# Why does a mirage disappear as you approach it?

Related: why do mirages only appear on hot day?

The title says it all: why does a mirage disappear as you approach it? The related question does not answer this.

• Is this the related question you mention? Why do mirages only appear on hot days? For future reference, the "related" sidebar is subject to change based on whatever algorithm is employed and any new content added to the site. Any internal links in posts or comments, on the other hand, automatically get permanently added to the "linked" sidebar.
– user10851
Aug 25 '13 at 3:08
• @Chris: If I read you correctly, because it is an internal link (which the software that creates the question did not allow me to do) I don't have reference it because it automatically comes up on the sidebar.
– Juan
Aug 25 '13 at 9:23

## 1 Answer

In the textbook mirage, a ray originating in the sky is refracted sufficiently to arrive at your eye from "below" so that it appear to come from the ground.

Now, if we put your eye at a height $h$ and look to see if a mirage appears at a distance $d$ in front of you that light ray has to refract through an angle slightly more than $\theta = \arctan \frac{h}{d}$. We you look at a close spot (or move toward the "location" of the mirage, $d$ gets smaller and $theta$ gets larger. For any given temperature gradient there is a maximum angle at which the ray still originates from the sky and therefore a minimum distance at which you still see the mirage.

• Great explanation!
– Juan
Aug 25 '13 at 21:16