3) can
be written as
n1 sin i = n2 sin r
(10 6)
This is the Snell’s law of refraction Further, if  l1 and l 2 denote the
wavelengths of light in medium 1 and medium 2, respectively and if the
distance BC is equal to l 1 then the distance AE will be equal to l 2 (because
if the crest from B has reached C in time t, then the crest from A should
have also reached E in time t ); thus,
1
1
2
2
BC
AE
v
v
λλ
=
=
or
1
2
1
2
v
v
λ
=λ
(10 7)
CHRISTIAAN HUYGENS (1629 – 1695)
Christiaan Huygens
(1629 – 1695) Dutch
physicist, astronomer,
mathematician and the
founder of the wave
theory of light