A light with a wavelength of 500 nm (red color) travels from a laser to a photocell in 15 ns. When a thin glass slab is placed normally in front of the photocell it takes the light 18 ns to travel from the laser to the photocell? When the Laser beam passes through the glass slab it changes to blue in color (λ = 660 nm). What is the thickness of the glass slab?

A light with a wavelength of 500 nm (red color) travels from a laser to a photocell in 15 ns. When a thin glass slab is placed normally in front of the photocell it takes the light 18 ns to travel from the laser to the photocell? When the Laser beam passes through the glass slab it changes to blue in color (λ = 660 nm). What is the thickness of the glass slab?