The wavelengths for the light of red and blue colours are nearly 7.8 x 10^-7 m and 4.8 x 10^-7 m respectively.

(a) Which colour has the greater speed in vacuum?

(b) Which colour has greater speed in glass?

(a) In vacuum, both have the same speed.

(b) As speed of light increases with increase in wavelength of light. When we observe the given values, we find that the wavelength of red colour (7.8 x 10^-7 m) is more than that of blue colour (4.8 x 10^-7 m). Hence, in glass, red light has greater speed.