# Critical Angle: Vacuum vs. Air Refraction

• Cheman
In summary, the critical angle for refraction in a vacuum is 90 degrees, while in air it is slightly less than 90 degrees due to the difference in refractive index. This critical angle plays a significant role in determining the behavior of light in both vacuum and air and has practical applications in fields such as optics, telecommunications, and astronomy. The critical angle can also be manipulated by changing the refractive index of the medium, either by altering the properties of the material or the environmental conditions.
Cheman
Critical angle...

Is the formula:

sin(critical angle) = 1/ mu, only true light is moving from one medium into air/ a vacuum?

Thanks in advance.

Correct. (Assuming "mu" is the index of refraction.) To generalize it to any two media (as long as $n_2 < n_1$) go to Snell's law: $n_1 \sin \theta_1 = n_2 \sin \theta_2$. The critical angle arises when $\sin \theta_2 = 1$, thus:
$$\sin \theta_{cr} = \frac {n_2}{n_1}$$

When the second medium is air or vacuum, $n_2 = 1$.

Last edited:

The formula for critical angle, sin(critical angle) = 1/ mu, is true for any situation where light is moving from a medium with a higher refractive index (mu) to a medium with a lower refractive index. This includes cases where light is moving from a medium into air or a vacuum. The critical angle is the angle at which light is refracted at a 90 degree angle, and any angle greater than the critical angle will result in total internal reflection. This is why it is often used in fiber optics, where light travels through a medium with a higher refractive index (such as glass) and then into air or a vacuum. In this case, the critical angle is crucial in determining the maximum angle at which light can enter the fiber before it is reflected back. So, in short, the formula for critical angle is applicable in both situations, whether light is moving from a medium into air or a vacuum.

## 1. What is the critical angle for refraction in a vacuum versus air?

The critical angle for refraction in a vacuum is 90 degrees, while the critical angle for refraction in air is slightly less than 90 degrees, depending on the refractive index of the air.

## 2. Why is the critical angle different in a vacuum versus air?

The critical angle is different in a vacuum versus air because the refractive index of air is slightly greater than 1, while the refractive index of a vacuum is exactly 1. This difference in refractive index causes the critical angle to be slightly less in air.

## 3. How does the critical angle affect the behavior of light in a vacuum versus air?

The critical angle plays a significant role in determining the behavior of light in both a vacuum and air. In a vacuum, light will always refract at 90 degrees, while in air, the angle of refraction can vary depending on the angle of incidence and the refractive index of air.

## 4. What is the practical application of understanding the critical angle in vacuum versus air?

Understanding the critical angle in both vacuum and air is important in various scientific and technological fields, such as optics, telecommunications, and astronomy. It helps in the design and functioning of lenses, mirrors, and other optical instruments.

## 5. Can the critical angle be manipulated in vacuum or air?

Yes, the critical angle can be manipulated by changing the refractive index of the medium. In air, this can be achieved by changing the temperature, humidity, or composition of the air. In a vacuum, the critical angle can be manipulated by changing the properties of the material that the light is passing through, such as a prism or lens.

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