# Optics. A laser beam is refracted in a piece of glass.

• yglo98
In summary, the distance between the focal point and the back side of the glass is 11 cm. The effect being focused in the focal point is P of 1mW.f

#### yglo98

Homework Statement
Want help with physics question from my book.
Relevant Equations
My idea for 1. is to use the formula for refraction in spherical surfaces: (n1/2) + (n2/s') = (n2-n1)/R), where n is refraction index for glass and air. First I would apply it for the first front surface of the glass, then take the image of that and use it as the object for the second refraction in the back side. I would have to take into account for the sign convention and thickness of the lens.

For 2 I think that maybe we can approximate the 2 refractions as right angles and then use the formula: T=(1-R)*2, where the (initial intensity/transmitted intensity) = T, and R=(n1-n2)^2/(n1+n2)^2. It would be used two times, as in the 2 refractions. But exactly how I would write the intensity, effect and area in the right way to solve this I don't know.
A red laser with the diameter of 3mm is directed towards a piece of glass. It has the effect, P of 1mW. The angle of incidence is 90 degrees. The glass has refraction index n2= 1,5 and thickness 2 cm. Its front side is convex with a convergence radius,R, of 11 cm. The back side is plane.

1. Whats the distance between the focal point and the back side of the glass?
2. What effect is being focused in the focal point?
Appreciate all kinds of help! Am I thinking right?

Last edited:
Hello @yglo98 , ##\qquad##!​

So what do we have here

Homework Statement:: A red laser with the diameter of 3mm is directed towards a piece of glass. It has the effect om 1mW. The angle of incidence is 90 degrees. The glass has refraction index 1,5 and thickness 2 cm. Its front side is convex with a convergence radius of 11 cm. The back side is plane.
1. Whats the distance between the focal point and the back side of the glass?
2. What effect is being focused in the focal point?
Relevant Equations:: T=(1-R)*2

(Since you are new here, I thought it a good idea to show how the template is intended to be used -- see the PF guidelines)

And your work so far (without repetition) is
My idea for 1. was to use the forum for refraction in spherical surfaces and use the concept of intermediate image for the two refractions that take place.

For 2 I think that maybe we can approximate the 2 refractions as right angles and then use the formula: T=(1-R)*2

Now,
Your relevant equation has no relationship with the variables that appear in the problem statement.
You don't explain what ##T## and ##R## are, but I guess Transmission and Reflectance.
Since the exercise comes from your book, you might want to try and find relevant equations there that have to do with focal length, thick lenses, etc.

A sketch of the situation showing some of the known variables might be a good thing to post as well ...

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• fresh_42 and yglo98