Physics [5 Mark Question Answer]

Light rays from the sun while coming towards earth suffers successive refractions from a rarer to a denser medium and so it bends towards the normal at each refraction. These rays on earth appear to be coming from an apparent position of the sun which is higher than its actual position. Thus, the sun is seen even when it is slightly below the horizon during sunset and sunrise.

Stars are very far from us. The light ray coming from a star undergoes successive refractions on entering the earth’s atmosphere and bends towards the normal. Hence, an apparent image of the star is seen. Simultaneously due to the movement of air, the refractive index of layers of atmosphere keeps on changing, i.e., the apparent position of star keeps changing. Thus, the star appears to twinkle.

Critical angle for diamond is quite less and is only 24°. Besides these diamonds are cut very sharp, making a large number of the refracting surfaces. Hence when a ray of light enters a diamond, it suffers a series of total internal reflections on account of a very small critical angle. This makes a ray of light literally trapped within the diamond for a little time and this trapped light energy makes a diamond sparkle.

(i) Ray diagram of monochromatic light

(ii) Incident ray and emergent rays are parallel to each other.
(iii) Lateral displacement is marked by d in the diagram.

In rectangular glass slab, the ray undergoes the only refraction and emerges out parallel. In a prism, the emerging ray is not parallel but split due to change in wavelength of different colour of light. The shape of the glass slab with two prisms up and down splits light but recombines it into one.

As shown in diagram, a beam of light is incident on face AB of the prism normally so it passes undeviated and strikes the face AC where it makes an angle of 45° with the normal to AC. Because here the incident angle is more than critical angle so rays suffer total internal reflection and reflect at angle of 45°. The beam then strikes face BC, where it is incident normally and so passes undeviated. As a result the incident beam gets deviated through 90°.Such a prism is used in periscope.

A fish can see everything above it in clear water irrespective of its position in different depths. The critical angle for water is 48.5°. The angle of the cone of vision for the fish is twice this angle.

Hence it is 97°. All the objects outside water but in this cone of vision Will be seen by the fish. Outside this range, the fish will see all the objects in the surrounding area (see figure). Those objects which are at the bottom will be seen by the fish due to total internal reflection taking place at the water-air interface.