Physics [ 2 Marks Questions ]

i) Polarisation by selective absorption
ii) Polarisation by reflection
iii) Polarisation by double refraction
iv) Polarisation by scattering

When a ray of unpolarised light is incident on a calcite crystal, two refracted rays are produced. Hence, two images of a single object are formed. This phenomenon is called double refraction or birefringence.

The secondary focus F2 is defined as a point where all the parallel rays travelling close to the principal axis converge to form an image on the principal axis.

The primary focus F1 is defined as a point where an object should be placed to give parallel emergent rays to the principal axis.

To ensure the critical angle incidence in the core-cladding boundary inside the optical fiber, the light should be incident at a certain angle at the end of an optical fiber while entering it. This is called an acceptance angle.

There is a method to test the two-way mirror. Place the fingernail against the mirror surface. If there is a gap between the nail and its image, then it is a regular mirror. If the fingernail directly touches its image then it is a two-way mirror.

The phenomenon in which a part of the light from a source undergoing reflection and the other part of the light from the same source undergoing refraction at the same surface is called simultaneous reflection or simultaneous refraction.

Refractive index of a transparent medium is defined as the ratio of the speed of light in vacuum (or air) to the speed of light in that medium.

Optical path of a medium is defined as the distance d’ light travels in vacuum in the same time it travels a distance d in the medium.

a) Concave mirror
b) Convex mirror
c) Plane mirror

The rays travelling far away from the principal axis and fall on the mirror far away from the pole are called as marginal rays.

Centre of curvature:
The centre of the sphere of which the mirror is a part is called the center of curvature (C) of the mirror.

Principal axis:
The line joining the pole and the centre of curvature is called the principal axis of the mirror. The light ray travelling along the principal axis towards the mirror after reflection travels back along the same principal axis. It is also called optical axis.

Focal length:
The distance between the pole and the focus is called the focal length (f) of the mirror.

Focal plane:
The plane through the focus and perpendicular to the principal axis is called the focal plane of the mirror.

Focus (or) Focal point:
Light rays travelling parallel and close to the principal axis when incident on a spherical mirror, converge at a point for concave mirror or appear to diverge from a point for convex mirror on the principal axis. This point is called the focus or focal point (F) of the mirror.

Radius of curvature:
The radius of the sphere of which the spherical mirror is a part is called the radius of curvature (R) of the mirror.

The middle point on the spherical surface of the mirror (or) the geometrical center of the mirror is called pole (P) of the mirror.

The bouncing back of light into the same medium when it encounters a reflecting surface is called reflection of light.