# Why does a mirage reflect the ground and area in front of it? E.G a car traveling on the highway where you can see the reflection on the road.

2.23K views

Why does a mirage reflect the ground and area in front of it? E.G a car traveling on the highway where you can see the reflection on the road.

In: Physics

Light Rays speed up as they move through air at higher temperatures. This happens naturally as light enters the atmosphere.

As a light ray approaches a hot road at an angle, the bottom of the Ray speeds up first causing it to bend upwards. At hot enough temperatures, like those above an ashphalt road in the summer, it bends up very sharply. This results in you seeing light Rays that never touched the ground.

There is another time that light curves upwards sharply too. When it hits a reflection, like a puddle of water. Because the two are visually identical, your brain assumes they are both puddles.

BUT, because the mirage light bends at a very specific angle, it disappears as you get closer (as your angle to the mirage changes), unlike a reflection. On a perfectly flat road, this would result in a mirage that seemed to stay at a fixed distance from you, like a rainbow.

It is a complicated concept which uses refraction, critical angles, snells law, etc.

First you need to understand refraction. This happens when lights moves from one medium to another. Say water to air. This you can notice in a pool, where if you put a stick half way in water and half outside, you will see it bend. This bending of light is called refraction. This is caused due to differing densities of the two mediums.

When light travels from a medium of higher to lower density. It bends away from the vertical. When it goes from lower to higher density it bends towards the vertical. This is explained in more detail using snells law.

We are interested in the first case where light bends away. The angle at which it bends( angle of refraction) is proportional to the angle at which the light is hitting the transition surface (agngle of incidence) between the two mediums. Now as that angle of incidence keeps increases the angle of refraction keeps increasing as well. Now since the light bends away from the vertical in this case the angle of refraction will always be greater than the angle of incidence. As the angle of incidence gets close to the horizontal (or the horizon where you usually see a mirage) this angle becomes so big that the angle of refraction is now parallel to the ground. So the angle of refraction now starts to just come back into the same medium where it started. Hope that makes sense.

So a mirage is just that but inverted, where the hotter air closer to the ground has lesser density than the cooler air above it. So when light comes from the cooler(more denser) air to the hotter (less denser) air it starts to refract. As you watch the objects closer to the horizon, the angle of incidence increase which proportionally increases the angle of refraction, and when you reach the critical angle, it just starts reflecting back (like a mirror).