The two effects of light on plants are:
i) Plants use light for the preparation of their own food through the process of photosynthesis.
ii) Some plants (like sunflower) turn in the direction of light.

To see a thing, there must be light which comes after reflection from the object to our eyes. In dark room there is no light, so we are not able to see our face.

If an object is of two different colours, then the colour of shadow will be black.

Limbra is called the dark part of shadow.

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

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 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.

Position A of the torch will form the image (a) and position B of the torch will form the image (b) of the football.

Place the object in front of a mirror and put the light source behind the object as shown in figure.

The image/shadow will be formed by the object as well as the image of the object by mirror.

No, they do not cast any shadow on the ground because they are so high.
They can cast shadow only if they are at some lower height, i.e. if they are near to the ground, we can see their shadows.

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 image can be observed behind wall using a periscope with the help of plane mirror by reflection of light.

So, when rays of light coming from tree behind the wall fall on plane mirror (P₁), then after reflection, it goes to the plane mirror (P₂), which again reflects the light rays towards viewer. Therefore, we can see the tree which is behind wall.