# Light refracted trough a prism

• xinlan
In summary, the problem involves finding the largest value of angle alpha for which no light is refracted out of a glass prism with a refractive index of 1.47 when it is immersed in air. This can be determined using the equation n1*sin(theta1) = n2*sin(theta2) and the critical angle formula sin critical alpha = (n2/n1)*sin 90. After realizing that the refractive index for air is 1.00, the correct value for alpha can be found by subtracting the angle between the light ray and the normal to surface A-C from 90 degrees. Drawing a diagram can help visualize the triangles involved in solving this problem.

## Homework Statement

Light is incident along the normal to face AB of a glass prism of refractive index 1.47

Find the alpha max, the largest value of the angle alpha such that no light is refracted out of the prism at face AC if the prism is immersed in air.

## Homework Equations

n1*sin thetha1 = n2 * sin thetha2

## The Attempt at a Solution

since no light is refracted out of the prism, then total internal reflection is involved.
sin critical alpha = (n2/n1)*sin 90
= 1.33/147
alpha = 64.8

but I got it wrong,
thanks..

#### Attachments

• yf_Figure_33_36.jpg
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What you found is the angle that the incident ray makes with the normal to surface A-C. That's not alpha, but you can use it to find alpha.

I just got it..
I just realized that the n for air should be 1.00 not 1.33.
then to get the alpha --> 90 - the angle that the incident ray makes with the normal to surface A-C..

anyway.. thanks.. :)

xinlan said:
I just got it..
Good!
I just realized that the n for air should be 1.00 not 1.33.
D'oh! (I didn't even notice that. I must have been sleeping.)
then to get the alpha --> 90 - the angle that the incident ray makes with the normal to surface A-C..
Right.

can someone explain a little more abour this problem ... i do no understand why is necessary to use 90 ... i just got one answer but its wrong this is the feedbak ... You have given the angle between the light ray and the normal to the surface AB, not the angle for alpha

Last edited:
blue_shark said:
can someone explain a little more abour this problem ... i do no understand why is necessary to use 90 ... i just got one answer but its wrong this is the feedbak ... You have given the angle between the light ray and the normal to the surface AB, not the angle for alpha
What you gave, I presume, is the critical angle, which is not the angle alpha. You've got to use the critical angle to figure out alpha. Draw a clear diagram and look for the triangles.

## 1. What is refraction?

Refraction is the bending of light as it passes through a different medium, such as air to water or air to glass.

## 2. What is a prism?

A prism is a transparent object with flat, polished surfaces that can refract light. It is usually in the shape of a triangular prism, with two triangular ends and three rectangular faces.

## 3. How does a prism refract light?

When light enters a prism, it is bent or refracted at different angles depending on the wavelength of the light. This results in the separation of white light into its component colors, creating a rainbow effect.

## 4. Why does light bend when it passes through a prism?

The bending of light is caused by the change in speed of the light as it passes through different mediums. This change in speed causes the light to change direction, resulting in refraction.

## 5. What are some practical applications of light refracted through a prism?

Prisms are commonly used in optical instruments such as binoculars, microscopes, and cameras. They are also used in spectrometers to analyze the composition of light. Prisms are also used in creating rainbows in art and design.

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