# Radiation intensity and distance?

## Homework Statement

A light ray enters the plastic block in the illustration at an angle of 33.0° and then exits the opposite side. The long side of the block has a length of 0.670 m and its index of refraction is 1.70. For how much time is the light ray inside the block?

https://www.physicsforums.com/attachment.php?attachmentid=15094&stc=1&d=1219187822

## Homework Equations

Snell's law:
nisin$$\theta$$i=nrsin$$\theta$$r
n=index of refraction
$$\theta$$=incident angle from normal

n plastic=1.70
n air = 1.00

## The Attempt at a Solution

According to Snell's law:
(1.70)*sin(33)=(1.00)*sin($$\theta$$)
0.92588636=sin($$\theta$$)
arcsin(0.92588636)=$$\theta$$ = 67.8023923 degrees

And the speed of light is 2.998*10^8 m/s.

But after that I am not sure where to proceed! Can anyone help me figure this one out?

## Answers and Replies

G01
Homework Helper
Gold Member
You need to know two things to solve this problem.

1. The speed of light in the plastic block.

Do you know how to find this?

2.You need to know the length of the path traveled through the block.

You should be able to find this using the angle obtained from Snell's law and some trigonometry, but I can't say for sure if you don't post the actual figure in question.

Got it. Speed of light in plastic. Roger. Appreciate it

Last edited:
alphysicist
Homework Helper
How do I determine the speed of light inside the plastic?

The only property you're given about the material is its index of refraction. What is the definition of the index of refraction?