Question

EXAMPLE 7 The velocity of light in air is 3 x 10
cm/sec. Find the velocity and wave-length of sodium
light ( = 5893 Å) in glass of refractive index 1.658.
​

Answer 1

In air velocity of light is 3×10

8

m/s and wavelength is 6000A

o

=6000×10

−10

m

We know that c=λ×f , f=frequency

3×10

8

=6000×10

−10

×f

f=5×10

14

Hz

We know that refractive index n=

velocityoflightinmedium

velocityoflightinvacuum

=

v

c

Here c=3×10

8

m/s

So 1.5=

v

3×10

8

⟹v=2×10

8

m/s

Since frequency does not change when light enters from one medium to other.

v=λf

2×10

8

=λ×5×10

14

λ=4×10

−7

m=4000A

o