Why lens is used to measure the speed of light?

Answer 1

The light refracts or bends as it changes from one medium to another and ist speed changes accordingly. So the speed of light depends on the medium it travels through.

In 1621, a Dutch physicist named Willebrord Snell (1591-1626), derived the relationship between the different angles of light as it passes from one transperent medium to another. When light passes from one transparent medium to another, it bends according to Snell's law which states:
Ni * Sin(Ai) = Nr * Sin(Ar),
where:
Ni is the refractive index of the medium the light is leaving,
Ai is the incident angle between the light ray and the normal to the meduim to medium interface,
Nr is the refractive index of the medium the light is entering,
Ar is the refractive angle between the light ray and the normal to the meduim to medium interface.
And it all relates to the speed of light through the refractive index
Refractive Index = Speed of light in Vacuum(C) / Speed of light in Medium (v)
so
n = C / V

The focal distance of the lens is directly related to the geometry and material the lens is made out of. Since you can measure the geometry of a lens and the refraction index of the material you can use above equations to compute the speed of light.

Answer 2

Basically to get a collimated beam of light. That is a beam of light that will not diverge over distance. Otherwise the light would loose intensity over distance and would be difficult to make measurements on.

Answer 3

the focal point of a lens depends on the refraction of light through it, this refraction depends on the speed of light(dont remember formula) thats why

Answer 4

To make the light brighter at a distance.