Dear Shalini as U R in Grade 10 U need to learn only this thing in order to caluclate the wavelength that is:
k = c/v
Wavelength of light is already been calclated for different light/color.
The cathode ray tube experiment is used for the different purposes like photelectric effect etc.
Too calculate the wwavelength u simply need the speed of the light or frequency nd apply formulae:
k = c/v
where c is the speed of light v is the wave speed and k is the wavength.
Put all the values in meter as their SI units.