Why do Lasers Reflect Off Mirrors?

The reason why lasers bounce off of mirrors has to do with the laws of reflection and refraction. The law of reflection states that when light reflects off a surface, it changes its direction by an angle equal to the one at which it hits the surface.

For example, if a beam of light hits a mirror perpendicularly, then the reflected beam hits a perpendicular plane as well.

The law of refraction, on the other hand, states that when light goes from one medium to another (for example, air to glass), it changes its direction by an angle proportional to the difference in indices of refraction and the distance traveled.

Since a laser is a beam of light and all beams do basically what reflection and refraction state above, they bounce off of mirrors.

This is summed up perfectly with Snell’s law.

Snell’s law states that, given an incident ray and a refracted ray, the ratio between their sines is equal to the ratio of their respective indices of refraction.

This law can be represented symbolically as sin(i) / sin(r) = n_(1)*n_(2), where i represents the incident ray, r represents the refracted ray, and n_(1) and n_(2) represent the indices of refraction for each medium.

In other words, when a beam from one medium hits a surface in another medium, it will either be reflected or transmitted at an angle that is equal to its incident angle. Since lasers are beams of light, they will either be reflected or refracted when they hit a surface.

If they are reflected, then these beams will bounce off of mirrors. If they are refracted, the beam will follow Snell’s law and either stay within the same medium or go to another medium with an angle equal to its incident angle.

Could a Laser go Through a Mirror?

If a laser is powerful enough, it will go through the mirror. If we think about Snell’s law again and how it can be represented as sin(i) / sin(r) = n_(1)*n_(2), then we can set i to 90 degrees (since this is perpendicular to the surface).

Thus, sin(90 degrees) = n_(2), and n_(2) can be written as 1.

Since we set i to 90 degrees, we will get the same value for r as sin(90): 1 (since sin(90)=1). This means that since r=1, if this was all done mathematically, the refracted ray would be the same as the incident ray, meaning that no refraction would occur.

This means that powerful lasers, will go through the mirror because there is no difference between their angles on either side of the surface.

To understand this even better, it can be said that if a laser’s power is so great to overcome the index of refraction, it will go through the mirror.

How Many Watts for a Laser to go Through a Mirror?

A laser would have to be 100 watts or more for it to go through a common mirror.

That’s a very powerful laser.

A common laser pointer outputs 5 milliwatts of power while a high-end laser can output 300-500 watts. A typical laser used for industrial purposes might output anywhere from 200 to 1000 watts. A 1000 watt focused beam of light would be able to cut through a number of materials, including steel.

Does a Mirror Absorb Energy from a Laser?

Yes, it does. When a laser bounces off of a mirror, some percentage of the light is absorbed by the mirror because mirrors can not completely reflect light.

Thus, if you think about this, there is more energy in the laser than what was received at the other side of the mirror since some energy went to powering the mirror.

The energy that was used to power the mirror goes out as heat, so there is less energy in the laser than what you initially fired.

Do Lasers Lose Power When bounced off of a Mirror?

Yes, lasers bounce off of mirrors with less power than what they had when they were initially fired. This occurs because the energy in the laser is conserved, but some is lost to powering the mirror.

Energy is lost through an increase in the index of refraction (which means that the beam needs more power to overcome this). The beam will bounce off, but with less intensity, than it had before.

Thus, there will be less power behind the beam after it hits a high index of refraction material.

More specifically, there is more energy lost in the laser because it requires more power to overcome higher indices of refraction, so some energy goes towards powering the mirror and not towards affecting the medium through which the beam travels.

A lower intensity laser will bounce off of a high index of refraction material more easily than a higher intensity laser.

Final Words

As you now know a laser will reflect off mirrors because they can’t overcome the index of refraction, otherwise the laser will go through the mirror. The laser would have to be very powerful to accomplish that.


Here’s a very fascinating video where a laser pointer is used with a mirror and a speaker to show the shapes of different sounds.