The diagram shows the dispersion of white light as it passes from air into glass. Red light travels at speed c in air, but only 2c/3 in glass. Red light has a wavelength of λ in air. What is the frequency of red light, and the speed of blue light in glass?

Answer:

23) C

The equation for speed (c) with relation to wavelength (λ) and frequency (ƒ) is:

c = ƒ x λ
ƒ = c/λ

Speed of red light in glass = 2c/3
Now, wavelength is directly proportional to speed. Since speed decreases, wavelength also decreases by by same amount.
Wavelength of red light in air = λ
Wavelength of red light in glass = 2λ/3
Frequency = (2c/3) ÷ (2λ/3) = 2c/3 x 3/2λ  = 6c/6λ = c/λ 

Since blue light refracts more than red light, speed of blue light in glass must be smaller than speed of red light in glass.
c (blue light) < c (red light)
c (blue light) < 2c/3

Hence, C is the answer