# Multiple images in a thick mirror

• Fiona Rozario
In summary, the amount of light reflected back from a glass surface in a thick glass mirror is determined by the refractive index of the glass. A higher refractive index leads to a higher percentage of light being reflected, while a lower refractive index leads to a lower percentage. Surface irregularities and the angle of incidence also play a role in the amount of light reflected. Front-surface reflective mirrors are essential for accurate reflection in applications such as astronomy and laser work.

#### Fiona Rozario

I understand that 4% of the light is reflected back from the glass surface in a thick glass mirror. But why 4%? Why not 5% or 10%? Will this percentage change if the glass has a lower refractive index?

Where did you read that ? do you have a reference ?

I would expect the percentage to be reflected back from the air/glass boundary surface to be dependant on the quality of the glass and the smoothness of the surface
the rest would be transmitted through the glass and reflected off the rear reflection surface and back out again ( there would be transmission losses in the glass as part of the double traverse the light does as well)

lets see what others have to say :)

Dave

Fiona Rozario said:
I understand that 4% of the light is reflected back from the glass surface in a thick glass mirror. But why 4%? Why not 5% or 10%? Will this percentage change if the glass has a lower refractive index?

Neglecting surface irregularities and assuming the light strikes at a normal angle (perpendicular to the surface), the amount of light reflected by transparent surface is given by the equation R=(1-n/1+n)2, where R is the amount of light reflected and n is the refractive index. Glass with a refractive index of 1.5 gives us: R=(1-1.5/1+1.5)2, which comes out to be R=0.04, which is 4%.

With a refractive index of 2: R=(1-2/1+2)2, or R=0.111..., which is about 11.1%.

With a refractive index of 1.1: R=1-1.1/1+1.1)2, or R=0.002268, about 0.2%.

Finding the amount of light reflected when the light is striking at an angle other than normal is much more complicated.

Fiona Rozario and davenn
Drakkith said:
Neglecting surface irregularities and assuming the light strikes at a normal angle (perpendicular to the surface), the amount of light reflected by transparent surface is given by the equation R=(1-n/1+n)2, where R is the amount of light reflected and n is the refractive index. Glass with a refractive index of 1.5 gives us: R=(1-1.5/1+1.5)2, which comes out to be R=0.04, which is 4%.

With a refractive index of 2: R=(1-2/1+2)2, or R=0.111..., which is about 11.1%.

With a refractive index of 1.1: R=1-1.1/1+1.1)2, or R=0.002268, about 0.2%.

Finding the amount of light reflected when the light is striking at an angle other than normal is much more complicated.

Thank you!

This is why accurate mirrors as are needed for astronomy and laser work are front-surface reflective.

Danger said:
This is why accurate mirrors as are needed for astronomy and laser work are front-surface reflective.

Indeed. To my knowledge, pretty much all telescope mirrors are front-surface reflective.

## 1. How do multiple images form in a thick mirror?

Multiple images form in a thick mirror due to the reflection and refraction of light rays. As the light rays enter the mirror, they are reflected at different angles and some are also refracted, resulting in multiple images being formed.

## 2. What is the difference between multiple images in a thick mirror and a thin mirror?

The main difference between multiple images in a thick mirror and a thin mirror is the thickness of the mirror. In a thin mirror, only a single image is formed due to the limited reflection and refraction of light. However, in a thick mirror, multiple images can be formed due to the various paths that the light rays can take.

## 3. Can the number of multiple images in a thick mirror be predicted?

Yes, the number of multiple images in a thick mirror can be predicted using the mirror equation. This equation takes into account the distance between the object and the mirror, the distance between the mirror and the observer, and the radius of curvature of the mirror.

## 4. Do all thick mirrors produce multiple images?

No, not all thick mirrors produce multiple images. The thickness and curvature of the mirror play a significant role in determining whether multiple images will be formed. A perfectly flat mirror, for example, will only produce a single image.

## 5. How does the size of the mirror affect the formation of multiple images?

The size of the mirror does not have a direct effect on the formation of multiple images. However, a larger mirror can potentially produce more and larger multiple images due to its ability to reflect and refract more light rays.