dµr=tan‌30∘=‌

1

√3

(where dµr, is the refractive index of rarer medium w.r.t. denser medium)
∴µd‌=√3
v‌=‌

c

µ

=‌

3×108

√3

=√3×108m∕ s
[Note : Also accept if a student solves it as follows]
‌µ=tan‌ip
‌µ=tan‌30∘=‌

1

√3

‌∴v=‌

3×108

1 √3

=3√3×108m∕ s