Multiple images in a thick mirror

Fiona Rozario · Dec 11, 2014

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?

davenn · Dec 11, 2014

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

Drakkith · Dec 11, 2014

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 · Dec 15, 2014

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!

Danger · Dec 15, 2014

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

Drakkith · Dec 15, 2014

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.

Log in or Sign up

Multiple images in a thick mirror

Fiona Rozario

davenn

Drakkith

Staff: Mentor

Fiona Rozario

Danger

Drakkith

Staff: Mentor

3D or 2D image in a Mirror (Replies: 6)

A question about mirrors and images (Replies: 1)

Images in plane mirror. (Replies: 2)

Image formation in concave mirrors (Replies: 4)

Image formation in concave mirror (Replies: 18)