# Why does light reflect at exactly 180 degrees?

I understand the molecular principles behind the reflection itself, however it presents many problems, such as the one above.
If you could explain it as easily as you can, I would be grateful.
:)

russ_watters
Mentor
The statement in the title is not generally true. Can you expand upon/clarify your understanding and question please.

Basically, light reflected at other angles cancels out, so it can appear to only reflect in a range around 180.

Check out QED by Richard Feynman. It explains some of the behavior of light when interacting with surfaces.

• Aureum_borealis
I'm probably misunderstanding the question, however from where I am sitting I can see a mirror on the wall.
In the mirror I can see the reflection of a ceiling light.
The angle between the ceiling light and the mirror is around 45 degrees, the angle between the mirror and me about 30 degrees.

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Angle of incidence equals the angle of reflection.
The angle of reflection from a surface can be anything up to 180 degrees but it is always the same angle as the incoming light being reflected.

Maybe what he is asking is why light striking normal to a surface ideally reflects normal to the surface 180 degrees from the incident direction, given that the process involved is absorption and re-radiation from molecules in the material.

Maybe what he is asking is why light striking normal to a surface ideally reflects normal to the surface 180 degrees from the incident direction, given that the process involved is absorption and re-radiation from molecules in the material.
Exactly!

Khashishi
This can be discussed at many levels, but here's one way to look at it. We can consider the re-radiation from atoms in the material to be random but the light will undergo constructive interference only in the direction expected by the law of reflection.

So imagine a beam of light coming from the left and striking a horizontal surface at some angle and consider two atoms at the surface. The leftmost atom is illuminated first and there is some delay before the rightmost atom is illuminated. Thus there is a phase difference between the light illuminating each atom. Upon reflection, in order to constructively interfere the light from the leftmost atom has to be delayed by this same amount and this can only happen in the specular direction given by the law of reflection.

sophiecentaur
Gold Member
Maybe what he is asking is why light striking normal to a surface ideally reflects normal to the surface 180 degrees from the incident direction, given that the process involved is absorption and re-radiation from molecules in the material.

This comes up so regularly. The light is not absorbed and re emitted by individual molecules. The whole structure is involved. Light passing through a gas interacts with individual atoms and is scattered and not affected coherently. You only get coherent reflection and refraction when a medium interacts as a whole.

• DrClaude
Also, check out http://physics.stackexchange.com/questions/83105/explain-reflection-laws-at-the-atomic-level

Look at the first diagram on the page, with the circles, using Huygen's Principle to make the same point. The leftmost scatterer is illuminated first, then the middle one and then the rightmost one. So at the instant shown, the circle representing the radiation from the leftmost one is largest, etc. The new wavefront is tangent to all three wavelets and is in the expected direction.

sophiecentaur
Gold Member
Also, check out http://physics.stackexchange.com/questions/83105/explain-reflection-laws-at-the-atomic-level

Look at the first diagram on the page, with the circles, using Huygen's Principle to make the same point. The leftmost scatterer is illuminated first, then the middle one and then the rightmost one. So at the instant shown, the circle representing the radiation from the leftmost one is largest, etc. The new wavefront is tangent to all three wavelets and is in the expected direction.
You need to be careful here. Huygens is just a 'construction', based on an assumption which is not molecule based ( they didn't have QM in those days). The assumption is that optical media are continuous.

• davenn
You need to be careful here. Huygens is just a 'construction', based on an assumption which is not molecule based ( they didn't have QM in those days). The assumption is that optical media are continuous.

Huygen's Principle assumes every point which light illuminates becomes a source of a spherical waves. The referenced diagram shows three such sources. We don't have to identify them as molecules.

sophiecentaur
Gold Member
Huygen's Principle assumes every point which light illuminates becomes a source of a spherical waves. The referenced diagram shows three such sources. We don't have to identify them as molecules.
Absolutely but it is very easy to mistakenly imagine Huygens sources as being physical entities. PF is full of threads with that confusion.

Maybe what he is asking is why light striking normal to a surface ideally reflects normal to the surface 180 degrees from the incident direction, given that the process involved is absorption and re-radiation from molecules in the material.
I don't Quite understand what you are saying could you explain.

I don't Quite understand what you are saying could you explain.

In geometrical optics we have the law of reflection, "angle of reflection equals angle of incidence," we represent the light by rays that refract or reflect when encountering lenses and mirrors and those objects are represented as simple surfaces.

What is actually happening on a microscopic level is complicated and can be described in several ways. I will try to answer you question using physical optics, where light is a wave phenomenon and it interacts with the constituents of matter in the objects causing these to become new radiators of light. As that light is generally random in direction, the OP was asking how the reflected light "knows" to follow the law of reflection.

In geometrical optics we have the law of reflection, "angle of reflection equals angle of incidence," we represent the light by rays that refract or reflect when encountering lenses and mirrors and those objects are represented as simple surfaces.

What is actually happening on a microscopic level is complicated and can be described in several ways. I will try to answer you question using physical optics, where light is a wave phenomenon and it interacts with the constituents of matter in the objects causing these to become new radiators of light. As that light is generally random in direction, the OP was asking how the reflected light "knows" to follow the law of reflection.
What I don't understand is where the 180 degrees normal to the surface comes from.
How is that figure arrived at it's either 0 degrees if the incident direction is straight in front of the surface or mirror, or anything up to 180 degrees if the incident direction is to the side of the mirror but it's never 180 degrees from the incident direction it can't be.

What I don't understand is where the 180 degrees normal to the surface comes from.

If the light strikes the surface normally (0 deg angle of incidence to the normal) the reflected light has 0 deg angle of reflection from the normal, but it's 180 degrees from the incident direction. If something is moving to the right and then suddenly moves to the left, that's a 180 change in direction.

If the light strikes the surface normally (0 deg angle of incidence to the normal) the reflected light has 0 deg angle of reflection from the normal, but it's 180 degrees from the incident direction. If something is moving to the right and then suddenly moves to the left, that's a 180 change in direction.

Not quite it is probably a misunderstanding of the scale on my part.
Let us take a flat surface (the mirror) and a bouncy ball (the reflected light). If you throw the ball at any angle up to 180 degrees at the flat surface, in a straight line, it will bounce of it at the same angle (angle of incidence is equal to the angle of reflection).
However if you throw the ball at 180 degrees to the flat surface it will be parallel and won't make contact .

jbriggs444
Homework Helper
Not quite it is probably a misunderstanding of the scale on my part.
Let us take a flat surface (the mirror) and a bouncy ball (the reflected light). If you throw the ball at any angle up to 180 degrees at the flat surface
Is this angle measured from the normal? In that case, angles only up to 90 degrees are allowed. Any higher and you are hitting the surface from the back side.
Is this angle measured from the tangent plane? In that case, angles only up to 90 degrees are allowed. Any higher and the angle starts decreasing.
However if you throw the ball at 180 degrees to the flat surface it will be parallel and won't make contact .
A ball thrown at 180 degrees to the normal will hit the flat surface square on from the back side.

Edit:

If you are trying to measure the deflection angle made between the incident trajectory and the reflected trajectory then a deflection angle of 0 degrees would indicate no deflection and a deflection angle of 180 degrees would indicate dead-on reflection with an identical-but-reversed rebound trajectory.

A mirror can produce a deflection angle of 0 degrees by simply not being there.

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Not quite it is probably a misunderstanding of the scale on my part.
Let us take a flat surface (the mirror) and a bouncy ball (the reflected light). If you throw the ball at any angle up to 180 degrees at the flat surface, in a straight line, it will bounce of it at the same angle (angle of incidence is equal to the angle of reflection).
However if you throw the ball at 180 degrees to the flat surface it will be parallel and won't make contact .

We're getting hung up on definitions of angles here. When I posted "Maybe what he is asking is why light striking normal to a surface ideally reflects normal to the surface 180 degrees from the incident direction..." the OP confirmed that is what he meant. This "180 degrees" is measured from the incident direction. The OP was just considering incident light normal to a surface, a special case of the law of reflection.

The usual convention is to measure angles of incidence and reflection from the normal to the surface. In this case, the OP's question would be "why light incident at 0 degrees reflects at 0 degrees..."

• Buckleymanor
davenn
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