# What Happens to Light When It Reflects Off a Mirror?

• Zen2
In summary, the conversation discusses the properties of light and what happens when it reflects off a mirror. It is mentioned that light exhibits both wave and matter-like properties and that there must be some kind of deceleration for light, but in reality, light does not accelerate or decelerate. Instead, when light hits a mirror, it excites the electrons and is then re-emitted in all directions, with the light that reflects back towards the source being what we perceive as a reflection. This is all governed by a separate set of physical laws for electromagnetic radiation, which includes equations such as the conservation of charge and Maxwell's equations.

#### Zen2

Light is an electromagnetic wave that exhibits both wave and matter-like properties yes? I was wondering what happens to light when it reflects off a mirror.

If a tennis ball is thrown at a wall, the moment of contact between the ball and the wall is very brief, so the deceleration rate is very, very high right? Say the ball was traveling 20ms^-1 then bounces back at around -19ms^-1, making a difference of 39ms^-1. The length of time the ball contacts the wall is around 0.02s (for the sake of example). Then the deceleration would be 1950ms^-2. There is a point in time where the ball is simply stationary.

Light travels at around 299,792,458ms^-1. The moment of 'contact' is near negligible, most likely in the picoseconds. Even so, there must be a measure of deceleration for light whether it is a particle (matter) or wave (radiation). If this is the case, then what kind of properties would light have at the inevitable point in time of 0 velocity?

If I have it all wrong and there is a separate set of physical laws governing EM radiation, what is it?

In fact there is a separate set of physical laws governing EM radiation:
$$\vec{\nabla}\cdot\vec{D} = \rho_f$$
$$\vec{\nabla}\cdot\vec{B} = 0$$
$$\vec{\nabla}\times\vec{E} = -\frac{\partial\vec{B}}{\partial t}$$
$$\vec{\nabla}\times\vec{H} = \frac{\partial\vec{D}}{\partial t} + \vec{J}_f$$
The process of solving those equations to figure out what happens at a reflecting surface is kind of long and involved, but you do find that EM waves are reflected without undergoing any change in speed. There's no point at which the light is not traveling at 299792458 m/s (which is an exact value, by the way). That's consistent with relativity, which says that anything that ever travels at the speed of light will always travel at the speed of light.

Zen2 said:
Light is an electromagnetic wave that exhibits both wave and matter-like properties yes? I was wondering what happens to light when it reflects off a mirror.

<snip>

If I have it all wrong and there is a separate set of physical laws governing EM radiation, what is it?

Light can exhibit both wavelike and particlelike behavior; this should not be confused with ponderable matter. Light may act as localized (particle) or nonlocalized (wave) objects.

Light, when scattering off an interface (including a mirror), does indeed transfer momentum to the scattering object.

Zen2 said:
... Even so, there must be a measure of deceleration for light whether it is a particle (matter) or wave (radiation). If this is the case, then what kind of properties would light have at the inevitable point in time of 0 velocity? ...
Very very erroneous thinking! Light doesn't accelerate, neither decelerate. In the moment it irradiates the mirror, light excites the electrons that make the mirror and, if you wish, stops existing. Electrons stay excited for about a few nanoseconds, and then release their excess energy in the form of light. It happens so that this light will come toward the source, thus reflection (in fact, electrons emit light in all directions, but light in forward direction cancels, while that returning doesn't).

## 1. What is reflection of light?

Reflection of light is the bouncing back of light rays when they hit a surface, such as a mirror. This phenomenon occurs due to the smoothness and flatness of the surface, which allows the light to bounce off at the same angle that it hits the surface.

## 2. Why does light reflect off of mirrors?

Light reflects off of mirrors because of the principle of reflection, which states that the angle of incidence (the angle at which the light hits the surface) is equal to the angle of reflection (the angle at which the light bounces off the surface). This is possible because mirrors have a highly reflective surface that does not absorb light.

## 3. What happens to the color of light when it reflects off a mirror?

The color of light remains the same when it reflects off a mirror. Mirrors do not change the color of light, they simply reflect it. The color of an object we see in a mirror is determined by the color of the light that is reflected off of it.

## 4. Does the angle of incidence affect the angle of reflection?

Yes, according to the law of reflection, the angle of incidence does affect the angle of reflection. The angle of reflection is always equal to the angle of incidence, meaning that if the angle of incidence changes, the angle of reflection will change as well.

## 5. Can light be reflected multiple times off a mirror?

Yes, light can be reflected multiple times off a mirror. This is known as multiple reflections and it occurs when the light reflects off of a series of surfaces. Each time the light reflects, the angle of incidence and angle of reflection will change, resulting in a different path for the light rays.