VIII.Optics of Retinoscopy-Contd.

Light rays entering subject's eye. (Illumination Stage)

Consider only the immediate source of illumination in front of the subject's eye . This may be a virtual image or real image of an original source of illumination behind the subject. When the immediate source is towards one side, the other side of the retina will be illuminated. When the source is shifted to the other side, illumination will be shifted to the opposite side of the retina.
This will be the situation in all states of refraction of the eye. (but rays coming out of the eye behave differently in different types of refraction).Type of mirror we are using to reflect the rays from the original source of illumination is not the primary factor deciding the optics.

If we are using a Plane mirror, the virtual image of the original source of illumination is formed as far behind the mirror as the original source is in front of it. So, the tilt of the mirror to one side will shift the image (immediate source of illumination) to the other side. Tilt of the plane mirror and the shift of illuminated patch on the retina are in the same direction.

If we are using a Concave mirror, the real image of the original source of illumination is formed in front of the mirror ( position depends on the focal length of the mirror). So, the tilt of the mirror to one side will shift the image (immediate source of illumination) to the same side itself. Tilt of the concave mirror and the shift of illuminated patch on the retina are in opposite directions.

In Practice we are considering the movement of the mirror .
When we use plane mirror we get movement of the mirror and movement of the retinal illumination in the same direction.( because of the movement of immediate source of illumination - ie virtual image formed behind the mirror - in the opposite direction of movement of the mirror).
When we use concave mirror we get movement of the mirror and movement of retinal illumination in opposite directions (because of the movement of immediate source of illumination - ie real image formed in front of the mirror – in the same direction of movement of the mirror).