The light always travels in a straight line. This can be shown by performing a simple activity. For this, first of all let us take three cardboards of equal size. Now, a hole is made'at the centre of each cardboard at the same level. The three cardboard are placed on a flat table and aligned linearly as shown in the figure.
A candle is lighted at one end of the table with its flame at the level of the holes and is seen at the other end. It is observed that the light of the candle is seen effortlessly. Now, the middle one of the three cardboards is displaced from its position. Now, we are unable to see the light. The reason behind this is that the light travelling in the straight line is obstructed due the misalignment of the cardboards. This proves that light always travels in a straight line

Luminous objects are those objects which emit the light of their own e.g. the stars, the sun, burning candle, etc. While non-luminous objects are those objects which do not emit light of their own, e.g. Blackboard, moon, earth, etc.

The two ways through which a transparent glass sheet can be converted into a translucent sheet are
(i) If one side of butter paper gets covered, then a transparent glass sheet turns into a translucent one.
(ii) By smearing a thin layer of oil.

The dark patch of light is known as a shadow which is formed when there is an opaque object in the path of light.
When an image is the reflection of light. By seeing image, we can identify the object but shadow can mislead you from actual object.

Yes, there are many things that could give a circular shadow if held in one way and a rectangular shadow, if held in another way, e.g. a cylindrical object, a pencil, a candle, etc.
(i) Getting a circular shadow of a candle (which is cylindrical in shape), when it is laid down horizontally

(ii) Getting a rectangular shadow of a candle while in standing or in vertical position.

If we make a metal red hot, it emits light. This is called incandescent body

The pinhole images are inverted while the shadow is different from it.

Shadow of flying aeroplane cannot be seen on the earth because umbra region cannot able to reach the earth.

The object which partially allows the light to pass through it is a translucent Object, e.g. wax paper.

Opaque objects do not allow light to pass through them. When an opaque object is placed in the path of light, then the light is obstructed by the object leading to the formation of a dark patch known as the shadow of that object, which can be taken on a screen placed behind it.
Shadow formation: To form a shadow, there must be (i) a light source (ii) a screen (iii) an opaque object placed between the light source and the screen. For shadow formation, we can follow the given steps
Step 1: Take an opaque object.
Step 2: Throw light on it from a light source.
Step 3: Put a screen behind the object. In this way, a shadow of an object can be obtained on the screen.
In this way, a shadow of an object can be obtained on the screen.

The shadow of the object cannot be caught if the screen and the object are very far from each other. This is the same case, the shadow of the football cannot be seen on the ground when it is kicked high in the air. This happens because if we take the object away and away from the screen, the image becomes smaller and smaller and a time comes when it disappears from the screen.