Physics Questions and Answers
by Aristotle

Start Your Free Trial

An optic fiber is made of clear plastic with index of refraction of 1.50.  For what angles of incidence will light remain within the plastic?

Expert Answers info

Tushar Chandra eNotes educator | Certified Educator

calendarEducator since 2010

write12,551 answers

starTop subjects are Math, Science, and Business

When a beam of light passes from one medium to another the beam bends if the two mediums do not have the same refractive index.

If the angle of incidence is A and the angle of refraction is B, the two are related by `(sin A)/(sin B) = (n2)/(n1)` where n1 is the refractive index of the first medium and n2 is the refractive index of the second.

The optic fiber is made of a material with refractive index 1.5, the refractive index of air is 1. A beam of light undergoes total internal reflection when the angle of incidence is equal to `sin^-1((n2)/(n1))` . For n2 = 1 and n1 = 1.5, `sin^-1(1/1.5) ~~ 41.81` degrees.

If the angle of incidence is equal to 41.81 or greater the beam of light does not leave the optic fiber.

check Approved by eNotes Editorial



pramodpandey | Student

Let crictical angle be `theta_c` ,so that light remain in plastic

We know

`sin(theta_c)=eta_2/eta_1`

where `eta_2 =1.0`refreactive index of the air , and

`eta_1=1.5` ,refractive index of the plastic.

Thus

`sin(theta_c)=1/1.5=.6666`

`theta_c=41.8^o`

For angles greater that 41.8o with respect to the normal .

Or  angles smaller than (90o - 41.8o) = 48.2o with respect to the surface .