Friday 25 March:

Target: Light

Index of Refraction: 

• The lower the number the faster it goes

• n = 1 in air or vacuum

• water: n = 1.333

• n = c/v   (3x10⁸) / velocity in that substance

Question: What is the minimum depth of a oil sheen that reflects max intensity of red light:

Given:

•  red l = 700 nm    (i.e., x 10^-9m)
• n = 2.0

answer:

Phase shift is 180⁰, so thickness (t) must be a multiple of 1/2 l.

The wave must travel there and back and be in phase

i.e., 2t = 1/2 l --> t = 1/4 l

v = fl --> f = v/l

(n = c/v_oil ) --> so,     v_oil = (c/n_oil)

f_oil  =  f_air   -->    v_oil/l_{oil  =}  v_air/l_air

 _{(c/noil) /} l_{oil = c /} l_air

--> (1/_{noil) /} l_{oil = 1/} l_air

 -->  l_{oil  =}  l_air /  n_oil

l_oil  =  700 x 10^-9 /  2  =  350 x 10^-9 m

  t = (1/4)l_oil

    t =  350 x 10^-9 /4 = 87.5 x 10^-9 m

Films:

http://www.youtube.com/watch?v=oDLjd2CZYt8&feature=related

Secret Of The Pyramids

http://www.youtube.com/watch?v=PotS7hPQZTU&feature=related

Mystery of the Sphinx