A wind turbine comprising: a hub rotatable about an axis; an elongated blade body extending along a longitudinal axis and having a blade root at one longitudinal end of the blade body and a blade tip at another longitudinal end of the blade body, wherein the blade body is connected to the hub via the blade root; a leading edge and a trailing edge, both extending from the blade root to the blade tip, the trailing edge spaced from the leading edge in a span-wise direction of the blade body; an upper skin and a lower skin extending from the leading edge to the trailing edge and from the blade root to the blade tip, the lower skin spaced from the upper skin in a thickness direction of the blade body, a system for detecting an ice layer on the blade body, the system comprising: a light source for emitting a light beam; a light splitting optical element optically connected to the light source so as to receive the light beam emitted from the light source, the optical element adapted to split the light beam received from the light source into a reference light beam and a detecting light beam; a boundary area which is arranged at the blade body so as to be exposed to the outer surroundings of the blade body and which is optically connected to the light splitting optical element such as to receive the detecting light beam and to reflect an internal reflected part of the detecting light beam at the boundary area and to transmit a transmitting part of the detecting light beam to the outer surroundings of the blade body through the boundary area and to allow an external reflected part of the transmitting part of the detecting light beam, which has been reflected from outside of the blade body, to be re-transmitted through the boundary area; a light measuring device optically connected to the light splitting optical element so as to receive the reference light beam, the internal reflected part and the external reflected part of the detecting light beam, wherein the light measuring device is configured to analyze the received light and to determine the presence and/or thickness of the ice layer on the blade body dependent on the analysis. a hub rotatable about an axis; an elongated blade body extending along a longitudinal axis and having a blade root at one longitudinal end of the blade body and a blade tip at another longitudinal end of the blade body, wherein the blade body is connected to the hub via the blade root; a leading edge and a trailing edge, both extending from the blade root to the blade tip, the trailing edge spaced from the leading edge in a span-wise direction of the blade body; an upper skin and a lower skin extending from the leading edge to the trailing edge and from the blade root to the blade tip, the lower skin spaced from the upper skin in a thickness direction of the blade body, a system for detecting an ice layer on the blade body, the system comprising: a light source for emitting a light beam; a light splitting optical element optically connected to the light source so as to receive the light beam emitted from the light source, the optical element adapted to split the light beam received from the light source into a reference light beam and a detecting light beam; a boundary area which is arranged at the blade body so as to be exposed to the outer surroundings of the blade body and which is optically connected to the light splitting optical element such as to receive the detecting light beam and to reflect an internal reflected part of the detecting light beam at the boundary area and to transmit a transmitting part of the detecting light beam to the outer surroundings of the blade body through the boundary area and to allow an external reflected part of the transmitting part of the detecting light beam, which has been reflected from outside of the blade body, to be re-transmitted through the boundary area; a light measuring device optically connected to the light splitting optical element so as to receive the reference light beam, the internal reflected part and the external reflected part of the detecting light beam, wherein the light measuring device is configured to analyze the received light and to determine the presence and/or thickness of the ice layer on the blade body dependent on the analysis. a light source for emitting a light beam; a light splitting optical element optically connected to the light source so as to receive the light beam emitted from the light source, the optical element adapted to split the light beam received from the light source into a reference light beam and a detecting light beam; a boundary area which is arranged at the blade body so as to be exposed to the outer surroundings of the blade body and which is optically connected to the light splitting optical element such as to receive the detecting light beam and to reflect an internal reflected part of the detecting light beam at the boundary area and to transmit a transmitting part of the detecting light beam to the outer surroundings of the blade body through the boundary area and to allow an external reflected part of the transmitting part of the detecting light beam, which has been reflected from outside of the blade body, to be re-transmitted through the boundary area; a light measuring device optically connected to the light splitting optical element so as to receive the reference light beam, the internal reflected part and the external reflected part of the detecting light beam, wherein the light measuring device is configured to analyze the received light and to determine the presence and/or thickness of the ice layer on the blade body dependent on the analysis.