Question

Red light always has a lower frequency than blue light.

(a) In a vacuum, these two colors are known to travel at the same speed. How do their wavelengths compare in vacuum? Choose one answer only.
wavelength of blue > wavelength of red
wavelength of red = wavelength of blue
wavelength of red > wavelength of blue
(b) In a certain type of glass, these colors are known to have the same wavelength. How do their wave speeds compare in this glass? Choose one answer only.
average speed of red > average speed of blue
average speed of blue > average speed of red
average speed of red = average speed of blue

Answer 1

Lower frequency ---> longer wavelength
because
T = 1/f = distance/speed


then for part B
we know the frequency of red is lower.
The frequency and period DO NOT CHANGE (there is no place to store extra waves)
Therefore if the wavelength of red gets shorter it had to cover less ground in a period. It went slower.