Question

3. With the aid of a well labelled diagram, describe the features of an image formed by a prism. 4. Briefly describe three ways by which prisms are used in the assessment of squint and heterophoria.

Answer 1

GLASS PRISM

In a rectangular glass slab, the emergent light rays are parallel to the incident light rays because the opposite faces of a rectangular glass slab (where refraction takes place) are parallel to one another.
The triangular glass prism is a transparent object made of glass having two triangular ends and three rectangular sides (or rectangular faces).The opposite faces of a triangular glass prism are not parallel to one another.
They are inclined at an angle to one another.The angle between its opposite faces is called the angle of the prism.
For the sake of convenience in drawing ray-diagrams, a triangular glass prism is represented by drawing a triangle. A triangular glass prism is usually called glass prism.
In refraction through a glass slab, the emergent ray is parallel to the incident ray but in refraction through a glass prism, the emergent ray is not parallel to the incident ray because the opposite faces of the glass prism (where refraction takes place) are not parallel to one another. In refraction through a glass prism, the emergent ray is deviated from its original direction by a certain angle.

REFRACTION OF LIGHT THROUGH A GLASS PRISM

M Angle of deviation N -NM Emergent ray D Incident ray A R Glass prism

When a ray of light passes through a glass prism, refraction (or bending) of light occurs both, when it enters the prism as well as when it leaves the prism. Since the refracting surfaces (PQ and PR) of the prism are not parallel, therefore, the emergent ray and incident ray are not parallel to one another.
(1) A glass prism PQR has been kept on its base QR.
(2) A ray of light AB is incident on the face PQ of the prism. The incident ray AB is going from air (rarer medium) into glass (denser medium), so it bends towards the normal BN' and goes along the direction BC inside the glass prism. Thus, BC is the refracted ray of light which bends towards the base QR of the prism.
(3) When the ray of light BC travelling in the glass prism comes out into air at point C, refraction takes place again. Since the ray BC is going from glass (denser medium) into air (rarer medium), so it bends away from the normal MC and goes along the direction CD in the form of emergent ray.
(4) The emergent ray of light CD bends towards the base QR of the prism. When a ray of light passes through a prism, it bends towards the base of prism.
The angle between incident ray and emergent ray is called angle of deviation. The angle EOD is the angle of deviation. It is the peculiar shape (triangular shape) of the glass prism which makes the emergent ray bend with respect to the incident ray.