NCERT 8th Class (CBSE) Science: Light

Question: What is a kaleidoscope? State the principle on which it is based.

Answer: The principle of forming multiple images by mirrors at an angle to each other is used in a toy called kaleidoscope. It consists of three plane mirrors inclined at an angle of 60° to each other. The arrangement is kept in a cylindrical cardboard box with a few pieces of colored glass at one end. The mirrors form multiple images of these glass pieces, resulting in beautiful patterns when seen from the other end. On rotating the cylindrical box, the patterns change continuously as the position of the glass pieces shifts.

Question: List the main characteristics of the image formed by a plane mirror.

Answer: Characteristics of an image formed with a plane mirror:

1. The image formed is erect.
2. The image is of the same size as the object.
3. The image is laterally inverted, that is, the right side of the object appears as the left side of the image.
4. The image is as far behind the mirror as the object is in front of it.
5. The image formed is virtual, that is, it cannot be caught on a screen. There is no actual meeting of the light rays (incident and reflected).

Question: State three uses of a plane mirror.

Answer: Uses of plane mirrors:

1. We commonly use plane mirrors in our homes to look at our reflection. In beauty parlours, plane mirrors kept at an angle are used to view the side of the head. Plane mirrors parallel to each other are
   used to view the back of the head.
2. Plane mirrors are used to reflect light on an object. For example, during outdoor shooting of a film, metal sheets are used as plane mirrors to reflect sunlight on the actors.
3. They are also used in solar cookers to reflect light on the food being cooked.
4. They are used in periscopes. From a submarine under the sea, a sailor can see objects and enemy ships on the surface of the sea by using a periscope.

Question: Two mirrors are kept perpendicular to each other. A ray strikes one mirror at an angle of 45° to the normal. Find the direction of the ray after reflection by the second mirror, by drawing a ray diagram.

Answer:

Question: In which direction will a light ray falling along the normal to a mirror be reflected?

Answer: If a light ray falls along the normal to a mirror it will be reflected back along the same path.

Question: A ray incident on a plane mirror is reflected in a direction such that the angle between the incident and the reflected ray is 90°. What are the angles of incidence and reflection?

Answer: If the angle between the incident and reflected ray is 90°, the angle of incidence and reflection are 45° each, i.e. half of 90°.