.3µm OPL