Angle of deviation

The idea

In a prism, light bends towards the normal entering the glass and away from it on leaving; because the faces are inclined, the emergent ray bends at an angle to the incident ray, the angle of deviation.

Light through a prism

In a glass slab, the light that comes out is parallel to the light that went in. A prism has faces that are not parallel to each other.

Will the ray that leaves the prism be parallel to the ray that entered?

• No, it leaves bent at an angle
• Yes, it is parallel to the entering ray
• It does not leave the prism

Send a ray

Change the angle of incidence and watch the ray. The glass here is only an example with a refractive index of 1.5.

Bent twice

The ray bends towards the normal when it enters the glass and away from the normal when it leaves. Because the two faces are inclined, the two bends add up, and the ray comes out at an angle to the incident ray.

In a prism, light bends towards the normal entering the glass and away from it on leaving; because the faces are inclined, the emergent ray bends at an angle to the incident ray, the angle of deviation.

Notes

The prism bends light towards the normal on entering and away on leaving; its faces are inclined, so the emergent ray makes the angle of deviation with the incident ray.

Check yourself

A ray enters the prism from air to glass at the first face. It bends…

At the second face, the ray goes from glass to air. It bends…

The angle between the incident ray and the emergent ray is called…

Why does the emergent ray bend at an angle to the incident ray?

• towards the normal — correct. Yes!
• away from the normal. Going from air to glass the ray bends towards the normal.
• not at all. At the first surface the light ray on refraction has bent towards the normal.
• towards the normal. Going from glass to air the ray bends away from the normal.
• away from the normal — correct. Yes!
• not at all. At the second surface the ray enters air from glass and bends away from the normal.
• the angle of the prism. The angle of the prism is the angle between two lateral faces.
• the angle of incidence. The angle of incidence is between the incident ray and the normal at the surface.
• the angle of deviation — correct. Yes!
• Because of the peculiar shape of the prism, with inclined faces — correct. Yes!
• Because its faces are parallel, as in a glass slab. With parallel faces, as in a glass slab, the emergent ray is parallel to the incident ray.
• Because the glass absorbs part of the light. The bend comes from the peculiar shape of the prism, whose surfaces are inclined to each other.