. The index of refraction is the ratio of the speed of light .

in a material to the density of the material

in the material to the speed of light in a vacuum

in a vacuum to the speed of light in the material

in a vacuum to the speed of light in the material

Explanation:

The index of refraction, also known as the refractive index, is the ratio of the speed of light in a vacuum to the speed of light in the material.

When light rays travel travels from one medium to another, it bends as a result of a change in the speed of traveling from the first medium to the second medium.

The refractive index (n) of a material is calculated by:

                     sin i/sin r

Where i = the incidence angle of the traveling ray in the vacuum and r = angle of refraction.

The refractive index (n) is also given as:

                         c/v

Where c = velocity of light in the vacuum and v = velocity of light in the material.

answer: the blood-brain barrier is a layer of specialized membranes and cell processes designed to keep undesirable substances from passing from the bloodstream into the precious tissues of the brain. while the blood-brain barrier does keep most bad things out of the central nervous system, it is not impermeable.

explanation:

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.