Usually what kind of lens are used in mobile phones. I mean of the types similar to convex, concave etc.. Are they fixed focal length usually?
How do we get the focal length of camera from digital photo? (Programmatically using MATLAB for instance)
You might find this page interresting, discussing the technical feat of the pureview 808 lens.
The highlight is that in order to get good resolution beyond diffraction limits they needed to make lens elements of funny shapes, partially concave partially convex aspherical, which would be impossible to do for larger lenses for glass as they'd have to be ground. And they cannot use plastic for big lenses due to thermal expansion.
Also note the last elements near the sensor, where the lenses seemingly turn the wrong way, which is typical for wide angle lenses.
If you cant see the focal length in the data sheet and exif data you can find it in matlab with bouquet's camera calibration toolkit.
The result will be in pixels, but if you take the sensor size divided by resolution multiplied the focal pixel size you get it in mm (if the sensor size was given in mm).
It depends on the camera design, but they are generally small fixed focal length lenses that are wide and relatively fast to allow lots of light to the sensor. This can vary depending on what the designers were trying to accomplish though. There are even a (very) limited number of phones that include zoom lenses.
To determine the focal length, the easiest way would be to look it up. If this isn't an option then you would need to figure out the field of view and sensor size and work backwards to determine the optical characteristics using a field of view calculator.