Question

In a physics lab, light with a wavelength of 560 nm travels in air from a laser to a photocell in a time of 17.2 ns . When a slab of glass with a thickness of 0.810 m is placed in the light beam, with the beam incident along the normal to the parallel faces of the slab, it takes the light a time of 20.8 ns to travel from the laser to the photocell.

Required:
What is the wavelength of the light in the glass?

Answer 1

Distance traveled = 3 * 10E8 * 17.2 * 10E-9 = 5.16 m

.81 / 3 * 10E8 = 2.7 * 10E-9 normal time thru glass

(20.8 - 17.2) * E10-9 = 3.6 * 10E-9 additional time due to glass

c tg = c n ta where tg and ta are the times spent in glass and air

(Note you can also write Va = n Vg or D / ta = n D / tg)

n = tg / ta = 3.6 / 2.7 = 1.33 the index of refraction of the glass

Wavelength (air) = Wavelength (glass) * n

Wavelenght = 560 nm / 1.33 = 421 nm