I saw the definition of 'lens' in an online portal as
Lens is a type of mirror which refracts the light instead of reflection
Is that true to say 'type of mirror'? Mirrors have an opaque surface in general but lenses have both surfaces transparent. Definition of Lens found in the web.
Physically speaking, no. However, some formula like Snell's law look a lot alike for lens and mirror, you can switch from one to the other by (formally) changing the sign of the optical index.
Briefly going to electromagnetic wave theory, it's easy to see that this is equivalent to changing the traveling direction of a wave:
The generalized definition of the optical index is:
$$n=\frac{k}{k_0}$$
with $k$ the angular wavenumber in matter and $k_0$ its value in the void. With this, a wave that travels in the positive direction of axis $Ox$ can be written as:
$$s=S_0e^{i(\omega t-nk_0x)}$$
Changing the sign of the optical index yields:
$$s=S_0e^{i(\omega t+nk_0x)}$$
which describes a wave traveling in the negative direction of the axis. This explains why the old trick of changing the sign of $n$ to switch from mirror to lens even has a chance to work.
I saw the definition of 'lens' in an online portal as Lens is a type of mirror which refracts the light instead of reflection. Is that true to say 'type of mirror'?
No, by definition, a lens is a transparent material and a mirror is a reflective material.
A lens is a transparent material of two-curved surfaces which is usually made up of glass or plastic(maybe) that used refraction to form an image of an object.
Where a mirror has a curved surface designed to reflect rays and also form images.
